BASIC MANUFACTURING PROCESS

Transcription

1 BASIC MANUFACTURING PROCESS By Prof.(Dr) MANOJ KUMAR PRADHAN B.E, (University College of Engineering, Burla, Odisha) M.Tech, (National Institute of Technology, Rourkela) Ph.D, UCE, Sambalpur University, Odisha Professor and Head Department of Mechanical Engineering Gandhi Institute for Technological Advancement (GITA) Bhubaneswar, Odisha

2 B.Tech (Mechanical Engineering) detail Syllabus for Admission Batch thSemester PME4I102 - BASIC MANUFACTURING PROCESS MODULE - I (10 LECTURES) 1. Foundry: a. Types of patterns, pattern materials and pattern allowances. b. Molding Materials - sand molding, metal molding, investment molding, shell molding. c. Composition of molding sand, Silica sand, Zircon sand, binders, additives, Binders - clay, binders for CO2. sand, binder for shell molding, binders for core sand. d. Properties of molding sand and sand testing. e. Melting furnaces - cupola, resistance furnace, induction and arc furnace. f. Solidification of castings, design of risers and runners, feeding distance, centre line freezing resistance chills and chaplets. g. Degasification and inoculation of metals. h. Casting methods like continuous casting, centrifugal casting, disc casting. i. Casting defects. MODULE II (8 LECTURES) 2. Welding and cutting: Introduction to gas welding, cutting, Arc welding and equipment s. TIG (GTAW) and MIG (GMAW) welding, resistance welding and Thermit welding. Weldablity Modern Welding methods like plasma Arc, Laser Beam, Electron Beam, Ultrasonic, Explosive and friction Welding, edge preparation in butt welding. Brazing and soldering, welding defects. Destructive and non-destructive testing of castings and welding. MODULE III (08 LECTURES) 3. Brief introduction to powder metallurgy processes. 4. Plastic deformation of metals: Variables in metal forming and their optimization. Dependence of stress strain diagram on Strain rate and temperature. Hot and cold working of metals, classification of metal forming processes. 5. Rolling: Pressure and Forces in rolling, types of rolling mills, Rolling defects. 6. Forging: Smith Forging, Drop and Press forging, M/c forging, Forging defects. MODULE IV (08 LECTURES) 7. Extrusions: Direct, Indirect, Impact and Hydrostatic extrusion and their applications, Extrusion of tubes. 8. Wire drawing methods and variables in wire-drawing, Optimum dies shape for extrusion and drawing. 9. Brief introduction to sheet metal working: Bending, Forming and Deep drawing, shearing. 10. Brief introduction to explosive forming, coating and deposition methods. TEXT BOOKS 1. Manufacturing technology by P.N.Rao, Tata McGraw Hill publication. 2. Welding Technology by R.A. Little, TMH 3. Manufacturing Science by A.Ghosh and A K Malick, EWP REFERENCE BOOKS 1. Fundamentals of metal casting technology by P.C. Mukherjee, Oxford PIBI. 2. Mechanical Metallurgy by Dieter, Mc-Graw Hill 3. Processes and Materials of Manufacture by R.A Lindberg, Prentice hall (India) 4. A Text Book of Production Engineering by P.C.Sharma, S.Chand

3 MODULE-I

4 INTRODUCTION Casting is the process of producing metal parts by pouring molten metal into the mould cavity of the required shape and allowing the metal to solidify. The solidified metal piece is called as casting. Manufacture of a machine part by heating a metal or alloy above its melting point and pouring the liquid metal/alloy in a cavity approximately of same shape and size as the machine part is called casting process. After the liquid metal cools and solidifies, it acquires the shape and size of the cavity and resembles the finished product required. The department of the workshop, where castings are made is called foundry. The manufacture of a casting requires: (a) Preparation of a pattern, (b) Preparation of a mould with the help of the pattern, (c) Melting of metal or alloy in a furnace, (d) Pouring of molten metal into mould cavity, (e) Breaking the mould to retrieve the casting, (f) Cleaning the casting and cutting off risers, runners etc., (this operation is called fettling ), and (g) Inspection of casting. Castings are made in a large number of metals and alloys, both ferrrous and non-ferrous. Grey cast iron components are very common; steel castings are stronger and are used for components subject to higher stresses. Bronze and brass castings are used on ships and in marine environment, where ferrous items will be subjected to heavy corrosion. Aluminium and aluminium-magnesium castings are used in automobiles. Stainless steel castings are used for making cutlery items. Types of casting Conventional Methods Unconventional Methods 1 CO 2 Moulding (Strong mould) 1 Green sand mould 2 Permanent (Metal mould) 2 Dry sand mould 3 Shell Moulding (Thinn mould) MOULDING AND CASTING PROCESS 4 Investment casting (Precision) 5 Centrifugal (without core) 6 Continuous Casting (Open)

5 1.1 Casting Terms i Flask: A metal or wood frame, without fixed top or bottom, in which the mold is formed. Depending upon the position of the flask in the molding structure, it is referred to by various names such as drag - lower molding flask, cope - upper molding flask, cheek - intermediate molding flask used in three piece molding. ii. Pattern: It is the replica of the final object to be made. The mold cavity is made with the help of pattern. iii. Molding sand: Sand, which binds strongly without losing its permeability to air or gases. It is a mixture of silica sand, clay, and moisture in appropriate proportions. iv. Facing sand: The small amount of carbonaceous material sprinkled on the inner surface of the mold cavity to give a better surface finish to the castings. v. Core: A separate part of the mold, made of sand and generally baked, which is used to create openings and various shaped cavities in the castings. vi. Pouring basin: A small funnel shaped cavity at the top of the mold into which the molten metal is poured. vii. Sprue: The passage through which the molten metal, from the pouring basin, reaches the mold cavity. In many cases it controls the flow of metal into the mold. viii. Runner: The channel through which the molten metal is carried from the sprue to the gate. ix. Gate: A channel through which the molten metal enters the mold cavity. x. Parting line: This is the dividing line between the two molding flasks that makes up the mold. xi. Chaplets: Chaplets are used to support the cores inside the mold cavity to take care of its own weight and overcome the metallostatic force. xii. Riser: A column of molten metal placed in the mold to feed the castings as it shrinks and solidifies. Also known as "feed head". xiii. Vent: Small opening in the mold to facilitate escape of air and gases.

6 Fig-1 1.2:Steps in Making Sand Castings There are six basic steps in making sand castings: Patternmaking Core making Molding Melting and pouring Cleaning Pattern making The pattern is a physical model of the casting used to make the mold. The mold is made by packing some readily formed aggregate material, such as molding sand, around the pattern. When the pattern is withdrawn, its imprint provides the mold cavity, which is ultimately filled with metal to become the casting. If the casting is to be hollow, as in the case of pipe fittings, additional patterns, referred to as cores, are used to form these cavities. Core making Cores are forms, usually made of sand, which are placed into a mold cavity to form the interior surfaces of castings. Thus the void space between the core and mold-cavity surface is what eventually becomes the casting. Molding Molding consists of all operations necessary to prepare a mold for receiving molten metal. Molding usually involves placing a molding aggregate around a pattern held with a supporting frame, withdrawing the pattern to leave the mold cavity, setting the cores in the mold cavity and finishing and closing the mold. Melting and Pouring

7 The preparation of molten metal for casting is referred to simply as melting. Melting is usually done in a specifically designated area of the foundry, and the molten metal is transferred to the pouring area where the molds are filled. Cleaning Cleaning refers to all operations necessary to the removal of sand, scale, and excess metal from the casting. Burned-on sand and scale are removed to improved the surface appearance of the casting. Excess metal, in the form of fins, wires, parting line fins, and gates, is removed. Inspection of the casting for defects and general quality is performed. 1.3Pattern The pattern is the principal tool during the casting process. It is the replica of the object to be made by the casting process, with some modifications. The main modifications are the addition of pattern allowances, and the provision of core prints. If the casting is to be hollow, additional patterns called cores are used to create these cavities in the finished product. The quality of the casting produced depends upon the material of the pattern, its design, and construction. The costs of the pattern and the related equipment are reflected in the cost of the casting. The use of an expensive pattern is justified when the quantity of castings required is substantial Functions of the Pattern 1. A pattern prepares a mold cavity for the purpose of making a casting. 2. A pattern may contain projections known as core prints if the casting requires a core and need to be made hollow. 3. Runner, gates, and risers used for feeding molten metal in the mold cavity may form a part of the pattern. 4. Patterns properly made and having finished and smooth surfaces reduce casting defects. 5. A properly constructed pattern minimizes the overall cost of the castings Pattern Material Patterns may be constructed from the following materials. Each material has its own advantages, limitations, and field of application. Some materials used for making patterns are: wood, metals and alloys, plastic, plaster of Paris, plastic and rubbers, wax, and resins. To be suitable for use, the pattern material should be: 1. Easily worked, shaped and joined 2. Light in weight 3. Strong, hard and durable 4. Resistant to wear and abrasion 5. Resistant to corrosion, and to chemical reactions 6. Dimensionally stable and unaffected by variations in temperature and humidity

8 7. Available at low cost The usual pattern materials are wood, metal, and plastics. The most commonly used pattern material is wood, since it is readily available and of low weight. Also, it can be easily shaped and is relatively cheap. The main disadvantage of wood is its absorption of moisture, which can cause distortion and dimensional changes. Hence, proper seasoning and upkeep of wood is almost a pre-requisite for largescale use of wood as a pattern material. 1.4-Pattern Allowances Pattern allowance is a vital feature as it affects the dimensional characteristics of the casting. Thus, when the pattern is produced, certain allowances must be given on the sizes specified in the finished component drawing so that a casting with the particular specification can be made. The selection of correct allowances greatly helps to reduce machining costs and avoid rejections. The allowances usually considered on patterns and core boxes are as follows: 1. Shrinkage or contraction allowance 2. Draft or taper allowance 3. Machining or finish allowance 4. Distortion or camber allowance 5. Rapping allowance Shrinkage or Contraction Allowance All most all cast metals shrink or contract volumetrically on cooling. The metal shrinkage is of two types: i. Liquid Shrinkage: it refers to the reduction in volume when the metal changes from liquid state to solid state at the solidus temperature. To account for this shrinkage; riser, which feed the liquid metal to the casting, are provided in the mold. ii. Solid Shrinkage: it refers to the reduction in volume caused when metal loses temperature in solid state. To account for this shrinkage allowance is provided on the patterns. The rate of contraction with temperature is dependent on the material. For example steel contracts to a higher degree compared to aluminum. To compensate the solid shrinkage, a shrink rule must be used in laying out the measurements for the pattern. A shrink rule for cast iron is 1/8 inch longer per foot than a standard rule. If a gear blank of 4 inch in diameter was planned to produce out of cast iron, the shrink rule in measuring it 4 inch would actually measure 4-1/24 inch, thus compensating for the shrinkage. The various rate of contraction of various materials are given in Table 1. Table 1 : Rate of Contraction of Various Metals 1.4Material Dimension Shrinkage allowance (inch/ft)

9 Grey Cast Iron Cast Steel Aluminum Magnesium Up to 2 feet 2 feet to 4 feet over 4 feet Up to 2 feet 2 feet to 6 feet over 6 feet Up to 4 feet 4 feet to 6 feet over 6 feet Up to 4 feet Over 4 feet EXERCISE 1 The casting shown is to be made in cast iron using a wooden pattern. Assuming only shrinkage allowance, calculate the dimension of the pattern. (All Dimensions are in Inches) Solution 1 The shrinkage allowance for cast iron for size up to 2 feet is o.125 inch per feet (as per Table 1) For dimension 18 inch, allowance = 18 X / 12 = inch» 0.2 inch For dimension 14 inch, allowance = 14 X / 12 = inch» 0.15 inch For dimension 8 inch, allowance = 8 X / 12 = inch» inch For dimension 6 inch, allowance = 6 X / 12 = inch» inch The pattern drawing with required dimension is shown below: Draft or Taper Allowance

10 By draft is meant the taper provided by the pattern maker on all vertical surfaces of the pattern so that it can be removed from the sand without tearing away the sides of the sand mold and without excessive rapping by the molder. Figure 3 (a) shows a pattern having no draft allowance being removed from the pattern. In this case, till the pattern is completely lifted out, its sides will remain in contact with the walls of the mold, thus tending to break it. Figure 3 (b) is an illustration of a pattern having proper draft allowance. Here, the moment the pattern lifting commences, all of its surfaces are well away from the sand surface. Thus the pattern can be removed without damaging the mold cavity. Draft allowance varies with the complexity of the sand job. But in general inner details of the pattern require higher draft than outer surfaces. The amount of draft depends upon the length of the vertical side of the pattern to be extracted; the intricacy of the pattern; the method of molding; and pattern material. Table 2 provides a general guide lines for the draft allowance. Table 2 : Draft Allowances of Various Metals Pattern material Height of the given surface (inch) Draft angle (External surface) Draft angle (Internal surface) to Wood 2 to to to to Metal and plastic 2 to to to

11 Machining or Finish Allowance The finish and accuracy achieved in sand casting are generally poor and therefore when the casting is functionally required to be of good surface finish or dimensionally accurate, it is generally achieved by subsequent machining. Machining or finish allowances are therefore added in the pattern dimension. The amount of machining allowance to be provided for is affected by the method of molding and casting used viz. hand molding or machine molding, sand casting or metal mold casting. The amount of machining allowance is also affected by the size and shape of the casting; the casting orientation; the metal; and the degree of accuracy and finish required. The machining allowances recommended for different metal is given in Table 3. Table 3 : Machining Allowances of Various Metals Metal Dimension (inch) Allowance (inch) Up to Cast iron Cast steel Non ferrous 12 to to 40 Up to 6 6 to to 40 Up to 8 8 to to EXERCISE 2 The casting shown is to be made in cast iron using a wooden pattern. Assuming only machining allowance, calculate the dimension of the pattern. (All Dimensions are in Inches.)

12 Solution 2 The machining allowance for cast iron for size, up to 12 inch is o.12 inch and from 12 inch to 20 inch is 0.20 inch (Table 3) For dimension 18 inch, allowance = 0.20 inch For dimension 14 inch, allowance = 0.20 inch For dimension 8 inch, allowance = 0.12 inch For dimension 6 inch, allowance = 0.12 inch The pattern drawing with required dimension is shown in Figure below Distortion or Camber Allowance Sometimes castings get distorted, during solidification, due to their typical shape. For example, if the casting has the form of the letter U, V, T, or L etc. it will tend to contract at the closed end causing the vertical legs to look slightly inclined. This can be prevented by making the legs of the U, V, T, or L shaped pattern converge slightly (inward) so that the casting after distortion will have its sides vertical (Figure 4). The distortion in casting may occur due to internal stresses. These internal stresses are caused on account of unequal cooling of different section of the casting and hindered contraction. Measure taken to prevent the distortion in casting includes: i. Modification of casting design ii. Providing sufficient machining allowance to cover the distortion affect iii. Providing suitable allowance on reflection) the pattern, called camber or distortion allowance (inverse

13 Figure 4: Distortions in Casting Rapping Allowance Before the withdrawal from the sand mold, the pattern is rapped all around the vertical faces to enlarge the mold cavity slightly, which facilitate its removal. Since it enlarges the final casting made, it is desirable that the original pattern dimension should be reduced to account for this increase. There is no sure way of quantifying this allowance, since it is highly dependent on the foundry personnel practice involved. It is a negative allowance and is to be applied only to those dimensionss that are parallel to the parting plane.

14 FACTORS EFFECTING SELECTION OF PATTERN MATERIAL The following factors must be taken into consideration while selecting pattern materials. 1. Number of castings to be produced. Metal pattern are preferred when castings are required large in number. 2. Type of mould material used. 3. Kind of molding process. 4. Method of molding (hand or machine). 5. Degree of dimensional accuracy and surface finish required. 6. Minimum thickness required. 7. Shape, complexity and size of casting. 8. Cost of pattern and chances of repeat orders of the pattern 1.5-Core and Core Prints Castings are often required to have holes, recesses, etc. of various sizes and shapes. These impressions can be obtained by using cores. So where coring is required, provision should be made to support the core inside the mold cavity. Core prints are used to serve this purpose. The core print is an added projection on the pattern and it forms a seat in the mold on which the sand core rests during pouring of the mold. The core print must be of adequate size and shape so that it can support the weight of the core during the casting operation. Depending upon the requirement a core can be placed horizontal, vertical and can be hanged inside the mold cavity. A typical job, its pattern and the mold cavity with core and core print is shown in Figure 5.

15 Figure 5: A Typical Job, its Pattern Types of Core A core is made by core sand and prepared separately in a core box. It is used to form a desired recess and cavity in casting. Different types of cores are used in foundry work and are employed according to their shape and their position in the mould. The main types of cores are described below : Horizontal Core It is the simplest type of core which is placed horizontally at the parting line of the mould. As per cross section, it may be of any shape but cylindrical shaped core is mostly used as shown in Figure fig Vertical Core It is similar to horizontal core, only differs in its position. Vertical core is placed in the mould with its axis vertical. Normally, top and bottom ends of the core are provided with a toper as shown in (Figure

16 fig Balanced Core It is suitable to produce a blind hole along a horizontal axis in casting. The overhanging length of the core is supported by means of chaplets as shown in Figure fig Hanging or Cover Core The core which has no support at the bottom and hangs vertically from the cope (Figure ) is known as hanging core. In this case, the entire mould cavity is prepared in the drag only. Types of core prints Core prints may be of the following types : Horizontal Core Print This forms seat for a horizontal core. Horizontal core print is often found on the split or two-piece pattern. Vertical Core Print It forms seat to support a vertical core in the mould. Balancing Core Print

17 It forms seat on one side of the mould and the core is supported at one end only, i.e. the core remains partly in this formed seat and partly in the mould cavity. The print of core in the mould cavity should balance the part which rests in the core seat. Cover or Handing Core Print It is used when the whole surface of pattern is rammed in the drag and the core is suspended from top of the mould. Wing Core Print At that place, where the cavity to be cored is above or below the parting line in the mould, wing core print is referred. Parting Line By parting line we mean a line or the plane of a pattern corresponding to the point of separation between the cope and drag portions of a sand mold. Parting lines must be flat or drafted so that the mold can be opened, the pattern removed and then closed for pouring without damage to the sand Pattern: A Pattern is a model or the replica of the object to be cast. Except for the various allowances a pattern exactly resembles the casting to be made. Patterns may be in two or three pieces, where as casting are in a single piece. A pattern is required even if one object has to be cast. The quality of casting and the final product will be effected to a great extent by the planning of pattern Functions of Patterns: A Pattern prepares a mould cavity for the purpose of making a casting. A Pattern may contain projections known as core prints if the casting requires a core and need to be made hollow. Risers, runners and gates may form a part of the pattern. Patterns properly made and having finished and smooth surfaces reduce casting defects. Properly constructed patterns minimize overall cost of the casting Selection of Pattern Materials: The following factors assist in selecting proper pattern material:

18 No. of castings to be produced. Metal to be cast. Dimensional accuracy & surface finish. Shape, complexity and size of casting. Casting design parameters. Type of molding materials. The chance of repeat orders. Nature of molding process. Position of core print The pattern material should be: Easily worked, shaped and joined. Light in weight. Strong, hard and durable. Resistant to wear and abrasion. Resistant to corrosion, and to chemical reactions. Dimensionally stable and unaffected by variations in temperature and humidity. Available at low cost Materials for making patterns: Metal Wood Plastic Plaster Wax A-Metal Patterns: These are employed where large no. of castings have to be produced from same patterns. Advantages: Do not absorb moisture More stronger Possess much longer life Do not wrap, retain their shape Greater resistance to abrasion Accurate and smooth surface finish Good machinability Limitations: Expensive Require a lot of machining for accuracy Not easily repaired Ferrous patterns get rusted Heavy weight, thus difficult to handle Commonly used metals for making patterns: Cast iron

19 Aluminium and its alloys Steel White metal Brass etc B-Wood Patterns: These are used where the no. of castings to be produced is small and pattern size is large. Advantages: Inexpensive Easily available in large quantities Easy to fabricate Light in weight They can be repaired easily Easy to obtain good surface finish Limitations: Susceptible to shrinkage and swelling Possess poor wear resistance Abraded easily by sand action Absorb moisture, consequently get wrapped Cannot withstand rough handling Life is very short Commonly used woods for making patterns: Teak Pine Mahogony Deodar etc C-Plastic Patterns: Advantages: Durable Provides a smooth surface Moisture resistant Does not involve any appreciable change in size or shape Light weight Good strength Wear and corrosion resistance Easy to make Abrasion resistance Good resistance to chemical attack Limitations: Plastic patterns are Fragile These are may not work well when subject to conditions of severe shock as in machine molding (jolting) D-Plaster Patterns: Advantages: It can be easily worked by using wood working tools. Intricate shapes can be cast without any difficulty. It has high compressive strength.

20 Plaster may be made out of Plaster of paris or Gypsum cement. Plaster mixture is poured into a mould made by a sweep pattern or a wooden master pattern, in order to obtain a Plaster pattern E-Wax patterns: Advantages: Provide very good surface finish. Impart high accuracy to castings. After being molded, the wax pattern is not taken out of the mould like other patterns; rather the mould is inverted and heated; the molten wax comes out and/or is evaporated. Thus there is no chance of the mould cavity getting damaged while removing the pattern. Wax patterns find applications in Investment casting process Types of patterns depend upon the following factors: The shape and size of casting No. of castings required Method of moulding employed Anticipated difficulty of moulding operation 1.7-Types of Patterns: Single piece pattern. Split piece pattern. Loose piece pattern. Match plate pattern. Sweep pattern. Gated pattern. Skeleton pattern Follow board pattern. Cope and Drag pattern Single piece (solid) pattern: Made from one piece and does not contain loose pieces or joints. Inexpensive. Used for large size simple castings. Pattern is accommodated either in the cope or in the drag. Examples: Bodies of regular shapes. stuffling box of steam engine Split piece pattern: Patterns of intricate shaped castings cannot be made in one piece because of the inherent difficulties associated with the molding operations (e.g. withdrawing pattern from mould). The upper and the lower parts of the split piece patterns are accommodated in the cope and drag portions of the mold respectively. Parting line of the pattern forms the parting line of the mould. Dowel pins are used for keeping the alignment between the two parts of the pattern. Examples: Hallow cylinder Taps and water stop cocks etc.,

21 1.7-3.Loose piece pattern: Certain patterns cannot be withdrawn once they are embedded in the molding sand. Such patterns are usually made with one or more loose pieces for facilitating from the molding box and are known as loose piece patterns. Loose parts or pieces remain attached with the main body of the pattern, with the help of dowel pins. The main body of the pattern is drawn first from the molding box and thereafter as soon as the loose part are removed, the result is the mold cavity Match plate pattern: It consists of a match plate, on either side of which each half of split patterns is fastened. A no. of different sized and shaped patterns may be mounted on one match plate. The match plate with the help of locator holes can be clamped with the drag. After the cope and drag have been rammed with the molding sand, the match plate pattern is removed from in between the cope and drag. Match plate patterns are normally used in machine molding. By using this we can eliminate mismatch of cope and drag cavities Sweep pattern: A sweep pattern is just a form made on a wooden board which sweeps the shape of the casting into the sand all around the circumference. The sweep pattern rotates about the post. Once the mold is ready, Sweep pattern and the post can be removed. Sweep pattern avoids the necessity of making a full, large circular and costly three-dimensional pattern. Making a sweep pattern saves a lot of time and labour as compared to making a full pattern. A sweep pattern is preferred for producing large casting of circular sections and symmetrical shapes.

22 Gated pattern: The sections connecting different patterns serve as runner and gates. This facilitates filling of the mould with molten metal in a better manner and at the same time eliminates the time and labour otherwise consumed in cutting runners and gates. A gated pattern can manufacture many casting at one time and thus it is used in mass production systems. Gated patterns are employed for producing small castings Skeleton pattern: A skeleton pattern is the skeleton of a desired shape which may be S-bend pipe or a chute or something else. The skeleton frame is mounted on a metal base The skeleton is made from wooden strips, and is thus a wooden work. The skeleton pattern is filled with sand and is rammed. A strickle (board) assists in giving the desired shape to the sand and removes extra sand. Skeleton patterns are employed for producing a few large castings. A skeleton pattern is very economical, because it involves less material costs Follow board pattern:

23 A follow board is a wooden board and is used for supporting a pattern which is very thin and fragile and which may give way and collapse under pressure when the sand above the pattern is being rammed. With the follow board support under the weak pattern, the drag is rammed, and then the fallow board is with drawn, The rammed drag is inverted, cope is mounted on it and rammed. During this operation pattern remains over the inverted drag and get support from the rammed sand of the drag under it. Follow boards are also used for casting master patterns for many applications Cope and Drag patterns: A cope and drag pattern is another form of split pattern. Each half of the pattern is fixed to a separate metal/wood plate. Each half of the pattern(along the plate) is molded separately in a separate molding box by an independent molder or moulders. The two moulds of each half of the pattern are finally assembled and the mould is ready for pouring. Cope and drag patterns are used for producing big castings which as a whole cannot be conveniently handled by one moulder alone.

24 (a)split pattern (b) Follow-board (c) Match Plate (d) Loose-piece (e) Sweep (f) Skeleton pattern 1.8-Classification of molding (casting) Processes Casting processes can be classified into following FOUR categories: i. Conventional Molding Processes a. Green Sand Molding b. Dry Sand Molding c. Flask less Molding ii. Chemical Sand Molding Processes a. Shell Molding b. Sodium Silicate Molding c. No-Bake Molding iii. Permanent Mold Processes a. Gravity Die casting b. Low and High Pressure Die Casting iv. Special Casting Processes a. Lost Wax b. Ceramics Shell Molding c. Evaporative Pattern Casting d.vacuum Sealed Molding e. Centrifugal Casting 1.9-Green Sand Molding Green sand is the most diversified molding method used in metal casting operations. The process utilizes a mold made of compressed or compacted moist sand. The term "green" denotes the presence of moisture in the molding sand. The mold material consists of silica sand mixed with a suitable bonding agent (usually clay) and moisture.

25 Advantages and limitations. Advantages are: 1. Great flexibility as a production process. Mechanical equipment can be utilized for performing molding and its allied operations. Furthermore, green sand can be reused many times by reconditioning it with water, clay, and ether materials. The molding process can be rapid and repetitive. 2. Usually, the meat direct route from pattern to mold ready for pouring is by green -sand molding. 3. Economy, green sand molding is ordinarily the least costly method of molding. Limitations in the use of green-sand molding are: 1. Some casting designs require the use of other casting processes. Thin, long projections of green sand in a mold cavity are washed away by the molten metal or may not even be moldable. Cooling fins on air-cooled-engine cylinder blocks and head are an example. Greater strength is then required of the mold. 2. Certain metals and some castings develop defects if poured into molds containing moisture. 3. The dimensional accuracy and surface finish of green-sand castings may not be adequate. 4. Large castings require greater mold strength and resistance to erosion than are available in green sands Sand Mold Making Procedure The procedure for making mold of a cast iron wheel is shown in (Figure 8 (a), (b), (c)). The first step in making mold is to place the pattern on the molding board. The drag is placed on the board ((Figure 8 (a)). Dry facing sand is sprinkled over the board and pattern to provide a non sticky layer. Molding sand is then riddled in to cover the pattern with the fingers; then the drag is completely filled. The sand is then firmly packed in the drag by means of hand rammers. The ramming must be proper i.e. it must neither be too hard or soft. After the ramming is over, the excess sand is leveled off with a straight bar known as a strike rod. With the help of vent rod, vent holes are made in the drag to the full depth of the flask as well as to the pattern to facilitate the removal of gases during pouring and solidification. The finished drag flask is now rolled over to the bottom board exposing the pattern.

26 Cope half of the pattern is then placed over the drag pattern with the help of locating pins. The cope flask on the drag is located aligning again with the help of pins ((Figure 8 (b)). The dry parting sand is sprinkled all over the drag and on the pattern. A sprue pin for making the sprue passage is located at a small distance from the pattern. Also, riser pin, if required, is placed at an appropriate place. The operation of filling, ramming and venting of the cope proceed in the same manner as performed in the drag. The sprue and riser pins are removed first and a pouring basin is scooped out at the top to pour the liquid metal. Then pattern from the cope and drag is removed and facing sand in the form of paste is applied all over the mold cavity and runners which would give the finished casting a good surface finish. The mold is now assembled. The mold now is ready for pouring (see ((Figure 8 (c) ) Dry-sand Molds: Dry-sand molds are actually made with molding sand in the green condition. The sand mixture is modified somewhat to favor good strength and other properties after the mold is dried. Dry-sand molding may be done the same way as green-sand molding on smaller sizes of castings. Usually, the mold-cavity surface is coated or sprayed with a mixture which, upon drying, imparts greater hardness or refractoriness to the mold. The entire mold is then dried in an oven at 300 to 650 F or by circulating heated air through the mold. The time-consuming drying operation is one inherent disadvantage of the dry-sand mold. Advantages

27 Dry sand molds are generally stronger than green sand molds and therefore can withstand much additional handling. Better dimension control than if they were molded in green sand. The improved quality of the sand mixture due to the removal of moisture can result in a much smoother finish on the castings than if made in green sand molds. Where molds are properly washed and sprayed with refractory coatings, the casting finish is further improved. Disadvantages This type of molding is much more expensive than green sand molding and is not a highproduction process. Correct baking (drying) times are essential Metallic Molding Metallic mold is also known as permanent mold because of their long life. The metallic mold can be reused many times before it is discarded or rebuilt. Permanent molds are made of dense, fine grained, heat resistant cast iron, steel, bronze, anodized aluminum, graphite or other suitable refractoriness. The mold is made in two halves in order to facilitate the removal of casting from the mold. Usually the metallic mold is called as dies and the metal is introduced in it under gravity. Some times this operation is also known as gravity die casting. When the molten metal is introduced in the die under pressure, then this process is called as pressure die casting. The mold walls of a permanent mold have thickness from 15 mm to 50 mm. The thicker mold walls can remove greater amount of heat from the casting. Although the metallic mold can be used both for ferrous and nonferrous castings but this process is more popular for the non-ferrous castings, for examples aluminum alloys, zinc alloys and magnesium alloys. Usually the metallic molds are made of grey iron, alloy steels and anodized aluminium alloys. Advantages (i) Fine and dense grained structure in casting is achieved using such mold. (ii) The process is economical. (iii) Because of rapid rate of cooling, the castings possess fine grain structure. (iv) Close dimensional tolerance is possible. (v) Good surface finish and surface details are obtained. (vi) Casting defects observed in sand castings are eliminated. (vii) Fast rate of production can be attained. (viii) The process requires less labor.

28 Disadvantages (i) The surface of casting becomes hard due to chilling effect. (ii) High refractoriness is needed for high melting point alloys. (iii) The process is impractical for large castings. Applications 1 This method is suitable for small and medium sized casting. 2 It is widely suitable for non-ferrous casting Shell Molding Shell mold casting is recent invention (Germany during the Second World War) in molding techniques for mass production and smooth finish. It is a process in which, a thin mold is made around a heated metallic pattern plate. The molding material is a mixture of dry, fine silica sand (clay content should be kept very low), and 3-8% of a thermosetting resin like phenol formaldehyde or silicon grease. Conventional dry mixing techniques are used for obtaining the moulding mixture. Specially prepared resin coated sands are also used. When the molding mixture drops on to the pattern plate, which is heated to a temperature of 35 to 700^F (18 to 375 C), a shell of about 6 mm thickness is formed. In order to cure the shell completely, it must be heated to 440 to 650 F (230 to 350t) for about 1-3 minutes. The shell is then released from the pattern plate by ejector pins. To prevent sticking of the baked shell to the pattern plate, a silicone release agent is applied to the latter before the molding mixture drops on to it. Shell molding is suitable for mass production of thin walled, grey cast iron (and aluminium alloy) castings having a maximum weight between 35 and 45 pounds (15 to 20 kg.) However, castings weighing up to 1000 pounds can be made by shell molding on an individual basis.

29 Advantages (i) High suitable for thin sections like petrol engine cylinder. (ii) Excellent surface finish. (iii) Dimensional accuracy of order of to mm. (iv) Negligible machining and cleaning cost. (v) Occupies less floor space. (vi) Skill-ness required is less. (vii) Molds formed by this process can be stored until required. (viii) Better quality of casting assured. (ix) Mass production. Disadvantages The main disadvantages of shell molding are: 1. Higher pattern cost. 2. Higher resin cost. 3. Not economical for small runs. 4. Dust-extraction problem. 5. Complicated jobs and jobs of various sizes cannot be easily shell molded. 6. Specialized equipment is required. 7. Resin binder is an expensive material. 8. Limited for small size 1.13-Investment molding The investment casting process also called lost wax process begins with the production of wax replicas or patterns of the desired shape of the castings. A pattern is needed for every casting to be produced. The patterns are prepared by injecting wax or polystyrene in a metal dies. A numbe r of patterns are attached to a central wax sprue to form a assembly. The mold is prepared by surrounding the pattern with refractory slurry that can set at room temperature. The mold is then heated so that pattern melts and flows out, leaving a clean cavi ty behind. The mould is further hardened by heating and the molten

30 metal is poured while it is still hot. taken out. When the casting is solidified, the mold is broken and the casting The basic steps of the investment casting process are ( Figure see below ) : Advantages Formation of hollow interiors in cylinders without cores Less material required for gate Fine grained structure at the outer surface of the casting free of gas and shrinkage cavities and porosity Disadvantages More segregation of alloy component during pouring under the forces of rotation Contamination of internal surface of castings with non -metallic inclusions. Inaccurate internal diameter Types of Moulding Sand Moulding sands are classified according to their use. These are classified and described below : Green Sand It is a mixture of silica sand with 18 to 30 percent clay, having quantity of water 6 to 8 percent. Green sand in its natural state contains enough moisture to give it sufficient bonding property. It is soft, light, porous and retains the shape easily when squeezed in the hand. Moulds prepared by this sand are known as green sand moulds which are used for small and medium castings only. Dry Sand When moisture from green sand mould is removed, it is known as dry sand mould and is used for large size of casting. By drying the mould in moulding box it becomes stronger and compact.

31 Facing Sand It is used directly next to the surface of pattern. When the mould is poured with the molten metal it comes directly in contact with the molten metal. As it is subjected to most severe conditions, it must possess high strength and refractoriness. It is made of silica sand and clay in fine powder form. Loam Sand It is a mixture of clay (about 50%), sand and water (about 18-20%) to obtain a thin plastic paste which is used to plaster on moulds with soft bricks and hardens on drying. This is particularly employed for loam moulding usually for rough and large castings. Backing Sand It is the sand obtained from mould and is used again and again. Due to its black colour which is due to burning and addition of coal dust, it is also known as black sand. Parting Sand It is fine sharp dry sand used to keep the green sand from sticking to the pattern and also to keep the moulding boxes (drag and cope) separated. Core Sand This is silica sand mixed with core oil which is composed of linseed oil, light mineral oil, resin and other binding materials. For the sake of economy, pitch or flours and water may also be used in case of large cores Properties of Moulding Sand The moulding sand should posses the following properties: 1. Porosity or permeability: It is also termed as porosity of the molding sand in order to allow the escape of any air, gases or moisture present or generated in the mold when the molten metal is poured into it. All these gaseous generated during pouring and solidification process must escape otherwise the casting becomes defective. Permeability is a function of grain size, grain shape, and moisture and clay contents in the molding sand. The extent of ramming of the sand directly affects the permeability of the mold. Permeability of mold can be further increased by venting using vent rod Adhesiveness: It is the property of sand due to which it adhere or cling to the sides of the moulding box. Good sand must have sufficient adhesiveness so that heavy sand masses can be successfully held in moulding box without any danger of its falling out when the box is removed Cohesiveness It is property of molding sand by virtue which the sand grain particles interact and attract each other within the molding sand. Thus, the binding capability of the molding sand gets enhanced to increase the green, dry and hot strength property of molding and core sand Green strength The green sand after water has been mixed into it, must have sufficient strength and toughness to permit the making and handling of the mold. For this, the sand grains must be adhesive, i.e. they must be capable of attaching themselves to another body and. therefore, and sand grains having high adhesiveness will cling to the sides of the molding box. Also, the sand grains must have the property

32 known as cohesiveness i.e. ability of the sand grains to stick to one another. By virtue of this property, the pattern can be taken out from the mold without breaking the mold and also the erosion of mold wall surfaces does not occur during the flow of molten metal. The green strength also depends upon the grain shape and size, amount and type of clay and the moisture content Dry strength As soon as the molten metal is poured into the mold, the moisture in the sand layer adjacent to the hot metal gets evaporated and this dry sand layer must have sufficient strength to its shape in order to avoid erosion of mold wall during the flow of molten metal Flowability or plasticity It is the ability of the sand to get compacted and behave like a fluid. It will flow uniformly to all portions of pattern when rammed and distribute the ramming pressure evenly all around in all directions. Generally sand particles resist moving around corners or projections. In general, flow ability increases with decrease in green strength, an, decrease in grain size. The flow ability also varies with moisture and clay content Refractoriness Refractoriness is defined as the ability of molding sand to withstand high temperatures without breaking down or fusing thus facilitating to get sound casting. It is a highly important characteristic of molding sands. Refractoriness can only be increased to a limited extent. Molding sand with poor refractoriness may burn on to the casting surface and no smooth casting surface can be obtained. The degree of refractoriness depends on the SiO2 i.e. quartz content, and the shape and grain size of the particle. The higher the SiO2 content and the rougher the grain volumetric composition the higher is the refractoriness of the molding sand and core sand. Refractoriness is measured by the sinter point of the sand rather than its melting point Collapsibility After the molten metal in the mold gets solidified, the sand mold must be collapsible so that free contraction of the metal occurs and this would naturally avoid the tearing or cracking of the contracting metal. In absence of this property the contraction of the metal is hindered by the mold and thus results in tears and cracks in the casting. This property is highly desired in cores Miscellaneous properties In addition to above requirements, the molding sand should not stick to the casting and should not chemically react with the metal. Molding sand should be cheap and easily available. It should be reusable for economic reasons. Its coefficients of expansion should be sufficiently low Types of Moulding Sand (i) Silica Sand: The sand which forms the major portion of the moulding sand (up to 96%) is essentially silica grains, the rest being the other oxides such as alumina, sodium (Na2O +K2O) and

33 magnesium oxide (MgO + CaO). These impurities should be minimized to about 2% since they affect the fusion point of the silica sand. The main source is the river sand which is used with or without washing. Ideally the fusion point of sands should be about 1450 C for cast irons and about 1550 C for steels. In the river sand, all sizes and shapes of grains are mixed. The sand grains may very in size from a few micrometers to a few millimeters. Shape of the grain may be round, sub-angular, angular and very angular. The size and shapes of these sand grains greatly affect the properties of the moulding sands. (ii) Zircon Sands: It is basically a zirconium silicate (ZrSiO4). The typical composition is ZrO %, SiO %, Al2O3 1.92%, Fe2O3 0.74% and traces of other oxides. It is very expensive. In India, it is available in the Quilon beach of Kerala. It has a fusion point of about 2400 C and also a low coefficient of thermal expansion. The other advantages are high thermal conductivity, high chilling power and high density. It requires a very small amount of binder (about 3%). It is generally used to manufacture precision steel casting requiring better surface finish and for precision investment casting. Chromite sand is crushed from the chrome ore whose typical composition is Cr2O3 44%, Fe2O3 28%, SiO2 2.5%, CaO 0.5%, and Al2O3 + MgO 25%. The fusion point is about 1800 C. It also requires a very small amount of binder (about 3%). It is also used to manufacture heavy steel castings requiring better surface finish. It is best suited to austenitic manganese steel castings. (iii) Olivine Sand: Contains the minerals fosterite (Mg2SiO4) and fayalita (Fe2SiO4). It is very versatile sand and the same mixture can be used for a range of steels. Comparative properties relevant for moulding of these various base sands. 15. Composition of Moulding Sand The principal constituent of moulding sand are silica sand, binder, additives and water. These are described below : Silica Sand As per composition, silica sand is the main constituent of moulding sand. It is a product of the breaking up of quarry stone or decomposition of granite. Silica sand imparts permeability, chemical resistivity and refractoriness to the moulding sand. Silica sand is specified according to the average shape and size of its grains. Binder The main function of binder is to impart the sufficient strength and cohesiveness of the moulding sand, so that it may retain its shape after ramming. The common binders may be divided as (i) organic binders, and (ii) inorganic binders. The organic binders such as molasses, dextrin, linseed oil and resins are usually used in core making while in the inorganic group the common binders are portland cement, clay and sodium silicate. Amongst all, the clay binders are widely used. Additives

34 Materials which are added to the moulding sand to improve its existing properties or to include certain new properties, are known as additives. As per demand coal dust, wood flour, mollases, cornflour and pitch may be used as an additive. Water When water is added to clay it furnishes the bounding action of clay. It penetrates the mass of clay and forms a microfilm. The bonding quality of clay totally depends on the maximum thickness of microfilm it can hold. In general, water quantity varies from 2 to 8 percent. BINDERS USED IN MOLDING SANDS Binders are added to give cohesion to molding sands. Binders provide strength to the molding sand and enable it to retain its shape as mold cavity. Binders should be added in optimum quantity as they reduce refractoriness and permeability. An optimal quantity of binders is needed, as further increases have no effect on properties of foundry sand. The following binders are generally added to foundry sand: (i) Fireclay (ii) Illite (iii) Bentonite Sodium montmorillonite Calcium montmorillonite (iv) Limonite (iv) Kaolinite (i) Fireclay : It is usually found near coal mines. For use in the foundry, the hard black lumps of fireclay are taken out, weathered and pulverized. Since the size of fireclay particles is nearly 400 times greater than the size of bentonite particles, they give poor bonding strength to foundry sand. (ii) Illite: Illite is found in natural molding sands that are formed by the decomposition of micaceous materials due to weathering. Illite possesses moderate shrinkage and poor bonding strength than bentonite. (iii) Bentonite: It is the most suitable material used in molding sands. Limonite and Kaolinite are not commonly used as binders as they have comparatively low binding properties. SAND TESTING Molding sand and core sand depend upon shape, size composition and distribution of sand grains, amount of clay, moisture and additives.

35 The increase in demand for good surface finish and higher accuracy in castings necessitates certainty in the quality of mold and core sands. Sand testing often allows the use of less expensive local sands. It also ensures reliable sand mixing and enables a utilization of the inherent properties of molding sand. Sand testing on delivery will immediately detect any variation from the standard quality, and adjustment of the sand mixture to specific requirements so that the casting defects can be minimized. Generally the following tests are performed to judge the molding and casting characteristics of foundry sands: b. Moisture content Test c. Clay content Test d. Chemical composition of sand e. Grain shape and surface texture of sand. f. Grain size distribution of sand g. Refractoriness of sand h. Strength Test i. Permeability Test j. Flowability Test k. Shatter index Test l. Mould hardness Test. Moisture Content Test The moisture content of the molding sand mixture may determine by drying a weighed amount of 20 to 50 grams of molding sand to a constant temperature up to 100 C in a oven for about one hour. It is then cooled to a room temperature and then reweighing the molding sand. The moisture content in molding sand is thus evaporated. The loss in weight of molding sand due to loss of moisture, gives the amount of moisture which can be expressed as a percentage of the original sand sample. The percentage of moisture content in the molding sand can also be determined in fact more speedily by an instrument known as a speedy moisture teller. This instrument is based on the principle that when water and calcium carbide react, they form acetylene gas which can be measured and this will be directly proportional to the moisture content. This instrument is provided with a pressure gauge calibrated to read directly the percentage of moisture present in the molding sand.

36 Clay Content Test The amount of clay is determined by carrying out the clay content test in which clay in molding sand of 50 grams is defined as particles which when suspended in water, fail to settle at the rate of one inch per min. Clay consists of particles less than 20 micron, per inch in dia. Grain Fineness Test The AFS Grain Fineness Number (AFS-GFN) is one means of measuring the grain fineness of a sand system. GFN is a measure of the average size of the particles (or grains) in a sand sample. The grain fineness of molding sand is measured using a test called sieve analysis. The test is carried out in power-driven shaker consisting of number of sieves fitted one over the other. 1. A representative sample of the sand is dried and weighed, then passed through a series of progressively finer sieves (screens) while they are agitated and tapped for a 15-minute test cycle. The series are placed in order of fineness from top to bottom. 2. The sand retained on each sieve (grains that are too large to pass through) is then weighed and recorded. 3. The weight retained on each sieve is carried out through calculations to get the AFS-GFN. Refractoriness Test The refractoriness of the molding sand is judged by heating the A.F.S standard sand specimen to very high temperatures ranges depending upon the type of sand. The heated sand test pieces are cooled to room temperature and examined under a microscope for surface characteristics or by scratching it with a steel needle. If the silica sand grains remain sharply defined and easily give way to the needle. Sintering has not yet set in. In the actual experiment the sand specimen in a porcelain boat is placed into an electric furnace. It is usual practice to start the test from l000 C and raise the temperature in steps of 100 C to 1300 C and in steps of 50 above 1300 C till sintering of the silica sand grains takes place. At each temperature level, it is kept for at least three minutes and then taken out from the oven for examination under a microscope for evaluating surface characteristics or by scratching it with a steel needle. Strength Test This is the strength of tempered sand expressed by its ability to hold a mold in shape. Sand molds are subjected to compressive, tensile, shearing, and transverse stresses. The green compressive strength test and dry compressive strength is the most used test in the foundry.

37 Compression tests A rammed specimen of tempered molding sand is produced that is 2 inches in diameter and 2 inches in height. The rammed sample is then subjected to a load which is gradually increased until the sample breaks. The point where the sample breaks is taken as the compression strength. Shear tests The compressive loading system is modified to provide offset loading of the specimen. Under most conditions the results of shear tests have been shown to be closely related to those of compression tests, although the latter property increases proportionately more at high ramming densities. The tensile test A special waisted specimen is loaded in tension through a pair of grips. The transverse test A plain rectangular specimen is supported on knife edges at the ends and centrally loaded to fracture. Tensile and transverse tests are commonly applied to high strength sands, the conditions being especially relevant to the stresses incurred in cores during handling and casting. Permeability Test Permeability is determined by measuring the rate of flow of air through a compacted specimen under standard conditions. A cylinder sand sample is prepared by using rammer and die. This specimen (usually 2 inch dia & 2 inch height) is used for testing the permeability or porosity of molding and the core sand. The test is performed in a permeability meter consisting of the balanced tank, water tank, nozzle, adjusting lever, nose piece for fixing sand specimen and a manometer. The permeability is directly measured. Permeability number P is volume of air (in cm3) passing through a sand specimen of 1 cm 2 crosssectional area and 1 cm height, at a pressure difference of 1 gm/cm 2 in one minute. P = Vh /atp Where, P = permeability v = volume of air passing through the specimen in c.c. h = height of specimen in cm p = pressure of air in gm/cm 2 a = cross-sectional area of the specimen in cm 2 t = time in minutes.

38 Shatter Index Test In this test, the A.F.S. standard sand specimen is rammed usually by 10 blows and then it is allowed to fall on a half inch mesh sieve from a height of m. The weight of sand retained on the sieve is weighed. It is then expressed as percentage of the total weight of the specimen which is a measure of the shatter index. Mold Hardness Test This test is performed by a mold hardness tester. The working of the tester is based on the principle of Brinell hardness testing machine. In an A.F.S. standard hardness tester a half inch diameter steel hemispherical ball is loaded with a spring load of 980 gm. This ball is made to penetrate into the mold sand or core sand surface. The penetration of the ball point into the mold surface is indicated on a dial in thousands of an inch. The dial is calibrated to read the hardness directly i.e. a mold surface which offers no resistance to the steel ball would have zero hardness value and a mold which is more rigid and is capable of completely preventing the steel ball from penetrating would have a hardness value of 100. The dial gauge of the hardness tester may provide direct readings Compatibility and flow ability The compatibility test is widely accepted as both simple to perform and directly related to the behaviour of sand in molding, particularly when involving squeeze compaction. A fixed volume of loose sand is compacted under standard conditions and the percentage reduction in volume represents the compatibility.

39 CUPOLA FURNACE For many years, the cupola was the primary method of melting used in iron foundries. The cupola furnace has several unique characteristics which are responsible for its widespread use as a melting unit for cast iron. Cupola furnace is employed for melting scrap metal or pig iron for production of various cast irons. It is also used for production of nodular and malleable cast iron. It is available in good varying sizes. The main considerations in selection of cupolas are melting capacity, diameter of shell without lining or with lining, spark arrester. Shape A typical cupola melting furnace consists of a water-cooled vertical cylinder which is lined with refractory material. Construction The construction of a conventional cupola consists of a vertical steel shell which is lined with a refractory brick. The charge is introduced into the furnace body by means of an opening approximately half way up the vertical shaft. The charge consists of alternate layers of the metal to be melted, coke fuel and limestone flux. The fuel is burnt in air which is introduced through tuyeres positioned above the hearth. The hot gases generated in the lower part of the shaft ascend and preheat the descending charge.

40 The different steps involved in cupola operation are: (1) Preparation of cupola ( including repairs) (2) Lighting the fire into the coke bed (3) Charging of cupola (4) Melting (5) Slagging and metal tapping (6) Dropping down the cupola bottom Zones of Cupola The different zones of cupola are marked in fig. and they are explained as under. (i) Well: It is a sort of well of molten iron. The molten iron collects in this zone before being tapped. The well is situated between the tapered rammed sand bottom and the bottom of the tuyeres. (ii) Superheating Combustion or Oxidizing Zone: All the oxygen in the air blast is consumed here owing to the (actual) combustion taking place in this zone. Thus, a lot of heat is supplied from here to other zones. Oxidation of Mn and Si evolve still more heat. The chemical reactions which occur in this zone are:

41 C + O 2 (from air) = CO 2 + Heat...(1) 2Mn + O 2 (from air) = MnO 2 + Heat...(2) Si + O2 (from air) = SiO2 + Heat...(3) The temperature of combustion zone varies from 1550 C to 1850 C. (iii) Reducing Zone or Protectivee Zone: It extends from the top of combustion zone to the top of coke bed. It protects from oxidation. The metal charge above and that dropping through it. An endothermic reaction takes place in this zone, in which some of hot CO2 moving upward through hot coke gets reduced. CO 2 + C(Coke) = 2CO Heat...(4) This reduces the heat in the reducing zone and it has a temperature only of the orderr of 1200 C. (iv) Melting Zone: Iron melts in this zone. The temperature in the melting zone is around or above 1600 C. As per the following reaction taking place in this zone, the molten iron picks up carbon. 3Fe + 2CO = Fe3C + CO 2...(5) (v) Preheating Zone: Preheating zone starts from above the melting zone and extends up to the bottom of the charging door. This contain cupola charge as alternate layers of coke, limestone and metal. Gases like CO 2, CO, N 2 rising upwards from combustion and reducing zones preheated the cupola charge to about 1100 C. Thus preheated charge gradually moves down in the melting zone. (vi) Stack Zone: Stack zone extends from above the preheating zone to where the cupola shell ends and spark arrester is attached. Hot gases from cupola pass through the stack zone and escape to atmosphere. Stack gases (i.e. gasess passing through stack zone) will normally contain about equal amounts of CO 2 and CO which is 12% each and rest 76% is Nitrogen. Induction furnace Induction furnaces are widely used for melting non-ferrous and ferrous alloys. There are two types of induction furnaces: coreless induction furnaces and channel induction furnaces: Coreless induction furnace

42 Coreless induction furnace consists of: a water cooled helical coil made of a copper tube, a crucible installed within the coil and supporting shell equipped with trunnions on which the furnace may tilt. Alternating current passing through the coil induces alternating currents in the metal charge loaded to the crucible. These induced currents heat the charge. When the charge is molten, electromagnetic field produced by the coil interacts with the electromagnetic field produced by the induced current. The resulted force causes stirring effect helping homogenizing the melt composition and the temperature. The frequency of the alternating current used in induction furnaces may vary from the line frequency (50Hz or 60Hz) to high frequency 10,000Hz Channel induction furnace Channel type induction furnace consists of a steel shell lined with refractory materials and an inductor attached to the shell. There is a channel connecting the main body with the inductor. The inductor of the channel furnace works as a transformer. It has a ring-like iron core with a water- or air-cooled coil as a primary coil and a loop of the melt, circulating in the channel, as a secondary coil. Melt circulation has a stirring effect. Channel induction furnaces work at line frequency currents. Channel induction furnaces are commonly used as holding furnaces (furnace for maintaining a molten metal, poured from a melting furnace, at a proper temperature). Channel furnaces are also used for melting low melting point alloys and iron. For two or three shift operation channel furnaces are more economical than coreless furnaces. Channel furnaces of ratings up to the 10 s of MW and up to capacities of thousands of tonnes have been used for melting and superheating iron. Electric Arc Furnace (EAF) is a steel making furnace, in which steel scrap is heated and melted by heat of electric arcs striking between the furnace electrodes and the metal bath. Two kinds of electric current may be used in Electric Arc Furnaces: direct (DC) and Alternating (AC). Three-phase AC Electric Arc Furnaces with graphite electrodes are commonly used in steel making. The main advantage of the Electric Arc Furnaces over the Basic Oxygen Furnaces is their capability to treat charges containing up to 100% of scrap. About 33% of the crude steel in the world is made in the Electric Arc Furnaces (EAF).Capacity of Electric Arc Furnace may reach 400 t. Structure of an Electric Arc Furnace

43 The scheme of a Electric Arc Furnace (EAF) is presented in the picture. The furnace consists of a spherical hearth (bottom), cylindrical shell and a swinging water-cooled dome-shaped roof. The roof has three holes for consumable graphite electrodes held by a clamping mechanism. The mechanism provides independent lifting and lowering of each electrode. The water-cooled electrode holders serve also as contacts for transmitting electric current supplied by water-cooled cables (tubes). The electrode and the scrap form the star connection of three-phase current, in which the scrap is common junction. The furnace is mounted on a tilting mechanism for tapping the molten steel throughh a tap hole with a pour spout located on the back side of the shell. The charge door, through which the slag components and alloying additives are charged, is located on the front side of the furnace shell. The charge door is also used for removing the slag (de-slagging). The scrap is charged commonly from the furnace top. The roof with the electrodes is swung aside before the scrap charging. The scrap arranged in the charge basket is transferred to the furnace by a crane and then dropped into the shell. Refractory lining of an Electric Arc Furnace Refractory linings of Electric Arc Furnaces are made generally of resin-bonded magnesia-carbon bricks. Fused magnesite grains and flack graphite are used as raw materials. When the bricks are heated the bonding material is coked and turns into a carbon network binding the refractory grains, preventing wetting by the slag and protecting the lining the from erosion and chemical attack of the molten metal and slag. Melting Melting process starts at low voltage (short arc) between the electrodes and the scrap. The arc during this period is unstable. In order to improve the arc stability small pieces of the scrap are placed in the upper layer of the charge. The electrodes descend melting the charge and penetrating into the scrap forming bores. The molten metal flows down to the furnace bottom. When the electrodes reach the liquid bath the arc becomes stable and the voltage may be increased (long arc). The electrodes are lifting together with the melt level. Most of scrap (85%) melt during this period. Temperature of the arc reaches 6300ºF (3500ºC). GATING SYSTEM DESIGN Gating system nothing but the basic design, which is needed to construct a smooth and proper filling of the mold cavity of the casting without any discontinuity, voids or solid inclusions. A proper method of gating system is that it leads the pure molten metal to flow through a ladle to the casting cavity, which ensures proper and smooth filling of the cavity. This depends on the layout of the gating channels too, such as the direction and the position of the runner, sprue and ingates.

44 Objective of the Gating System : The four main points, which enables a proper gating system, are: Clean molten metal. Smooth filling of the casting cavity. Uniform filling of the casting cavity. Complete filling of the casting cavity. Elements of Gating System The gating systems refer to all those elements which are connected with the flow of molten metal from the ladle to the mould cavity. The elements of gating systems are Pouring Basin Sprue Sprue Base Well Runner Runner Extension Ingate Riser

45 Any gating system designed should aim at providing a defect free casting. This can be achieved by considering following requirements. The mould should be completely filled in the smallest possible time without having to raise neither metal temperature nor use of higher metal heads. The metal should flow smoothly into the mould without any turbulence. A turbulence metal flow tends to form dross in the mould. Unwanted materials such as slag, dross and other mould materials should not be allowed to enter the mould cavity. The metal entry into the mould cavity should be properly controlled in such a way that aspiration of the atmospheric air is prevented. A proper thermal gradient should be maintained so that the casting is cooled without any shrinkage cavities or distortions. Metal flow should be maintained in such a way that no gating or mould erosion takes place. The gating system should ensure that enough molten metal reaches the mould cavity. It should be economical and easy to implement and remove after casting solidification. The casting yield should be maximised. The liquid metal that runs through the various channels in the mould obeys the Bernoulli s theorem which states that the total energy head remains constant at any section. Ignoring frictional losses, we have

46 Where h = Potential Head, m P = Static Pressure, Pa v = Liquid Velocity, m / s ρg = w = Specific weight of liquid, N / m 2 g = Acceleration due to gravity, m / s 2 Though quantitatively Bernoulli s theorem may not be applied, it helps to understand qualitatively, the metal flow in the sand mould. As the metal enters the pouring basin, it has the highest potential energy with no kinetic or pressure energies. But as the metal moves through the gating system, a loss of energy occurs because of the friction between the molten metal and the mould walls. Heat is continuously lost through the mould material though it is not represented in the Bernoulli s equation. Another law of fluid mechanics, which is useful in understanding the gating system behaviour, is the law of continuity which says that the volume of metal flowing at any section in the mould is constant. The same in equation form is Q = A 1 V 1 = A 2 V 2 Where Q = Rate of flow, m 3 / s A = Area of cross section, m 2 V = Velocity of metal flow, m / s Pouring Time The main objective for the gating system design is to fill the mould in the smallest time. The time for complete filling of a mould is called pouring time. Too long a pouring time requires a higher pouring temperature and too less a pouring time means turbulent flow in the mould which makes the casting defect prone. The pouring time depends on the casting materials, complexity of the casting, section thickness and casting size. Steels lose heat very fast, so required less pouring time while for nonferrous materials longer pouring time is beneficial because they lose heat slowly and tend to form dross if metal is pour too quickly. Ratio of surface area to volume of casting is important in addition to the mass of the casting. Also gating mass is considered when its mass is comparable to the mass of the casting. For grey cast iron up to 450 Kg Pouring time, t = K { T} W seconds Where K = Fluidity of iron in inches / 40 T = Average section thickness, mm W = Mass of the casting, Kg For grey cast iron greater than 450 Kg Pouring time, t = K { T} seconds Typical pouring times for cast iron are Casting mass Pouring time in seconds 20 Kg 6 to 10

47 100 Kg 15 to 30 Steel Casting Pouring time, t = ( log W) seconds Shell moulded ductile iron( vertical pouring) Pouring time, t = K 1 W seconds Where K1 = for thinner sections = for sections 10 to 25 mm thick = for heavier sections Copper alloy castings Pouring time, t = K2 3 W seconds Where K 2 is a constant whose value is given by 1.30 for top gating, 1.80 for bottom gating, 1.90 for brass and 2.80 for tin bronze. Choke Area After calculation of pouring time, it is required to establish the main control area which meters the metal flow into the mould cavity so that the mould is completely filled within the calculated pouring time. The controlling area is the choke area. The choke area happens to be at the bottom of the sprue and hence the first element to be designed in the gating system is the sprue size and its proportions. The main advantage in having sprue bottom as the choke area is that proper flow characteristics are established early in the mould. The choke area can be calculated using Bernoulli s equation as Where A= Choke area, mm2 W= Casting mass, Kg t = Pouring time, s d = Mass density of the molten metal, Kg / mm3 g = acceleration due to gravity, mm /s 2 H = Effective metal head ( sprue height), mm C = Efficiency factor which is a function of the gating system used The effective sprue height H, of the mould depends on the casting dimensions and type of the gating used. It can be calculated using the following relations. Top gate, H= h Bottom gate H = h -2c and H = h pxp/2c Where h = Height of the sprue

48 p = Height of mould cavity in cope c = Total height of the mould cavity The efficiency coefficient of the gating system depends on the various sections that are normally used in a gating system. The elements of a gating system should be circular in cross section since they have lower surface area to volume ratio which would reduce heat loss and have less friction. Moreover, streamlining the various gating elements would greatly increase volumetric efficiency of the gating system and allow for smaller size gates and runners which would increase the casting yield. Whenever a runner changes direction or joins with another runner or gate, there is some loss in the metal head, all of which when taken properly into consideration would give the overall efficiency of the gating system. Sprue The sprues should be tapered down to take into account the gain in velocity of the metal as it flows down reducing the air aspiration. The exact tapering can be obtained by equation of continuity. Denoting the top and the choke sections of the sprue by the subscripts t and c respectively, we get AtVt = ACVC Or At = A C Vc /V t Since the velocities are proportional to the square of the potential heads, then from Bernoulli s equation At = AC The square roots suggest that the profile of the sprue should be parabolic if exactly done as per the above equation. But making a parabolic sprue is inconvenient in practice and therefore a straight taper is preferable.

49 Sprue and pouring basin height and area Other Gating Elements Pouring Basin The main function of a pouring basin is to reduce the momentum of the liquid flowing into the mould by settling first into it. In order that the metal enters into the sprue without any turbulence it is necessary that the pouring basin be deep enough, also the entrance into the sprue be a smooth radius of at least 25 mm. The pouring basin depth of 2.5 times the sprue entrance diameter is enough for smooth metal flow and to prevent vortex formation. In order that vortex is not formed during pouring, it is necessary that the pouring basin be kept full and constant conditions of flow are established. This can be achieved by using a delay screen or a strainer core. A delay screen is a small piece of perforated thin tin sheet placed in the pouring basin at the top of the down sprue. This screen usually melts because of the heat from the metal and in the process delays the entrance of metal into the sprue thus filling the pouring basin fully. This ensures a constant flow of metal as also exclude slag and dirt since only metal from below is allowed to go into the sprue. A similar effect is also achieved by a strainer core which is a ceramic coated screen with many holes. The strainer restricts the flow of metal into the sprue and thus helps in quick filling of the pouring basin. Pouring basins are most desirable for alloys which form troublesome oxide skins (aluminium, aluminium bronze, etc.

50 Figure: Pouring basin (1) Figure 6: Pouring basin (2) Sprue Base Well The provision of a sprue base welll at the bottom of the sprue helps in reducing the velocity of the incoming metal and also the mould erosion. A general guide line could be that the sprue base well area should be five times that of the sprue choke area and the well depth should be approximately equal to that of the runner. Gating Ratios- It refers to the proportion of the cross sectional areas between the sprue, runner and ingates and is generally denoted as sprue area : runner area : ingate area. Depending on the choke area there can be two types of gating systems: Non-pressurised Pressurised A non pressurised gating system having choke at the sprue base, has total runner area and ingate area higher than the sprue area. In this system there is no pressure existing in the metal flow system and

51 thus it helps to reduce turbulence. This is particularly useful for casting drossy alloys such as aluminium alloys and magnesium alloys. When metal is to enter the mould cavity through multiple ingates, the cross section of the runner should accordingly be reduced at each of a runner break-up to allow for equal distribution of metal through all ingates. A typical gating ratio is 1:4:4 The disadvantages of unpressurised gating are: The gating system needs to be carefully designed to see that all parts flow full. Otherwise some elements of the gating system may flow partially allowing for the air aspiration. Tapered sprues are invariably used with unpressurised system. The runners are maintained in drag while the gates are kept in cope to ensure that runners are full. Casting yield gets reduced because of large metal involved in the runners and gates. In the case of pressurised gating system normally the ingates area is the smallest, thus maintaining a back pressure throughout and generally flows full and thereby, can minimize the air aspiration even when a straight sprue is used. It provided higher casting yield since the volume of metal used up in the runners and gates is reduced. Because of turbulence and associated dross formation, this type of gating system is not used for light alloys but can be advantageously used for ferrous castings. A typical gating ratio is 1:2:1. While designing the runner system, care should be taken to reduce sharp corners or sudden change of sections since they tend to cause turbulence and gas entrapment. Though from heat loss factor circular cross section runners are preferable, traditionally trapezoidal runner sections are employed to reduce the turbulence. The approximate proportions are fro a square to rectangle with width twice as that of the depth of the runner. When multiple ingates are used, the runner cross section should be suitably restricted at the separation of each runner in the interest of uniform flow through all sections. FACTORS CONTROLING GATING DESIGN The following factors must be considered while designing gating system. (i) Sharp corners and abrupt changes in at any section or portion in gating system should be avoided for suppressing turbulence and gas entrapment. Suitable relationship must exist between different cross-sectional areas of gating systems. (ii) The most important characteristics of gating system besides sprue are the shape, location and dimensions of runners and type of flow. It is also important to determine the position at which the molten metal enters the mould cavity. (iii) Gating ratio should reveal that the total cross-section of sprue, runner and gate decreases towards the mold cavity which provides a choke effect. (iv) Bending of runner if any should be kept away from mold cavity. (v) Developing the various cross sections of gating system to nullify the effect of turbulence or momentum of molten metal. (vi) Streamlining or removing sharp corners at any junctions by providing generous radius, tapering the sprue, providing radius at sprue entrance and exit and providing a basin instead pouring cup etc. Riser

52 It is the task of casting designer to reduce all hot spots so that no shrinkage cavities occurred. Since solidification of the casting occurs by loosing heat from the surfaces and the amount of the heat is given by the volume of the casting, the cooling characteristics of a casting can be represented by the surface area to the volume ratio. Since the riser is almost similar to the casting in its solidification behaviour, the riser characteristics can also be specified by the ratio of its surface area to volume. If this ratio of casting is higher, then it is expected to cool faster. According to Chvorinov, solidification time can be calculated as Where ts = solidification time, s V = volume of the casting, SA = surface area K = mould constant which depends on pouring temperature, casting & mould thermal Characteristics The freezing ratio, X of a mould is defined as the ratio of cooling characteristics of casting to that of the riser. X = In order to feed the casting, the riserr should solidify last and hence its freezing ratio should be greater than unity. CAINE s Method X = { a / Y-b} + c Where Y = riser volume / casting volume a, b, c are constants whose values for different materials are given here Types of risers:- Various types of risers in use are Side riser(live or hot riser):it is filled last and contains the hottest metal. It receives the molten metal directly from the runner before it enters the mould cavity and effectivee than top riser. Top riser(dead or cold riser); It is located on the top of the casting and has advantages of additional pressure head and small feeding distance over the side riser which is placed adjacent to the casting Open riser: These risers are open to the atmosphere at the top surface of the mould. Advantages: Can be easily moulded

53 Since it is open to atmosphere,it will not draw metal from the casting as a result of partial vacuum in the riser Such risers serve as collectors of non-mettalic inclusions floating upto the surface. Limitations: The height should be adjust with height of the cope, this result partial vaccum in the risrer These are the holes through which foreign matter may get into the mould cavity. iii. Blind riser :the riser which is not open to atmosphere called as blind riser. Advantages: Can be removed more easily ROLE OF RISER IN SAND CASTING Metals and their alloys shrink as they cool or solidify and hence may create a partial vacuum within the casting which leads to casting defect known as shrinkage or void. The primary function of riser as attached with the mould is to feed molten metal to accommodate shrinkage occurring during solidification of the casting. As shrinkage is very common casting defect in casting and hence it should be avoided by allowing molten metal to rise in riser after filling the mould cavity completely and supplying the molten metal to further feed the void occurred during solidification of the casting because of shrinkage. Riser also permits the escape of evolved air and mold gases as the mold cavity is being filled with the molten metal. It also indicates to the foundry man whether mold cavity has been filled completely or not. The suitable design of riser also helps to promote the directional solidification and hence helps in production of desired sound casting. Considerations for Desiging Riser

54 While designing risers the following considerations must always be taken into account. (A) Freezing time 1 For producing sound casting, the molten metal must be fed to the mold till it solidifies completely. This can be achieved when molten metal in riser should freeze at slower rate than the casting. 2 Freezing time of molten metal should be more for risers than casting. The quantative risering analysis developed by Caine and others can be followed while designing risers. (B) Feeding range 1. When large castings are produced in complicated size, then more than one riser are employed to feed molten metal depending upon the effective freezing range of each riser. 2. Casting should be divided into divided into different zones so that each zone can be feed by a separate riser. 3. Risers should be attached to that heavy section which generally solidifies last in the casting. 4. Riser should maintain proper temperature gradients for continuous feeding throughout freezing or solidifying. (C) Feed Volume Capacity 1 Riser should have sufficient volume to feed the mold cavity till the solidification of the entire casting so as to compensate the volume shrinkage or contraction of the solidifying metal. 2 The metal is always kept in molten state at all the times in risers during freezing of casting. This can be achieved by using exothermic compounds and electric arc feeding arrangement. Thus it results for small riser size and high casting yield. 3 It is very important to note that volume feed capacity riser should be based upon freezing time and freezing demand. Riser system is designed using full considerations on the shape, size and the position or location of the riser in the mold. SOLIDIFICATION OF CASTINGS After molten metal is poured into a mould, a numbers of events takes place during the solidification of the casting and its cooling to atmospheric temperature. These events greatly influence the size, shape, uniformity, and chemical composition of the grains formed throughout the casting, which in turn influence its overall properties. The significant factors affecting these events are the type of metal, thermal properties of both the metal and the mould, the geometric relationship between volume and surface area of the casting, and shape of the mould. Nucleation and Grain Growth When the free energy of a parent phase is reduced by means of temperature or pressure then there is a driving force leading to crystallization. At the melting point, the thermal fluctuations result in the formation of tiny particles (containing only a few atoms) of the product phase within the parent volume. Such a tiny particle has an interface that separates it from the parent matrix. It grows by

55 transfer of atoms across its interface. The process of formation of the first stable tiny particle is called nucleation. And the process of increase in the sizes of these particles is called grain growth. Solidification of Pure Metal or Eutectic Alloy Cooling curve for metal Because a pure metal or eutectic alloy has a clearly defined melting or freezing point, it solidifies at a constant temperature. After the temperature of the molten metal drops to its freezing point, its temperature remains constant while the latent heat of fusion is given off. The solidification front (solid-liquid interface) moves through the molten metal, solidifying from the mould walls in toward the centre. Once solidification has taken place at any point, cooling resumes. The solidified metal, called casting, is taken out of the mould and is allowed to cool to ambient temperature. At the mould walls, which are at ambient temperature, the metal cools rapidly. Rapid cooling produces a solidified skin or shell. The grains grow in a direction opposite to that of the heat transfer through the mould. Those grains that have favourable orientation will grow preferentially and are called columnar grains. As the driving force of the heat transfer is reduced away from the mould walls, the grains become equiaxed and coarse. Those grains that have substantially different orientations are blocked from further growth. This grain development is called homogeneous nucleation, meaning that grains grow upon themselves, starting from the mould wall. Control of Solidification for obtaining Sound Castings (Centre line of freezing)

56 Solidification of a plate In order to obtain a sound casting with no shrinkage void along the centreline, two requirements must be satisfied as follows: 1. The longitudinal solidification must be progressive toward the riser from the point, or points, most distant from the riser. 2. The temperature gradient, in addition to being properly directed, must be sufficiently steep so that liquid metal can pass through the wedge-shaped channel to compensate for shrinkage as it occurs at the centreline. If the temperature gradient is not sufficiently steep, the included angle of the wedge-shaped channel will be too small and proper passage of feed metal is not possible. If there were no temperature gradient, the lateral solidification at all points would reach the centreline at the same time. The result in either case is a lack of metal at the centreline, which causes an elongated narrow void known as centreline shrinkage. In other casting sections, voids of various shapes are caused by the shrinkage of skin forming type of alloy. Solidification in alloys begins when the temperature drops below the liquidus temperature and is complete when it reaches the solidus temperature. Within this temperature range, the alloy is in a mushy or pasty state with columnar dendrites. The mushy zone is described in terms of a temperature difference, known as the freezing range, as follows: Freezing Range = T L T S Centreline Feeding Resistance

57 The freezing patterns of a chilled and an ordinary mould are shown in figure. The solidification starts at the centre line of the mould before the solidification is completed even at the mould face. In the chilled mould, on the other hand, due to rapid heat extraction, a narrow liquid zone quickly sweeps across the molten metal. The difficulty of feeding a given alloy in a mould is expressed by a quantity, called centre line feeding resistance (CFR). CFR= 100 In the above figure, we have CFR = 100% Normally, feeding is considered to be difficult if CFR > 70 %. Chills These are provided in the mould so as to increase the heat extraction capability of the sand mould. A chill normally provides a steeper temperature gradient so that directional solidification as required in a casting is obtained. These are metallic objects having a higher heat absorbing capability than the sand mould. The chills can be of two types: external and internal.

58 The external chills are placed adjoining the mould cavity at any required position. Providing a chill at the edge may not normally have the desired effect as the temperature gradient is steeper at the end of the casting since heat is removed from all sides. However, if it is placed between two risers it would have maximum effect. The chills when placed in the mould should be clean and dry, otherwise gas inclusions be left in the castings. Also, after placing the chills in the mould, they should not be kept for long since moisture may condense on the chills causing blow holes in the casting. Chaplets Chaplets are metallic support often kept inside the mould cavity to support the cores. These are of the same composition as that of the pouring metal so that the molten metal would provide enough heat to completely melt them and thus fuse with it during solidification.

59 Though the chaplet is supposed to fuse with parent metal, in practice it is difficult to achieve and normally it forms a weak joint in the casting. The other likely problems encountered in chaplets are the condensation of moisture which finally ends up as blow holes. So chaplets before they are placed in the mould should be thoroughly cleaned of any dirt, oil or grease. Because of the problems associated with chaplets, it is desirable to redesign the castings, as far as possible. GATING SYSTEM IN MOLD Fig shows the different elements of the gating system. Some of which are discussed as under. Goals of Gating System

60 The goals for the gating system are To minimize turbulence to avoid trapping gasses into the mold To get enough metal into the mold cavity before the metal starts to solidify To avoid shrinkage Establish the best possible temperature gradient in the solidifying casting so that the shrinkage if occurs must be in the gating system not in the required cast part. Incorporates a system for trapping the non-metallic inclusions 1. Pouring basin It is the conical hollow element or tapered hollow vertical portion of the gating system which helps to feed the molten metal initially through the path of gating system to mold cavity. It may be made out of core sand or it may be cut in cope portion of the sand mold. It makes easier for the ladle operator to direct the flow of molten metal from crucible to pouring basin and sprue. It helps in maintaining the required rate of liquid metal flow. It reduces turbulence and vertexing at the sprue entrance. It also helps in separating dross, slag and foreign element etc. from molten metal before it enters the sprue. 2. Sprue Fig. Gating System It is a vertical passage made generally in the cope using tapered sprue pin. It is connected at bottom of pouring basin. It is tapered with its bigger end at to receive the molten metal the smaller end is

61 connected to the runner. It helps to feed molten metal without turbulence to the runner which in turn reaches the mold cavity through gate. It some times possesses skim bob at its lower end. The main purpose of skim bob is to collect impurities from molten metal and it does not allow them to reach the mold cavity through runner and gate. 3. Gate It is a small passage or channel being cut by gate cutter which connect runner with the mould cavity and through which molten metal flows to fill the mould cavity. It feeds the liquid metal to the casting at the rate consistent with the rate of solidification. 4. Choke It is that part of the gating system which possesses smallest cross-section area. In choked system, gate serves as a choke, but in free gating system sprue serves as a choke. 5. Runner It is a channel which connects the sprue to the gate for avoiding turbulence and gas entrapment. 6. Riser It is a passage in molding sand made in the cope portion of the mold. Molten metal rises in it after filling the mould cavity completely. The molten metal in the riser compensates the shrinkage during solidification of the casting thus avoiding the shrinkage defect in the casting. Functions of Risers Provide extra metal to compensate for the volumetric shrinkage Allow mold gases to escape Provide extra metal pressure on the solidifying mold to reproduce mold details more exact Design Requirements of Risers 1. Riser size: For a sound casting riser must be last to freeze. The ratio of (volume / surface area) 2 of the riser must be greater than that of the casting. However, when this condition does not meet the metal in the riser can be kept in liquid state by heating it externally or using exothermic materials in the risers. 2. Riser placement: the spacing of risers in the casting must be considered by effectively calculating the feeding distance of the risers. 3. Riser shape: cylindrical risers are recommended for most of the castings as spherical risers, although considers as best, are difficult to cast. To increase volume/surface area ratio the bottom of the riser can be shaped as hemisphere. It also permits the escape of air and mould gases. It promotes directional solidification too and helps in bringing the soundness in the casting.

62 Forces acting on the Core and Moulding Flask The main force acting on the core when metal is poured into the mould cavity is due to buoyancy. The buoyant force can be calculated as the difference in the weight of the liquid metal to that of the core material of the same volume as that of the exposed core. It can be written as P = V (ρ d) Where P = buoyant force, N V = volume of the core in the mould cavity, cm 3 ρ = weight density of the liquid metal, N/ cm 3 d = weight density of the core material, N/ cm 2 = 1.65 x 10-2 N/ cm 3 The above equation is valid for horizontally placed core. But for vertically placed core the following equation has to be used. P = 0.25 π ( D 1 2 D 2 ) H ρ - Vd Where V = total volume of the core in the mould. Parting line The parting line is the boundary where the cope, drag and the part meet. If the surface of the cope and drag are planar, then the parting line is the outline of the cross-section of the part along that plane. You can easily see the parting line for many cast and molded parts that you commonly use. It is conventional that the parting line should be planar, if possible. A very small of metal will always leak outside the mold between the cope and the drag in any casting. This is called the flash. If the flash is along an external surface, it must be machined away by some finishing operation. If the parting line is along an edge of the part, it is less visible this is preferred. Special castings

63 1.6.1 DIE CASTING (PRESSURE DIE CASTING) It involves the preparation of components by injecting molten metal at high pressure (2 to 200 N/ mm2) into a metallic die. Because of high pressure involved in die casting, any narrow sections, complex shapes and fine surface details can easily be produced. The metallic die consists of two parts. One part called as stationary or cover die is fixed to the die casting machine while the other part called ejector die is moved out for the extraction of the casting. The casting cycle begins when the twp parts of the die are apart. The lubricant is sprayed on the die cavity manually or by the auto lubrication system. Then the two halves are closed and clamped. The required amount of metal is injected into the die. After the casting is solidified under pressure the die is opened and the casting is ejected. Die Casting Machine Die casting machine performs the following functions: Holding two die halves firmly together. Spraying lubricant on the die cavity. Closing the die. Injecting molten metal into the die. Opening the die. Ejecting the casting out of the die. Die casting machines are of two types: Hot chamber die casting machine in which the holding furnace for the liquid metal is integral with it. Cold chamber die casting machine in which the molten metal, which is melted in a separate furnace, is poured into the die casting machine with a ladle for each casting cycle. Hot Chamber Process In this, a gooseneck is used for pumping the liquid metal into the die cavity. The gooseneck, made of grey, alloy or ductile iron or cast steel, is submerged in the holding furnace containing the molten metal. A plunger made of alloy cast iron and which is hydraulically operated, moves up in the gooseneck to uncover the entry port for the intake of liquid metal into the gooseneck. The plunger can then develop the necessary pressure for forcing the metal into the die cavity through the nozzle.

64 Figure (a) Hot chamber die casting (b) Cold chamber die casting In another arrangement, the gooseneck container is operated by a lifting mechanism. Initially it is submerged in the molten metal and is filled by the gravity. Then it is raised so as to bring the nozzle in contact with the die opening and is locked in that position. Compressed air then forces the metal into the die and pressure is maintained till solidification. When solidification is complete, the gooseneck is lowered down and casting is removed by ejector pins after opening the dies. Operating Sequence: The cycle starts with the closing of the die, when the plunger is in the highest position in the gooseneck, thus facilitating the filling of the gooseneck by the liquid metal. The plunger then starts moving down to force the metal in the gooseneck to be injected into the cavity. The metal is then held at the same pressure till it is solidified. The die is opened, any cores, if present are also retracted. The plunger then moves back returning the unused liquid metal to the gooseneck. The casting which is in the ejector die is now ejected and at the same time the plunger uncovers the filling hole, letting the liquid metal from the furnace to enter the gooseneck. Cold Chamber Process The hot chamber process is used for most of the low melting temperature alloys such as zinc, lead and tin. For materials such as aluminiumm and brass, their high melting temperatures make it difficult to cast them by hot chamber process, because gooseneck of the hot chamber machine is continuously in contact with the molten metal. Also liquid aluminium would attack the gooseneck material and thus hot chamber process is not used with aluminium alloys. In cold chamber process, the molten metal is poured with a ladle into the shot chamber for every shot. This process reduces the contact time between the liquid metal and the shot chamber. Operating Sequence: The operation starts with the spraying of die lubricants through out the die cavity and closing of the die when molten metal is ladled into the shot chamber of the machine either manually by a hand ladle or

65 by means of an automatic robotic ladle. The metal volume and pouring temperature can be precisely controlled with a robotic ladle and hence the desired casting quality can be held. Then plunger forces the metal into the die cavity and maintains the pressure till it solidifies. After that the die opens and the casting is ejected. At the same time the plunger returns to its original position completing the operation. The main disadvantage of this process is the longer cycle time needed compared to the hot chamber process. Since the metal is ladled into the machine from the furnace, it may lose the superheat and sometimes may cause defects such cold shuts. Advantages Because of the use of the movable cores, it is possible to obtain fairly complex castings. Very small thickness (0.4 mm) can be easily filled because the liquid metal is injected at high pressure. Very high production rates (300 to 350 pieces per hour) can be achieved. Because of metallic dies, very good surface finish (1 micron) can be obtained. The surfaces generated by die casting can be directly electroplated without any further processing. Close dimensional tolerances of the order of ± 0.08 mm for small dimensions can be obtained. The die has a long life, which is the order of 3,00,000 pieces for zinc alloys and 1,50,000 for aluminium alloys. Die casting gives better mechanical properties compared to sand casting, because of the fine grained skin formed during solidification. Inserts can be readily cast in place. It is very economical for large scale production. It requires less floor space. The labour cost involved is less. The increased soundness and reduction of defects provide increased yield. Threads and other fine surface details can be easily obtained. Limitations The maximum size of casting is limited. The normal sizes are less than 4 Kg with a maximum of order of 15 Kg. This is not suitable for all materials because of the limitations on die materials. Normally, zinc, aluminium, magnesium and copper alloys are die cast. The air in the die cavity gets trapped inside the casting and is therefore a problem often with the die castings. Porosity causes reduction of mechanical properties. Heart treatment is usually not possible because at high temperature the metal becomes weaker, and the entrapped air expands, causing blisters to raise on the die casting surfaces.

66 The dies and the machines are very expensive and therefore, economy in production is possible only when large quantities are produced. The life of die decreases rapidly if metal temperature is high. Special skill is required for maintenance and supervision of dies. This technique requires comparatively longer time for going into production (set up time, preparation time, etc.). Applications The typical products made by die casting are carburetor, crank case, magnetos, handle bar housings, parts of scooters, motorcycles, zip fasteners, head lamp bezels, battery parts, light duty bearings, radiation shield, and many decorative parts CENTRIFUGAL CASTING Centrifugal casting is accomplished by rotating a mould rapidly about its central axis as the metal is poured into it. This is done principally to secure higher pressures upon the molten metal before and during its solidification. Denser metal is obtained, since relatively lighter impurities within the metal, such as oxide, sand, slag, and gas, will get separated and float more quickly toward the centre of rotation. There are three main types of centrifugal casting processes. They are True centrifugal casting Semi-centrifugal casting Centrifuging. True Centrifugal Casting This is used for the making of hollow pipes, tubes, hollow bushes, etc. which are axisymmetric with a concentric hole. Since the metal is always pushed outward because of centrifugal force, no core needs to be used for making the concentric hole. The axis of rotation can be horizontal, vertical or any angle in between. Very long pipes are normally cast with horizontal axis, whereas short pipes are more conveniently cast with a vertical axis. Figure : True centrifugal casting At first the moulding flask is properly rammed with sand to confirm to the outer contour of the pipe to be made. Any end details like spigot ends, or flanged ends are obtained with the help of dry sand cores

67 located in the ends. Then the flask is dynamically balanced so as to reduce the occurrence of undesirable vibrations during the casting process. The finished flask is mounted in between the rollers and the mould is rotated slowly. Now the molten metal in the requisite quantity is poured into the mould through the movable poring basin. The amount of metal poured determines the thickness of the pipe to be cast. After the pouring is complete, the mould is rotated at its operational speed till it solidifies, to form the requisite tubing. Then the mould is replaced by a new mould and the process is repeated. Metal mould can also be used in true centrifugal casting process for large quantity production. A water jacket is provided around the mould for cooling it. The casting machine is mounted on wheels with the pouring with the pouring ladle which has a long spout extending till the other end of the pipe to be made. Initially the mould is rotated with the metal being delivered at the extreme end of the pipe. The casting machine is slowly moved down the track allowing the metal to be deposited all along the length of the pipe. The machine is continuously rotated till the pipe is completely solidified. Afterwards, the pipe is extracted from the mould and the cycle is repeated. Castings with relatively short lengths are usually more conveniently cast in moulds rotating about a vertical or an inclined axis. The resulting central hole, instead of being cylindrical, will be slightly paraboloidal. However, the high spinning speeds used will produce central holes, which are nearly cylindrical. A centrifugal casting machine, which spins about a vertical or an inclined axis, should be strong and rigid, since the forces encountered when heavy moulds are rapidly rotated may be considerable. When pouring, the metal should be directed against the centre of the mould bottom where the movement is least. The moulds may be sand lined or permanent moulds made of metal, graphite, or other suitable materials. Advantages: The mechanical properties of centrifugal cast jobs are better compared to other processes, because the inclusions such as slag and oxides get segregated towards the centre and can be easily removed by machining. Also, the pressure acting on the metal throughout the solidification causes the porosity to be eliminated giving rise to dense metal. Up to a certain thickness of objects, proper directional solidification can be obtained starting from the mould surface to the centre. No cores are required for making concentric holes in the case of true centrifugal casting. There is no need for gates and runners, which increases the casting yield, reaching almost 100 %. Limitations: Only certain shapes which are axisymmetric and having concentric holes are suitable for true centrifugal casting. The equipment is expensive and thus is suitable only for large quantity production. Applications: Cylindrical parts ranging from 13 mm to 3 m in diameter and 16 m long can be cast with wall thickness ranging from 6 mm to 125 mm. In addition to pipes, typical parts made are bushings, engine cylinder liners, and bearing rings with or without flanges.

68 Semi-Centrifugal Casting This casting is used for jobs which are more complicated than those possible in true centrifugal casting, but are axisymmetric in nature. It is not necessary that these should have a central hole, which is to be obtained with help of a core. The moulds made of sand or metal are rotated about a vertical axis and the metal enters the mould through the central pouring basin. For larger production rates the moulds can be stacked one over the other, all feeding from the same central pouring basin. The rotating speeds used in this process are not as high as in the case of true centrifugal casting. The general practice is to rotate these moulds at rpm which will give a linear speed at the outside edge of the castings of about 200 m per minute. The typical products made by this process are wheels, gear blanks, sheaves etc. Figure Semi centrifugal casting Centrifuging When casting shapes are not axisymmetric, then centrifuging process is used. This is suitable for small jobs of any shape. A number of small jobs are joined together by means of radial runners with a central sprue on a revolving table. The jobs are uniformly placed on the table around the periphery so that their masses are properly balanced. The process is similar to semi centrifugal casting. Stacked or multiple moulds may be advantageously employed for castings required in large quantities. Continuous Casting Generally the starting point of any structural steel product is the ingot which is subsequently rolled through number of mills before a final product such as slab or bloom is obtained. However, the wide adoption of continuous casting has changed that scenario by directly casting slabs, billets and blooms without going through the rolling process. This process is fast and also economical. In this process, the liquid steel is poured into a double walled, bottomless water cooled mould made up copper where a solid skin is quickly formed and a semi-finished skin emerges from the open mould bottom. The skin formed in the mould is about 10 to 25 mm thick and is further solidified by intensive cooling with water sprays as casting moves downwards. A typical arrangement of continuous casting plant is shown in the figure. The molten steel is collected in a ladle and kept over a refractory lined intermediate pouring vessel named tundish. The steel is then poured into water cooled vertical moulds which are 450 to 750 mm long. Before starting the casting a dummy bar is placed in the mould bottom. After starting the casting process as the metal level rises in the mould to a desirable height, the starter bar is withdrawn at a rate equal to the steel pouring rate. The initial metal freezes onto the starter bar as well as the periphery of the mould. This solidified shell supports the liquid steel as it moves

69 downwards. This steel shell is mechanically supported (rollers) as it moves down through the secondary cooling zone where water is sprayed onto the shell surfaces to complete the solidification process. After the casting is completely solidified, it is cut to the desired lengths by a suitable cut off apparatus. Casting Defects Defects may occur due to one or more of the following reasons: Fault in design of casting pattern Fault in design on mold and core Fault in design of gating system and riser Improper choice of molding sand Improper metal composition Inadequate melting temperature and rate of pouring Moulding related defect Improper Closer Across parting plane: flash Along parting line: mismatch Filling-related Defects Improper filling; cold shut, misrun. Gaseous Entrapments: blow hole, gas porosity. Solid Inclusions: sand inclusion, slag inclusion

70 Solidification/Cooling related Defects Solidification Shrinkage:cavity,porosity, centreline, sink. Hindered Cooling Contraction: hot tear, crack, distortion. Classification of Casting Defects Surface Defects: Blow, Scar, Blister, Drop, Scab, Penetration, Buckle. Internal Defects:Blow holes, Porosity, Pin holes, Inclusions, Dross. Visible Defects: Wash, Rat tail, Swell, Misrun, Cold shut, Hot tear, Shrinkage/Shift. Surface Defects Blow is relatively large cavity produced by gases which displace molten metal from convex surface. Scar is shallow blow generally occurring on a flat surface. Blister is type of scar which is covered with a thin layer of metal over it. These are due to improper permeability or venting. Sometimes excessive gas forming constituents in moulding sand. Drop is an by dropping irregularly-shaped projection on the cope surface caused of sand. A scab when an up heaved sand gets separated from the mould surface and the molten metal flows between the displaced sand and the mold. Buckle is a v- by expansion of a Penetration occurs when the molten metal flows between the sand particles in the mould. These defects are due to inadequate strength of the mold and high temperature of the molten metal adds on it. shaped depression on the surface of a flat casting caused thin layer of sand at the mould face. Internal Defects

71 The internal defects found in the castings are mainly due to trapped gases and dirty metal. Gases get trapped due to hard ramming or improper venting. These defects also occur when excessive moisture or excessive gas forming materials are used for mould making. Blow holes are large spherical shaped gas bubbles Porosity indicates a large number of uniformly distributed tiny holes. Pin holes are tiny blow holes appearing just below the casting surface. Inclusions are the non-metallic particles in the metal matrix, Lighter impurities appearing the casting surface are dross. Insufficient mould strength, insufficient metal, low pouring temperature, and bad design of casting are some of the common causes. Wash is a low projection near the gate caused by erosion of sand by the flowing metal. expansion of Rat tail is a long, shallow, angular depression caused by the sand. Swell is the deformation of vertical mould surface due to hydrostatic pressure caused by moisture in the sand. Visible Defects Misrunand cold shut are caused by insufficient superheat provided to the liquid metal. Hot tear is the crack in the casting caused by high residual stresses. Shrinkage is essentially solidification contraction and occurs due to improper use of Riser. Shift is due to misalignment of two parts of the mould or incorrect core location.

72 MODULE-II

73 2.1 INTRODUCTION Welding is a process for joining two similar or dissimilar metals by fusion. It joins different metals/alloys, with or without the application of pressure and with or without the use of filler metal. The fusion of metal takes place by means of heat. The heat may be generated either from combustion of gases, electric arc, electric resistance or by chemical reaction. During some type of welding processes, pressure may also be employed, but this is not an essential requirement for all welding processes. Welding provides a permanent joint but it normally affects the metallurgy of the components. Weldability: It is the capacity of being welded into inseparable joints having specified properties such as definite weld strength, proper structure etc. Weldability depends on : (1) Melting point (2) Thermal conductivity (3) Thermal expansion (4) Surface condition (5) Change in Micro structure etc. These characteristics may be controlled / corrected by proper shielding atmosphere, proper fluxing material, proper filler material, proper welding procedure, proper heat treatment before and after deposition. 2.2 BASIC CONCEPT OF WELDING Welding is a process of joining two similar or dissimilar metals with the help of heat or pressure or by some other means. The cost of welding is very less as compared to other processes and forms a strong joint. For this reason it is largely used in the following fields of engineering: 1. Manufacturing of machine tools, auto parts, cycle parts, etc. 2. Fabrication of farm machinery & equipment. 3. Fabrication of buildings, bridges & ships. 4. Construction of boilers, furnaces, railways, cars, aeroplanes, rockets and missiles. 5. Manufacturing of television sets, refrigerators, kitchen cabinets, etc. A weld will inherit the common drawback of brittleness, which is a disadvantage. The emphasis in welding technique should, therefore, be preventing this brittleness to the maximum possible extent Advantages & Disadvantages:

74 2.3 CLASSIFICATION OF WELDING PROCESSES Many types of welding processes have been developed depending upon the field of their applications (Table 7.1). But the welding is broadly divided into following two groups. 1. Forge or Pressure Welding (Under pressure without additional filler metal) (a) Friction welding (b) Electric resistance welding (c) Blacksmiths forge welding (d) Cold pressure welding 2. Fusion or non-pressure welding (With additional filler material) (a) Gas welding (Heat created by Gas) (b) Electric arc welding (Heat created by electrically) (c) Thermite welding (Heat created by chemical Reaction)

75 2.4 GAS WELDING PROCESSESS A fusion welding process which joins metals, using the heat of combustion of an oxygen /air and fuel gas (i.e. acetylene, hydrogen propane or butane) mixture is usually referred as gas welding. The intense heat (flame) thus producedd melts and fuses together the edges of the parts to be welded, generally with the addition of a filler metal. Operation of gas welding is shown in Fig.. The fuel gas generally employed is acetylene; however gases other than acetylene can also be used though with lower flame temperature. Oxy-acetylene flame is the most versatile and hottest of all the flames produced by the combination of oxygen and other fuel gases. Fig. Gas welding operation

76 2.5.1 Oxy-Acetylene Welding Oxy-fuel welding, commonly referred to as oxy welding or gas welding is a process of joining metals by application of heat created by gas flame. The fuel gas commonly acetylene, when mixed with proper proportion of oxygen in a mixing chamber of welding torch, produces a very hot flame of about F. With this flame it is possible to bring any of the so-called commercial metals, namely: cast iron, steel, copper, and aluminum, to a molten state and cause a fusion of two pieces of like metals in such a manner that the point of fusion will very closely approach the strength of the metal fused. If more metal of like nature is added, the union is made even stronger than the original. This method is called oxy-acetylene welding Gas Welding Equipments An arrangement of oxy acetylene welding set up is shown in Fig. used for oxy-acetylene welding are following:. The basic tools and equipments Oxy-fuel apparatus consists of two cylinders (one oxygen and one acetylene) equipped with two regulators, pressure gauges, two lengths of hose, and a blow torch. The regulators are attached to cylinders and are used to reduce and maintain a uniform pressure of gases at the torch. The gases at reduced pressure are conveyed to the torch by the hoses. The regulators include high pressure and low pressure gauges to indicate the contents of the cylinder and the working-pressure on each hose. When the gases reach the torch they are there mixed and combustion takes place at the welding tip fitted to the torch. The basic equipments used to carry out gas welding are: 1. Oxygen gas cylinder (green)

77 2. Acetylene gas cylinder (maroon/red) 3. Oxygen pressure regulator 4. Acetylene pressure regulator 5. Oxygen gas hose(blue) 6. Acetylene gas hose(red) 7. Welding torch or blow pipe with a set of nozzles and gas lighter 8. Trolleys for the transportation of oxygen and acetylene cylinders 9. Set of keys and spanners 10. Filler rods and fluxes 11. Protective clothing for the welder (e.g., asbestos apron, gloves, goggles, etc. Acetylene and oxygen gas is stored in compressed gas cylinders. These gas cylinders differ widely in capacity, design and colour code. However, in most of the countries, the standard size of these cylinders is 6 to 7 m 3 and is painted black for oxygen and maroon for acetylene. An acetylene cylinder is filled with some absorptive material, which is saturated with a chemical solvent acetone. Acetone has the ability to absorb a large volume of acetylene and release it as the pressure falls. If large quantities of acetylene gas are being consumed, it is much cheaper to generate the gas at the place of use with the help of acetylene gas generators. Acetylene gas is generated by carbide-to-water method. Oxygen gas cylinders are usually equipped with about 40 litres of oxygen at a pressure of about 154 Kgf/cm 2 at 21 C. To provide against dangerously excessive pressure, such as could occur if the cylinders were exposed to fire, every valve has a safety device to release the oxygen before there is any danger of rupturing the cylinders. Fragile discs and fusible plugs are usually provided in the cylinders valves in case it is subjected to danger. Chemistry of Oxy Acetylene Process The most common fuel used in welding is acetylene. It has a two stage reaction; the first stage primary reaction involves the acetylene disassociating in the presence of oxygen to produce heat, carbon monoxide, and hydrogen gas. 2C 2 H 2 + 2O 2 = 4CO + 2H 2 + Heat (1) A secondary reaction follows where the carbon monoxide and hydrogen combine with more oxygen to produce carbon dioxide and water vapor. 4CO + 2H 2 + 3O 2 = 4CO 2 + 2H 2 O + Heat (2) When you combine equations (1) and (2) you will notice that about 5 parts of oxygen is necessary to consume 2 parts of acetylene 2C 2 H 2 + 5O 2 = 4CO 2 + 2H 2 O + Heat (3)

78 Gas pressure regulators Gas pressure regulators are employed for regulating the supply of acetylene and oxygen gas from cylinders. A pressure regulator is connected between the cylinder and hose leading to welding torch. The cylinder and hose connections have left-handed threads on the acetylene regulator while these are right handed on the oxygen regulator. A pressure regulator is fitted with two pressure gauges, one for indication of the gas pressure in the cylinder and the other for indication of the reduced pressure at which the gas is going out WELDING TORCH & BLOW PIPE A welding torch mixes oxygen and acetylene in the desired proportions, burns the mixture at the end of the tip, and provides a means for moving and directing the flame. There are two types of welding torches, namely: a) High pressure (or equal pressure) type b) Low pressure (or injector) type The high pressure torch also called the equal pressure torch is most commonly used because: a) It is lighter and simpler; b) It does not need an injector; c) In operation, it is less troublesome since it does not suffer from backfires to the same extent. Torch tips It is the portion of the welding apparatus through which the gases pass just prior to their ignition and burning. A great variety of interchangeable welding tips differing in size, shape and construction are available commercially. The tip sizes are identified by the diameter of the opening. The diameter of the tip opening used for welding depends upon the type of metal to be welded. Hose pipes The hose pipes are used for the supply of gases from the pressure regulators. The most common method of hose pipe fitting both oxygen and acetylene gas is the reinforced rubber hose pipe. Green is the standard color for oxygen hose, red for acetylene, and black hose for other industrially available welding gases.

79 Filler Metals: Filler metals are used to supply additional material to the pool to assist in filling the gap (or groove) and it forms an integral part of the weld. Filler rods have the same or nearly the same chemical composition as the base metal and are available in a variety of compositions (for welding different materials) and sizes. These consumable filler rods may be bare, or they may be coated with flux. The purpose of the flux is to retard oxidation of the surfaces of the parts being welded, by generating gaseous shield around the weld zone. The flux also helps to dissolve and remove oxides and other substances from the work piece and so contributes to the formation of a stronger joint. Characteristics of good flux The melting point of a flux must be lower than that of either the metal or the oxides formed, so that it will be liquid. The ideal flux has exactly the right fluidity when the welding temperature has been reached. The flux will protect the molten metal from atmospheric oxidation. flux will remain close to the weld area instead of flowing all over the base metal for some distance from the weld. Composition of Fluxes Fluxes differ in their composition according to the metals with which they are to be used. In cast iron welding, a slag forms on the surface of the puddle. The flux serves to break this up. Equal parts of a carbonate of soda and bicarbonate of soda make a good compound for this purpose. Nonferrous metals usually require a flux. Copper also requires a filler rod containing enough phosphorous to produce a metal free from oxides. Borax which has been melted and powdered is often used as a flux with copper alloys. A good flux is required with aluminum, because there is a tendency for the heavy slag formed to mix with the melted aluminum and weaken the weld. For sheet aluminum welding, it is customary to dissolve the flux in water and apply it to the rod. After welding aluminum, all traces of the flux should be removed. Characteristics of the oxy-acetylene welding process include: The use dual oxygen and acetylene gases stored under pressure in steel cylinders; Its ability to switch quickly to a cutting process, by changing the welding tip to a cutting tip; The high temperature the gas mixture attains (~5800 F); The use of regulators to control gas flow and reduce pressure on both the oxygen and acetylene tanks; The use of double line rubber hoses to conduct the gas from the tanks to the torch; Melting the materials to be welded together; The ability to regulate temperature by adjusting gas flow

80 Types of Welding Flames In oxy-acetylene welding, flame is the most important means to control the welding joint and the welding process. The correct type of flame is essential for the production of satisfactory welds. The flame must be of the proper size, shape and condition in order to operate with maximum efficiency. There are three basic types of oxy-acetylene flames. Neutral welding flame (Acetylene and oxygen in equal proportions). 2. Carburizing welding flame or reducing (excess of acetylene). 3. Oxidizing welding flame (excess of oxygen). The gas welding flames are shown in Fig Neutral Flame - A neutral flame is produced when approximately equal volumes of oxygen and acetylene are mixed in the welding torch and burnt at the torch tip. (More accurately the oxygen-toacetylene ratio is 1.1 to 1). The temperature of the neutral flame is of the order of about 3260ºC. The flame has a nicely defined inner cone which is light blue in colour. It is surrounded by an outer flame envelope, produced by the combination of oxygen in the air and superheated carbon monoxide and hydrogen gases from the inner cone. This envelope is usually a much darker blue than the inner cone. A neutral flame is named so because it effects no chemical change in the molten metal and therefore will not oxidize or carburize the metal. The neutral flame is commonly used for the welding of: (i) Mild steel (ii) Stainless steel (iii) Cast Iron (iv) Copper (v) Aluminium Reducing Flame - If the volume of oxygen supplied to the neutral flame is reduced, the resulting flame will be a carburising or reducing flame, i.e. rich in acetylene. A reducing flame can be recognized by acetylene feather which exists between the inner cone and the outer envelope. The outer flame envelope is longer than that of the neutral flame and is usually much brighter in colour. A reducing flame does not completely, consume the available carbon; therefore, its burning temperature is lower and the left over carbon is forced into the molten metal. With iron and steel it produces very hard, brittle substance known as iron carbide. This chemical change makes the metal unfit for many applications in which the weld may need to be bent or stretched. Metals that tend to absorb carbon should not be welded with reducing flame. A reducing flame has an approximate temperature of 3038 C. A reducing flame may be distinguished from a carburizing flame by the fact that a carburizing flame contains more acetylene than a reducing flame. A carburizing flame is used in the welding of lead and for carburizing (surface hardening) purposes.

81 A reducing flame, on the other hand, does not carburize the metal, rather it ensures the absence of the oxidizing condition. It is used for welding with low alloy steel rods and for welding those metals, (e.g. non ferrous) that do not tend to absorb carbon. This flame is very well used for welding high carbon steel Oxidising Flame - If, after the neutral flame has been established, the supply of oxygen is further increased, the result will be an oxidising flame. An oxidising flame can be recognized by the small white cone which is shorter, much bluer in colour and more pointed than that of the neutral flame. The outer flame envelope is much shorter and tends to fan out at the end on the other hand the neutral and carburizing envelopes tend to come to a sharp point. An oxidising flame burns with a decided loud roar. An oxidising flame tends to be hotter than the neutral flame. This is because of excess oxygen and which causes the temperature to rise as high as 3500 C. The high temperature of an oxidizing flame (O2: C2H2 = 1.5: 1) would be an advantage if it were not for the fact that the excess oxygen, especially at high temperatures, tends to combine with many metals to form hard, brittle, low strength oxides. Moreover, an excess of oxygen causes the weld bead and the surrounding area to have a scummy or dirty appearance. For these reasons, an oxidising flame is of limited use in welding. It is not used in the welding of steel. A slightly oxidising flame is helpful when welding most (i) Copper base metals (ii) Zinc base metals, and (iii) A few types of ferrous metals, such as manganese steel and cast iron The oxidizing atmosphere, in these cases, creates a base metal oxide that protects the base metal. For example, in welding brass, the zinc has a tendency to separate and fume away.

82 Use of flux: Flux is employed in the welding of such metal as cast iron. Some alloy steel and non-ferrous metals to dissolve such as: 1. Remove impurities. 2. Control surface tension. 3. Give protection from atmosphere. It is usually in the format paste in which the rod is dipped. Method of welding using oxy-acetylene welding process. 1. Back hand welding: In this method, the torch precedes the welding rod, as shown bellow. Back hand welding Forehand welding: In this method, the welding rod precedes the torch. The torch is held at approximately a 45 degree angle from the vertical in the direction of welding, as shown bellow. Forehand welding 3. Fillet welding: The fillet weld is the most popular of all types of welds because there is normally no preparation required. 4. Horizontal position welding: In horizontal welding, the weld axis is approximately horizontal, but the weld type dictates the complete definition. For a fillet weld, welding is performed on the upper side of an approximately horizontal surface and against an approximately vertical surface. For a groove weld, the face of the weld lies in an approximately vertical plane

83 5. Flat position welding: This type of welding is performed from the upper side of the joint. The face of the weld is approximately horizontal. 6. Vertical position welding: In vertical position, the plane of the workpiece is vertical and the weld is deposited upon a vertical surface. It is difficult to produce satisfactory welds in this position due to the effect of the force of gravity on the molten metal. The welder must constantly control the metal so that it does not run or drop from the weld. Vertical welding may be of two types viz., vertical-up and vertical-down. Vertical-up welding is preferred when strength is the major consideration. The verticaldown welding is used for a sealing operation and for welding sheet metal. 7. Over head position welding: The overhead position is probably even more difficult to weld than the vertical position. Here the pull of gravity against the molten metal is much greater. The force of the flame against the weld serves to counteract the pull of gravity. In overhead position, the plane of the workpiece is horizontal. But the welding is carried out from the underside. The electrode is held with its welding end upward. It is a good practice to use very short arc and basic coated electrodes for overhead welding.

84 Advantages of Oxyacetylene Process 1) Does not require electricity; 2) The equipment is portable, easy to transport; 3) Welder has considerable control over the rate of heat input, the temperature of the weld zone, and the oxidizing or reducing potential of the welding atmosphere; 4) Oxyacetylene process is ideally suited to the welding of thin sheet, tubes, and small diameter pipe. It is also used for repair work, maintenance and in body shops; 5) Dissimilar metals can easily be joined; 6) Can also be used for preheating, cutting metal, case hardening, soldering and annealing. Limitations 1. Acetylene becomes extremely dangerous if used above 15 pounds pressure. Pure acetylene is selfexplosive if stored in the free state under a pressure of 29.4 pounds per square inch (psi); 2. The process is typically slower than the electrical arc-welding processes; 3. Heavy sections cannot be joined efficiently. 4. For heavy sections proper penetration may not be achieved. 5. Slower speed of welding compared electric arc welding. 6. Flux used in the filler metal provides fumes which are irritating to the eyes, nose, throat and lungs. 7. More safety is recommended in gas welding. 8. Acetylene and oxygen are expensive gases. 9. Prolonged heating of the joint may results in large HAZ. Applications of Gas Welding - 1. For joining thin materials.

85 2. For joining materials in whose case excessively high temperatures or rapid heating and cooling of the job would produce unwanted or harmful changes in the metal. 3. For joining materials in whose case extremely high temperatures would cause certain elements in the metal to escape into the atmosphere. 4. For joining most ferrous and nonferrous metals, e.g., carbon steels, alloy steels, cast iron, aluminium, copper, nickel, magnesium and its alloys, etc. 5. In automotive and aircraft industries. In sheet metal fabricating plants,etc. Gas Cutting: It is possible to rapidly oxidise (burn) iron and steel when it is heated to a temperature between 800 to C. When a high pressure oxygen jet with a pressure of the order of 300 KPa is directed against a heated steel plate, the oxygen jet burns the metal and blows it away causing the cut. This process is used for cutting steel plates of various thicknesses (can go up to 2 m) mainly because the equipment required is simple and can be carried anywhere without handling the heavy steel plates. Oxy-acetylene gas cutting outfit is similar to that of the oxy-acetylene welding except for the torch tip. Here the torch tip has a provision for preheating the plate as well as providing the oxygen jet. Thus the tip has a central hole for oxygen jet with surrounding holes for preheating flames. The cutting tip should be chosen for the intended application. The size is normally dependent on the thickness of the plate which determines the amount of preheating as well as the oxygen jet flow required for cutting. After the steel is heated to the kindling temperature which is about 870 0C, it gets readily combined with oxygen giving iron oxide with the following reactions: 3 Fe + 2 O2 -- Fe3O MJ/Kg of iron 2Fe + O2 -- 2FeO MJ/Kg of iron 4 Fe + 3 O2 -- 2Fe2O MJ/Kg of iron All the above reactions are exothermic in nature and as such would provide a good amount of heat to preheat the steel. But this energy may not be sufficient to bring the steel to its kindling temperature, and hence preheating flames may have to be continued as somewhat lower rate. The heat generated causes the metal to melt and get blown away by the oxygen pressure. About 30 to 40 % of metal is simply blown away, while the rest is oxidised. The cutting can start at the edge or in the middle of the plate. After the plate has reached the kindling temperature, the operator should release the oxygen jet to start the cutting, moving the torch in the forehand direction to achieve the desired cut. Drag is the amount by which the lower edge of the drag line trails from the top edge.

86 A good cut is characterised by very small or negligible drag. When the torch is moved too rapidly, the metal at the bottom does not get sufficient heat to get oxidized and cut and hence there is a large drag. When the torch is moved slowly, all the preheated metal is burnt away by the oxygen jet and a large amount of slag is generated. Though the gas cutting is more useful with thick plates, thin sheets (less than 3 mm) can also be cut by this process taking special precautions. Tip size chosen should be as small as possible. If small tips are not available, then the tip is inclined at an angle of 15 to 20 degrees. Gas cutting can be done manually or by a machine. The manual cutting is used for general purpose work and for straight line cutting. In machine cutting the torch is mounted on a rail and both rail and the torch can move simultaneously along the two mutually perpendicular axes in the horizontal plane with the help of servo motors. There is provision in the machine to hold more than one torch so that large number of identical pieces can be cut at the same time. 2.6.Arc-Welding Introduction Arc welding is the fusion of two pieces of metal by an electric arc between the pieces being joined the work pieces and an electrode that is guided along the joint between the pieces. The electrode is either a rod that simply carries current between the tip and the work, or a rod or wire that melts and supplies filler metal to the joint. Principle of Arc The basic arc welding circuit is an alternating current (AC) or direct current (DC) power source connected by a work cable to the work piece and by a hot cable to an electrode. When the electrode is positioned close to the work piece, an arc is created across the gap between the metal and the hot cable electrode. An ionized column of gas develops to complete the circuit.

87 Figure Arc welding setup The arc produces a temperature of about 6000 C to 7000 C at the tip and melts part of the metal being welded and part of the electrode. This produces a pool of molten metal that cools and solidifies behind the electrode as it is moved along the joint. There are two types of electrodes. Consumable electrode tips melt, and molten metal droplets detach and mix into the weld pool. Non-consumable electrodes do not melt. Instead, filler metal is melted into the joint from a separate rod or wire. The strength of the weld is reduced when metals at high temperatures react with oxygen and nitrogen in the air to form oxides and nitrides. Most arc welding processes minimize contact between the molten metal and the air with a shield of gas, vapour or slag. Granular flux, for example, adds deoxidizers that create a shield to protect the molten pool, thus improving the weld. Arc Welding Equipment The main requirement in an arc welding setup is the source of electric power. They are essentially of two types: a) Alternating Current Machines 1. Transformer 2. Motor or engine driven alternator b) Direct Current Machines 1. Transformer with DC rectifier 2. Motor or engine driven generator In AC welding normally transformer is used. It has following operational characteristics. 1. No moving parts and less noise; 2. Less maintenance; 3. Higher efficiency; 4. Cheaper power source.

88 In DC arc welding a rectifier or a generator can be used to supply the required DC power. At first input voltage is stepped down to required voltage and then through silicon controlled rectifier (SCR) is converted from AC to DC. Its characteristics are 1. Compact setup 2. Highly reliable and efficient 3. Less noise 4. Costly setup The welding machine can be of two types. 1. Constant current welding machines or droopers 2. Constant voltage welding machines In constant current welding machine the change in arc current magnitude due to change in voltage across the electrodes is very small. This machine is very essential for manual arc welding processes since the maintenance of constant arc is nearly impossible by a human welder. With the variation of electrode distance from the base plate in manual arc welding the voltage across the arc gap changes continuously but the magnitude of current remains almost constant due to which good quality of weld can be made. In constant voltage welding machines small change in voltage makes for an extremely large change in the output currents. These machines are generally preferred in the automatic machines since they become self corrective. When the electrode comes a bit closer to the work, the arc voltage drops raising the output current to very high value. This current instantly melts the electrode and thus maintains the arc gap. Figure : Constant current characteristics

89 Figure : Constant voltage characteristics Though DC arc welding is more expensive than AC welding, it is generally preferred because of the control of the heat input offered by it. If more heat is required at the workpiece side, such as for thicker sheets or for the work materials which have higher thermal conductivity such as aluminium and copper, the workpiece can be made as anode, liberating large heat near it. This is termed as straight polarity or direct current electrode negative (DCEN). This gives rise to higher penetration of weld metal. For thinner materials where less heat input is required in the weld zone, the polarity could be reversed by making the workpiece as negative. This is termed as reversed polarity or direct current electrode positive (DCEP).In this case weld metal penetration is small. In case of AC welding the bead obtained is somewhere in between the above two types. DC arc welding is preferred for difficult tasks such as overhead welding, since it can maintain a stable arc. Figure : Weld penetration

90 A voltage of the order of 40 to 50 V should be enough for starting an arc, whereas for continuous welding 20 to 30 V is sufficient. The minimum voltage Vm can be calculated as Vm = I, where I is the load current in amperes. The rated current specifies the maximum current in amperes that a welding machine is capable of supplying at a given voltage. The preferred current ratings as per Indian standardd are 150, 200, 300, 400, 500, 600 and 900 A. Duty Cycle: Duty cycle is the ratio of arcing time to the weld cycle time multiplied by 100. Welding cycle time is either 5 minutes as per European standards or 10 minutes as per American standard and accordingly power sources are designed. It arcing time is continuously 5 minutes then as per European standard it is 100% duty cycle and 50% as per American standard. At 100% duty cycle minimum current is to be drawn i.e. with the reduction of duty cycle current drawn can be of higher level. The welding current which can be drawn at a duty cycle can be evaluated from the following equation; Duty cycle and associated currents are important as it ensures that power source remains safe and its windings are not getting damaged due to increase in temperature beyond specified limit. The maximum current which can be drawn from a power source depends upon its size of winding wire, type of insulation and cooling system of the power source.

91 Power sources produce DC with the electrode either positive or negative, or AC. The choice of current and polarity depends on the process, the type of electrode, the arc atmosphere and the metal being welded. Some Important Definitions Arc-on time: When the welder holds an arc between the electrode and the work piece Idling time: When welding equipment is ready for use but is not generating an arc Operating factor: The ratio of arc-on time to the total time worked, often expressed as a percentage: Work time: Convention is to assume total annual work time of 4000 hours (two shifts). Electrode Efficiency The efficiency of an electrode is the mass of metal actually deposited compared with the mass of that portion of the electrode consumed. It can be expressed as: efficiency % =mass of metal deposited/mass of metal of the electrode consumed x 100 With ordinary electrodes the efficiency varies from 75 to 95 % but with electrodes containing metallic components in the covering the efficiency can approach 200 %(e.g. electrodes containing iron powder).the electrodes are marked with a 6 digit numeral associated by a prefix and a suffix. The meaning of these and the various values are shown in figure. Fig. Example of electrode designation according to ISO-2560 Figure Designation of manual metal arc welding electrode for mild steel Arc Blow When current flows through a conductor, it produces a magnetic flux that circles around the conductor in perpendicular planes. The centres of the flux circles are located at the centre of the conductor. The magnetic flux is produced in the steel and across the arc gap. The arc column is mainly influenced by the lines of forces crossing the arc gap. As the weld joins the pieces together, there is less and less chance that the magnetic field will concentrate in the arc gap. As the weld is filling the gap of the joint, it pushes the magnetic flux ahead of the arc. As long as the flux can travel, no serious arc blow will

92 interrupt the weld. When flux ceases to move, it piles up and a magnetic field of considerable strength develops. The buildup of the flux causes a deflection of arc column as it pills away from this heavy concentration of magnetic forces. Ionized gases that carry the arc from the end of electrode to the workpiece are acting as flexible conductors. This concentration of flux that pulls the arc from its intended path is called Arc Blow. Spatter At the conclusion of a weld small particles or globules of metal may sometimes be observed scattered around the vicinity of the weld along its length. This is known as spatter and may occur through: 1. Arc blow making the arc uncontrollable. 2. The use of too long an arc or too high an arc voltage. 3. The use of excessive current. Power Efficiency Figure : Spatter

93 Welding power sources draw power when idling. Efficiency is greater when idling is reduced and the operating factor is close to 100 percent. The higher the operating factor, the more efficient the process. The following are ways to improve efficiency: Use the most efficient welding process. Use gas metal arc welding (GMAW) instead of shielded metal arc welding (SMAW). Typically, operating factors for SMAW fall between 10 to 30 percent; operating factors for GMAW fall between 30 to 50 percent. Use multi-process inverter power sources. Modern inverter power sources can be used for several welding processes and save time and effort when switching processes. For example, the Miller XTM 304 can be used for GMAW, FCAW, SMAW and GTAW. Automate when possible. Manage repetitive operations by applying advances in automation and computer programming. Reduce idling time. Cut the time spent on pre-welding tasks such as assembly, positioning, tacking and cleaning, and on follow-up operations, such as slag removal and defect repair. Power Source Performance Certain characteristics determine the energy efficiency of power sources: Power factor: Power factor is the ratio of real electrical power made available by the welding power source for producing a welding arc (the power you can use) to the "apparent" electrical power supplied by the utility (the power you pay for). The older technology of transformer-rectifier power sources can have power factors in the order of 75 percent; modern inverter power sources have power factors close to 100 percent. Arc-on power and idling power: Transformer-rectifier power sources use more power in arc-on and idling modes than modern inverter power sources do with the same output. To compare the performance of power sources use the following formula: 2.7.COMMON ELECTRIC ARC WELDING PROCESSES 2.7.1Shielded Metal Arc Welding (SMAW) SMAW is a manual arc welding process in which the heat for welding is generated by an electric arc between a flux-covered consumable electrode and the work. Figure shows a typical welding circuit for SMAW. The electrode tip, arc, molten weld metal and the adjacent areas of the work are protected from atmospheric contamination by the gaseous shield produced by the combustion and decomposition of the electrode covering. Additional shielding is provided for the molten weld metal by the molten flux (or slag) that forms. Filler metal is supplied by the core wire of the consumable electrode, or for certain electrode types, from metal powder mixed with the electrode covering. Figure shows the operating principles for the SMAW process.

94 Advantages SMAW is the simplest and most versatile of the arc welding processes. The simplicity and portability of SMAW equipment allow use of this process in a wide variety of applications from refinery piping to cross country pipelines, and even underwater to repair offshore structures. SMAW can be used in any position or location that can be reached with an electrode. Joints in blind areas can be welded, including the back sides of pipes in restricted areas that are inaccessible for most other welding processes, by using bent electrodes. SMAW is used to join a wide variety of ferrous and nonferrous materials including carbon and low alloy steels, stainless steels, nickel based alloys, cast iron, and some copper alloys SMAW is used to join a wide variety of ferrous and nonferrous materials including carbon and low alloy steels, stainless steels, nickel based alloys, cast iron, and some copper alloys. Disadvantages Even though SMAW is a highly versatile process, it has several characteristics that make the deposition rate lower than with semi-automatic or automatic processes Electrodes are of fixed length and welding must be stopped after each electrode has been consumed.

95 The stub of the electrode is lost, and time is lost for changing electrodes. The slag must be removed from the weld after each pass before subsequent passes can be deposited. These steps lower welding efficiency by about 50%. Smoke and fumes present a problem with SMAW, and ventilation is required in confined spaces. The view of the weld puddle is somewhat obscured by the protective slag that covers the freezing weld metal and by the smoke. Extra welder skill is needed to make radiograph-quality welds in pipe or plate when welded from one side Gas Tungsten Arc Welding (GTAW) In gas tungsten arc welding (GTAW), heat is generated by creating an arc, in an inert shielding gas, between a nonconsumable tungsten electrode and the work. GTAW melts the area of the work under the arc without melting the tungsten electrode. Figure shows the equipment for GTAW. The GTAW process can be used either manually or automatically. Filler metal can be added to the weld by introducing a bare rod into the zone of the arc. Welding techniques are similar to those for oxyfuel gas welding, but the arc and molten puddle are shielded from the atmosphere by a blanket of inert gas, usually argon, helium, or mixtures of these. Inert gas is fed through the torch and around the tungsten. Welds produced with the GTAW process have a smooth surface that is free of slag and low in hydrogen content. One variation of the GTAW process (pulsed GTAW) uses a power source that pulses the welding current. This permits a higher average current for better penetration and weld puddle control, particularly on root passes. Pulsed GTAW is especially useful for out-of-position pipe welding on stainless steel and nonferrous materials such as nickel based alloys.

96 GTAW has been adapted to automatic welding. Automation of the process requires a programmed power source and controls, a wire feeder, and machine guided travel. It has been used to make high quality tube-to-tubesheet seal welds and heat exchanger tube butt welds. Butt welding of large diameter thick walled pipe at utility power plants is another successful application of automatic GTAW. When GTAW uses automatic wire feed it is also referred to as cold wire TIG. Another automatic version of GTAW welding is called hot wire TIG, which has been developed to compete with other, higher deposition rate, welding processes. With hot wire TIG, the wire is resistance heated with low voltage AC current to increase the deposition rate. Advantages The GTAW process produces high quality welds without slag in a variety of ferrous and nonferrous materials. With proper welding technique, all atmospheric contaminants are excluded. A major advantage of the process is that it can be used to make high quality root passes from one side on a wide range of materials. Consequently, GTAW is used extensively for pipe welding. Welding current can be controlled over a wide range, from about 5 to 300 amps, providing greater ability to compensate for changing joint conditions such as root gap. For example, on thin walled (below 0.20-inch) pipe and sheet metal, the current can be adjusted low enough to control penetration and prevent burn-through more easily than can be done with processes that use coated electrodes. The lower speed of travel as compared to SMAW provides better visibility and makes it easier to control the weld metal during deposition and fusion. Disadvantages The main disadvantage of GTAW is its lower deposition rate compared with other processes such as SMAW. In addition, GTAW requires closer control of joint fit-up to produce high quality welds from one side. GTAW also needs better joint cleaning to remove oil, grease, rust, and other contaminants in order to avoid porosity and other weld defects. GTAW must be carefully shielded from air movements above about 5 mph in order to maintain the inert gas shield over the molten puddle. Applications GTAW is excellent for thin wall pipe and small diameter tubing of stainless steel, nickel alloys, copper alloys, and aluminum. On heavier wall piping, it is frequently used for the root pass on welds requiring high quality, such as for high pressure, high temperature hydrogen piping and return bends in furnace coils. It is also used for root passes where a smooth inside diameter surface is required, such as on piping in acid service. Because of the inert gas protection of the weld and excellent process control, GTAW is frequently used on reactive metals such as titanium and magnesium.

97 2.7.3GAS METAL ARC WELDING MIG(GMAW) Gas Metal Arc Welding is an arc welding process that uses the heat of an electric arc established between a consumable metal electrode and the work to be welded. The electrode is a bare metal wire that is transferred across the arc and into the molten weld puddle. The wire, the weld puddle, and the area in the arc zone are protected from the atmosphere by a gaseous shield. Inert gases, reactive gases, and gas mixtures are used for shielding. The metal transfer mode is dependent on shielding gas choice and welding current level. Figure 9 is a sketch of the process showing the basic features Gas metal arc welding is a versatile process that may be used to weld a wide variety of metals including carbon steels, low alloy steels, stainless steels, aluminum alloys, magnesium, copper and copper alloys, and nickel alloys. It can be used to weld sheet metal or relatively heavy sections. Welds may be made in all positions, and the process may be used for semiautomatic welding or automatic welding. In semiautomatic welding, the wire feed speed, voltage, amperage, and gas flow are all preset on the control equipment. The operator needs merely to guide the welding gun along the joint at a uniform speed and hold a relatively constant arc length. In automatic welding, the gun is mounted on a travel carriage that moves along the joint, or the gun may be stationary with the work moving or revolving beneath it. Practically all GMAW is done using DCEP (Electrode positive). This polarity provides deep penetration, a stable arc and low spatter levels. A small amount of GMAW welding is done with DCEN and although the melting rate of the electrode is high, the arc is erratic. Alternating current is not used for gas metal arc welding. Current Density - To understand why gas metal arc welding can deposit weld metal at a rapid rate, it is necessary that the term "current density" be understood. Figure shows a 1/4" coated electrode and a 1/16" solid wire drawn to scale. Both are capable of carrying 400 amperes. Notice that the area of the 1/16" wire is only 1/16 that of the core wire of the coated electrode. We can say that the current density of the 1/16" wire is 16 times. EQUIPMENT AND OPERATION - The equipment used for gas metal arc welding is more complicated than that required for shielded metal arc welding. Initial cost is relatively high, but the cost is rapidly amortized due to the savings in labor and overhead achieved by the rapid weld metal deposition. The equipment necessary for gas metal arc welding is listed below:

98 1) Power source 2) Wire feeder 3) Welding gun 4) Shielding gas supply 5) Solid electrode wire 6) Protective equipment The basic equipment necessary for semiautomatic gas metal arc welding is shown in Figure. SHIELDING GASES - In gas metal arc welding, there are a variety of shielding gases that can be used, either alone or in combinations of varying degrees. The choice is dependent on the type of metal transfer employed, the type and thickness of metal. carbon dioxide (CO 2 ) is often used for short circuiting arc welding because of its low cost. Mixing argon in proportions of 50-75% with carbon dioxide will produce a smoother arc and reduce spatter levels The 75% Argon/25 CO 2 mixture is often chosen for short circuit welding of thin sections, whereas the combination works well on thicker section Electrodes - The solid electrodes used in GMAW are of high purity when they come from the mill. Their chemistry must be closely controlled and some types purposely contain high levels of deoxidizers for use with CO 2 shielding. The electrode manufacturer draws down the electrode to a finished diameter that, with GMAW, is usually quite small. Diameters from.030" thru 1/16" are common. Most steel GMAW electrodes are copper plated as a means of protecting the surface. The copper inhibits rusting, provides smooth feeding, and helps electrical conductivity. Advantages

99 Faster than TIG Deeper penetration Both thick & thin jobs possible Easy to mechanize No flux Disadvantages Complex Air drafts may disrupt the gas shielding Higher base metal cooling rates Not for outdoors Applications Welding tool steels & dies Manufacturing refrigerator parts Aircraft, civil, automotive industry Non ferrous metals & their alloys 2.7.4RESISTANCE WELDING In resistance welding the metal parts to be joined are heated by their resistance to the flow of an electrical current. Usually this is the only source of heat, but a few of the welding operations combine resistance heating with arc heating, and possibly with combustion of metal in the arc. The process applies to practically all metals and most combinations of pure metals and those alloys, which have only a limited plastic range, are welded by heating the parts to fusion (melting). Some alloys, however, may welded without fusion; instead, the parts are heated to a plastic state at which the applied pressure causes their crystalline structures to grow together. The welding of dissimilar metals may be accomplished by melting both metals frequently only the metal with the lower melting point is melted, and an alloy bond is formed at the surface of the un melted metal. In resistance welding processes no fluxes are employed, the filler metal is rarely used and the joints are usually of the lap type. The amount of heat generated in the workpiece depend on the following factors: (1) Magnitude of the current, (2) Resistance of the current conducting path, and Mathematically, H = IVt = I(IR)t = I 2 Rt Where H = heat generated in joules

100 I = current in Amp. R = resistance in ohms t = time of current flow in seconds Types of Resistance welding The major types of resistance welding are given as under: (1) Spot Welding (2) Seam Welding (3) Projection Welding (4) Resistance Butt Welding (5) Flash Butt Welding (6) Percussion Welding (7) High Frequency Resistance Welding (8) High Frequency Induction Welding Some of the above important welding processes are discussed as under, Spot Welding In this process overlapping sheets are joined by local fusion at one or more spots, by the concentration of current flowing between two electrodes. This is the most widely used resistance welding process. A typical resistance spot welding machine is shown in Fig It essentially consists of two electrodes, out of which one is fixed. The other electrode is fixed to a rocker arm (to provide mechanical advantage) for transmitting the mechanical force from a pneumatic cylinder. This is the simplest type of arrangement. The other possibility is that of a pneumatic or hydraulic cylinder being directly connected to the electrode without any rocker arm. For welding large assemblies such as car bodies, portable spot welding machines are used. Here the electrode holders and the pneumatic pressurizing system are present in the form of a portable assembly which is taken to the place, where the spot is to be made. The electric current, compressed air and the cooling water needed for the electrodes is supplied through cables and hoses from the main welding machine to the portable unit. In spot welding, a satisfactory weld is obtained when a proper current density is maintained. The current density depends on the contact area between the electrode and the work-piece. With the continuous use, if the tip becomes upset and- the contact area increases, the current density will be lowered and consequently the weld is obtained over a large area. This would not be able to melt the metal and hence there would be no proper fusion. A resistance welding schedule is the sequence of events that normally take place in each of the welds. The events are: 1. The squeeze time is the time required for the electrodes to align and clamp the two work-pieces together under them and provide the necessary electrical contact. 2. The weld time is the time of the current flow through the work-pieces till they are heated to the melting temperature.

101 3. The hold time is the time when the pressure is to be maintained on the molten metal without the electric current. During this time, the pieces are expected to be forged welded. 4. The off time is time during which, the pressure on the electrode is taken off so that the plates can be positioned for the next spot. Spot welding electrodes Spot welding electrodes are made of materials which have (1) Higher electrical and thermal resistivities, and (2) Sufficient strength to withstand high pressure at elevated temperatures. Copper base alloys such as copper beryllium and copper tungsten are commonly used materials for spot welding electrodes. For achieving the desired current density, It is important to have proper electrode shape for which three main types of spot welding electrodes are used which are pointed, domed and flat electrodes. Applications of Spot Welding (i) It has applications in automobile and aircraft industries (ii) The attachment of braces, brackets, pads or clips to formed sheet-metal parts such as cases, covers or trays is another application of spot welding. (iii) Spot welding of two 12.5 mm thick steel plates has been done satisfactorily as a replacement for riveting. (iv) Many assemblies of two or more sheet metal stampings that do not require gas tight or liquid tight joints can be more economically joined by spot welding than by mechanical methods. (v) Containers and boxes frequently are spot welded Resistance Seam Welding It is a continuous type of spot welding wherein spot welds overlap each other to the desired extent. In this process coalescence at the faying surfaces is produced by the heat obtained from the resistance to electric current (flow) through the work pieces held together under pressure by circular electrodes. The

102 resulting weld is a series of overlapping resistance-spots welds made progressively along a joint by rotating the circular electrodes. The principle of seam welding is shown in Fig (a) and resistance seam welding process set up is shown in Fig.. The seam welding is similar to spot welding, except that circular rolling electrodes are used to produce a continuous air-tight seam of overlapping welds. Overlapping continuous spot welds seams are produced by the rotating electrodes and a regularly interrupted current. Applications 1. It is used for making leak proof joints in fuel tanks of automobiles. 2. Except for copper and high copper alloys, most other metals can be seam welded. 3. It is also used for making flange welds for use in watertight tanks Resistance Projection Welding Fig. shows the projection welding. This process is a resistance welding process in which two or more than two spot welds are made simultaneously by making raised portions or projections on predetermined locations on one of the work piece. These projections act to localize the heat of the welding circuit. The pieces to be welded are held in position under pressure being maintained by electrodes. The projected contact spot for welding should be approximately equal to the weld metal thickness. The welding of a nut on the automotive chasis is an example of projection welding. Advantages and disadvantages of resistance welding Advantages Simple, low power requirements High speed & low cost Not hazardous, no extra material cost Disadvantages

103 Only butt joint is possible Molten metal expulsion i.e. flash needs to be removed Very rigid machine is required Applications Combinations of metals can be welded Production of shafts, gears & valves Production of cutting tools & their bodies Welding together the small forgings Friction Welding In this process, the heat for welding is obtained from mechanically induced sliding motion between rubbing surfaces of work-pieces as shown in Fig In friction welding, one part is firmly held while the other (usually cylindrical) is rotated under simultaneous application of axial pressure. As these parts are brought to rub against each other under pressure, they get heated due to friction. When the desired forging temperature is attained, the rotation is stopped and the axial pressure is increased to obtain forging action and hence welded joint. Most of the metals and their dissimilar combinations such as aluminium and titanium, copper and steel, aluminium and steel etc. can be welded using friction welding. Friction welding process Advantages Simple, low power requirements High speed & low cost

104 Not hazardous, no extra material cost Disadvantages Only butt joint is possible Molten metal expulsion i.e. flash needs to be removed Very rigid machine is required Applications Combinations of metals can be welded Production of shafts, gears & valves Production of cutting tools & their bodies Welding together the small forgings Thermite Welding Thermite welding (TW) (sometimes called thermit welding) is a process which joins metals by heating them with super heated liquid metal from a chemical reaction between a metal oxide and aluminum or other reducing agent, with or without the application of pressure. Filler metal is obtained from the liquid metal. The heat for welding is obtained from an exothermic reaction or chemical change between iron oxide and aluminum. This reaction is shown by the following formula: 8A1 + 3fe304 = 9Fe + 4A Heat The temperature resulting from this reaction is approximately 2482 C. The super heated steel is contained in a crucible located immediately above the weld joint. The exothermic reaction is relatively slow and requires 20 to 30 seconds, regardless of the amount of chemicals involved. The parts to be welded are aligned with a gap between them. The super heated steel runs into a mold which is built around the parts to be welded. Since it is almost twice as hot as the melting temperature of the base metal, melting occurs at the edges of the joint and alloys with the molten steel from the crucible.normal heat losses cause the mass of molten metal to solidify, coalescence occurs, and the weld is completed. If the parts to be welded are large, preheating within the mold cavity may be necessary to bring the pats to welding temperature and to dry out the mold. If the parts are small, preheating is often eliminated. The thermit welding process is applied only in the automatic mode. Once the reaction is started, it continues until completion. Themite welding utilizes gravity, which causes the molten metal to fill the cavity between the parts being welded. It is very similar to the foundry practice of pouring a casting. The difference is the extremely high temperature of the molten metal. The making of a thermit weld is shown in figure. When the filler metal has cooled, all unwanted excess metal may be removed by oxygen cutting, machining, or grinding. The surface of the completed weld is usually sufficiently smooth and contoured so that it does not require additional metal finishing.

105 Thermite Welding Equipment (Tw) Thermite material is a mechanical mixture of metallic aluminum and processed iron oxide. Molten steel is produced by the thermite reaction in a magnesite-lined crucible. At the bottom of the crucible, a magnesite stone is burned, into which a magnesite stone thimble is fitted. This thimble provides a passage through which the molten steel is discharged into the mold. The hole through the thimble is plugged with a tapping pin, which is covered with a fire-resistant washer and refractory sand. The crucible is charged by placing the correct quantity of thoroughly mixed thermit material in it. In preparing the joint for thermite welding, the parts to be welded must be cleaned, alined, and held firmly in place. If necessary, metal is removed from the joint to permit a free flow of the thermite metal into the joint. A wax pattern is then made around the joint in the size and shape of the intended weld. A mold made of refractory sand is built around the wax pattern and joint to hold the molten metal after it is poured. The sand mold is then heated to melt out the wax and dry the mold. The mold should be properly vented to permit the escape of gases and to allow the proper distribution of the thermite metal at the joint. A thermite welding crucible and mold is shown in figure.

106 Advantages No costly power supply, on site repairs/welding is possible Disadvantages Economical for heavier sections & that too for ferrous metals only. Applications Rail-road repairs Repairing or welding of large crankshafts, machine frames Welding for cast pieces together For replacing broken teeth on large gears Welding Defects 1. Introduction Common weld defects include:

107 i. Lack of fusion ii. Lack of penetration or excess penetration iii. Porosity iv. Inclusions v. Cracking vi. Undercut vii. Lamellar tearing Any of these defects are potentially disastorous as they can all give rise to high stress intensities which may result in sudden unexpected failure below the design load or in the case of cyclic loading, failure after fewer load cycles than predicted. 2. Types of Defects i and ii. - To achieve a good quality join it is essential that the fusion zone extends the full thickness of the sheets being joined. Thin sheet material can be joined with a single pass and a clean square edge will be a satisfactory basis for a join. However thicker material will normally need edges cut at a V angle and may need several passes to fill the V with weld metal. Where both sides are accessible one or more passes may be made along the reverse side to ensure the joint extends the full thickness of the metal. Lack of fusion results from too little heat input and / or too rapid traverse of the welding torch (gas or electric). Excess penetration arises from to high a heat input and / or too slow transverse of the welding torch (gas or electric). Excess penetration - burning through - is more of a problem with thin sheet as a higher level of skill is needed to balance heat input and torch traverse when welding thin metal. ii. Porosity - This occurs when gases are trapped in the solidifying weld metal. These may arise from damp consumables or metal or, from dirt, particularly oil or grease, on the metal in the vicinity of the weld. This can be avoided by ensuring all consumables are stored in dry conditions and work is carefully cleaned and degreased prior to welding. iv. Inclusions - These can occur when several runs are made along a V join when joining thick plate using flux cored or flux coated rods and the slag covering a run is not totally removed after every run before the following run. v. Cracking - This can occur due just to thermal shrinkage or due to a combination of strain accompanying phase change and thermal shrinkage. In the case of welded stiff frames, a combination of poor design and inappropriate procedure may result in high residual stresses and cracking. Where alloy steels or steels with a carbon content greater than about 0.2% are being welded, self cooling may be rapid enough to cause some (brittle) martensite to form. This will easily develop cracks. To prevent these problems a process of pre-heating in stages may be needed and after welding a slow controlled post cooling in stages will be required. This can greatly increase the cost of welded joins, but for high strength steels, such as those used in petrochemical plant and piping, there may well be no alternative.

108 Solidification Cracking This is also called centreline or hot cracking. They are called hot cracks because they occur immediately after welds are completed and sometimes while the welds are being made. These defects, which are often caused by sulphur and phosphorus, are more likely to occur in higher carbon steels. Solidification cracks are normally distinguishable from other types of cracks by the following features: they occur only in the weld metal - although the parent metal is almost always the source of the low melting point contaminants associated with the cracking they normally appear in straight lines along the centreline of the weld bead, but may occasionally appear as transverse cracking solidification cracks in the final crater may have a branching appearance as the cracks are 'open' they are visible to the naked eye A schematic diagram of a centreline crack is shown below: On breaking open the weld the crack surface may have a blue appearance, showing the cracks formed while the metal was still hot. The cracks form at the solidification boundaries and are characteristically inter dendritic. There may be evidence of segregation associated with the solidification boundary. The main cause of solidification cracking is that the weld bead in the final stage of solidification has insufficient strength to withstand the contraction stresses generated as the weld pool solidifies. Factors which increase the risk include: insufficient weld bead size or inappropriate shape welding under excessive restraint material properties - such as a high impurity content or a relatively large shrinkage on solidification Joint design can have an influence on the level of residual stresses. Large gaps between conponents will increase the strain on the solidifying weld metal, especially if the depth of penetration is small. Hence weld beads with a small depth to width ratio, such as is formed when bridging a large wide gap with a thin bead, will be more susceptible to solidification cracking. In steels, cracking is associated with impurities, particularly sulphur and phosphorus and is promoted by carbon, whereas manganese and sulphur can help to reduce the risk. To minimise the risk of cracking, fillers with low carbon and impurity levels and a relatively high manganese content are preferred. As a general rule, for carbon manganese steels, the total sulphur and phosphorus content should be no greater than 0.06%. However when welding a highly restrained joint using high strength steels, a combined level below 0.03% might be needed. Weld metal composition is dominated by the filler and as this is usually cleaner than the metal being welded, cracking is less likely with low dilution processes such as MMA and MIG. Parent metal composition becomes more important with autogenous welding techniques, such as TIG with no filler.

109 Avoiding Solidification Cracking Apart from choice of material and filler, the main techniques for avoiding solidification cracking are: control the joint fit up to reduce the gaps clean off all contaminants before welding ensure that the welding sequence will not lead to a buildup of thermally induced stresses choose welding parameters to produce a weld bead with adequate depth to width ratio or with sufficient throat thickness (fillet weld) to ensure the bead has sufficient resistance to solidificatiuon stresses. Recommended minimum depth to width ratio is 0.5:1 avoid producing too large a depth to width ratio which will encourage segregation and excessive transverse strains. As a rule, weld beads with a depth to width ratio exceeds 2:1 will be prone to solidification cracking avoid high welding speeds (at high current levels) which increase segregation and stress levels accross the weld bead at the run stop, ensure adequate filling of the crater to avoid an unfavourable concave shape Hydrogen induced cracking (HIC) - also referred to as hydrogen cracking or hydrogen assisted cracking, can occur in steels during manufacture, during fabrication or during service. When HIC occurs as a result of welding, the cracks are in the heat affected zone (HAZ) or in the weld metal itself. Four requirements for HIC to occur are: a) Hydrogen be present, this may come from moisture in any flux or from other sources. It is absorbed by the weld pool and diffuses int o the HAZ. b) A HAZ microstructure susceptible to hydrogen cracking. c) Tensile stresses act on the weld d) The assembly has cooled to close to ambient - less than 150 o C HIC in the HAZ is often at the weld toe, but can be under the weld bead or at the weld root. In fillet welds cracks are normally parallel to the weld run but in butt welds cracks can be transverse to the welding direction. vi Undercutting - In this case the thickness of one (or both) of the sheets is reduced at the toe of the weld. This is due to incorrect settings / procedure. There is already a stress concentration at the toe of the weld and any undercut will reduce the strength of the join. vii Lamellar tearing - This is mainly a problem with low quality steels. It occurs in plate that has a low ductility in the through thickness direction, which is caused by non metallic inclusions, such as suphides and oxides that have been elongated during the rolling process. These inclusions mean that the plate can not tolerate the contraction stresses in the short transverse direction. Lamellar tearing can occur in both fillet and butt welds, but the most vulnerable joints are 'T' and corner joints, where the fusion boundary is parallel to the rolling plane. These problem can be overcome by using better quality steel, 'buttering' the weld area with a ductile material and possibly by redesigning the joint. 3. Detection Visual Inspection

110 Prior to any welding, the materials should be visually inspected to see that they are clean, aligned correctly, machine settings, filler selection checked, etc. As a first stage of inspection of all completed welds, visual inspected under good lighting should be carried out. A magnifying glass and straight edge may be used as a part of this process. Undercutting can be detected with the naked eye and (provided there is access to the reverse side) excess penetration can often be visually detected. Liquid Penetrant Inspection Serious cases of surface cracking can be detected by the naked eye but for most cases some type of aid is needed and the use of dye penetrant methods are quite efficient when used by a trained operator. This procedure is as follows: Clean the surface of the weld and the weld vicinity Spray the surface with a liquid dye that has good penetrating properties Carefully wipe all the die off the surface Spray the surface with a white powder Any cracks will have trapped some die which will weep out and discolour the white coating and be clearly visible X - Ray Inspection Sub-surface cracks and inclusions can be detected 'X' ray examination. This is expensive, but for safety critical joints - eg in submarines and nuclear power plants - 100% 'X' ray examination of welded joints will normally be carried out. Ultrasonic Inspection Surface and sub-surface defects can also be detected by ultrasonic inspection. This involves directing a high frequency sound beam through the base metal and weld on a predictable path. When the beam strikes a discontinuity some of it is reflected beck. This reflected beam is received and amplified and processed and from the time delay, the location of a flaw estimated. Porosity, however, in the form of numerous gas bubbles causes a lot of low amplitude reflections which are difficult to separate from the background noise. Results from any ultrasonic inspection require skilled interpretation. Magnetic Particle Inspection This process can be used to detect surface and slightly sub-surface cracks in ferro-magnetic materials (it can not therefore be used with austenitic stainless steels). The process involves placing a probe on each side of the area to be inspected and passing a high current between them. This produces a magnetic flux at right angles to the flow of the current. When these lines of force meet a discontinuity, such as a longitudinal crack, they are diverted and leak through the surface, creating magnetic poles or points of attraction. A magnetic powder dusted onto the surface will cling to the leakage area more than elsewhere, indicating the location of any discontinuities. This process may be carried out wet or dry, the wet process is more sensitive as finer particles may be used which can detect very small defects. Fluorescent powders can also be used to enhance sensitivity when used in conjunction with ultra violet illumination.

111 Welding Defects #1: Incomplete Penetration Incomplete penetration happens when your filler metal and base metal aren t joined properly, and the result is a gap or a crack of some sort. Check out the Figure below for an example of incomplete penetration. Welds that suffer from incomplete penetration are weak at best, and they ll likely fail if you apply much force to them. (Put simply, welds with incomplete penetration are basically useless.) Here s a list of the most common causes of incomplete penetration welding defects. The groove you re welding is too narrow, and the filler metal doesn t reach the bottom of the joint. You ve left too much space between the pieces you re welding, so they don t melt together on the first pass. You re welding a joint with a V-shaped groove and the angle of the groove is too small (less than 60 to 70 degrees), such that you can t manipulate your electrode at the bottom of the joint to complete the weld. Your electrode is too large for the metals you re welding. Your speed of travel(how quickly you move the bead) is too fast, so not enough metal is deposited in the joint. Your welding amperage is too low.if you don t have enough electricity going to the electrode, the current won t be strong enough to melt the metal properly Welding Defects #2: Incomplete Fusion Incomplete fusion occurs when individual weld beads don t fuse together, or when the weld beads don t fuse properly to the base metal you re welding, such as in below.

112 The most common type of incomplete fusion is called overlap and usually occurs at the toe(on the very top or very bottom of the side) of a weld. One of the top causes is an incorrect weld angle, which means you re probably holding the electrode and/or your filler rods at the wrong angle while you re making a weld; if you think that s the case, tweak the angle a little at a time until your overlap problem disappears. Here are a few more usual suspects when it comes to incomplete fusion causes. Your electrode is too small for the thickness of the metal you re welding. You re using the wrong electrode for the material that you re welding. Your speed of travel is too fast. Your arc length is too short. Your welding amperage is set too low. If you think your incomplete fusion may be because of a low welding amperage, crank up the machine! But be careful: You really need only enough amperage to melt the base metal and ensure a good weld. Anything more is unnecessary and can be dangerous. Contaminants or impurities on the surface of the parent metal(the metal you re welding) prevent the molten metal (from the filler rod or elsewhere on the parent metal) from fusing. Welding Defects #3: Undercutting Undercutting is an extremely common welding defect. It happens when your base metal is burned away at one of the toes of a weld. To see what I mean, look at Figure. When you weld more than one pass on a joint, undercutting can occur between the passes because the molten weld is already hot and takes less heat to fill, yet you re using the same heat as if it were cold. It s actually a very serious defect that can ruin the quality of a weld, especially when more than 1 32 inch is burned away. If you do a pass and notice some undercutting, you must remove it before you make your next pass or you risk trapping slag (waste material see the following section) into the welded joint (which is bad news). The only good thing about undercutting is that it s extremely easy to spot after you know what you re looking for. Here are a few common causes of undercutting: Your electrode is too large for the base metal you re welding. Your arc is too long. You have your amperage set too high. You re moving your electrode around too much while you re welding. Weaving your electrode back and forth is okay and even beneficial, but if

113 you do it too much, you re buying a one-way ticket to Undercutting City (which is of course the county seat for Lousy Weld County). Welding Defects #4: Slag Inclusions A little bit of slag goes a long way... toward ruining an otherwise quality weld. Slagis the waste material created when you re welding, and bits of this solid material can become incorporated (accidentally) into your weld, as in Figure. Bits of flux, rust, and even tungsten can be counted as slag and can cause contamination in your welds. Common causes of slag inclusions include Flux from the stick welding electrode that comes off and ends up in the weld Failure to clean a welding pass before applying the next pass Be sure to clean your welds before you go back in and apply a second weld bead. Slag running ahead of your weld puddle when you re welding a V-shaped groove that s too tight Incorrect welding angle Welding amperage that s too low Welding Defects #5 Flux Inclusions If you re soldering or brazing (also called braze welding), flux inclusions can be a real problem. If you use too much flux in an effort to float out impurities from your weld, you may very well end up with flux inclusions like those in Figure. (Head to Chapter 13 for more on brazing and soldering.) If you re working on a multilayer braze weld, flux inclusion can occur when you fail to remove the slag or glass on the surface of the braze before you apply the next layer. When you re soldering, flux inclusion can be a problem if you re not using enough heat. These inclusions are usually closely spaced, and they can cause a soldered joint to leak. If you want to avoid flux inclusions (and believe me, you do), make sure you do the following: Clean your weld joints properly after each pass.this task is especially

114 important when you re brazing. Don t go overboard with your use of flux. Make sure you re using enough heat to melt the filler or flux material. Welding Defects #6: Porosity If you read very much of this book, you quickly figure out that porosity(tiny holes in the weld) can be a serious problem in your welds (especially stick or mig welds). Your molten puddle releases gases like hydrogen and carbon dioxide as the puddle cools; if the little pockets of gas don t reach the surface before the metal solidifies, they become incorporated in the weld, and nothing can weaken a weld joint quite like gas pockets. Take a gander at Figure for an example of porosity. Following are a few simple steps you can take to reduce porosity in your welds: Make sure all your materials are clean before you begin welding. Work on proper manipulation of your electrode. Try using low-hydrogen electrodes. Welding Defects #7: Cracks Cracks can occur just about everywhere in a weld: in the weld metal, the plate next to the weld metal, or in any other piece affected by the intense heat of welding. Check out the example of cracking in Figure. Here are the three major types of cracks, what causes them, and how you can prevent them. Hot cracks: This type of crack occurs during welding or shortly after you ve deposited a weld, and its cause is simple: The metal gets hot too quickly or cools down too quickly. If you re having problems with hot cracking, try preheating your material. You can also postheat your material, which means that you apply a little heat here and there after you ve finished welding in an effort to let the metal cool down more gradually. Cold cracks: This type of crack happens well after a weld is completed and the metal has cooled off. (It can even happen days or weeks after a weld.) It generally happens only in steel, and it s caused by deformities in the structure of the steel.

115 You can guard against cold cracking by increasing the thickness of your first welding pass when starting a new weld. Making sure you re manipulating your electrode properly, as well as pre- and postheating your metal, can also help thwart cold cracking. Crater cracks: These little devils usually occur at the ending point of a weld, when you ve stopped welding before using up the rest of an electrode. The really annoying part about crater cracks is that they can cause other cracks, and the cracking can just kind of snowball from there. You can control the problem by making sure you re using the appropriate amount of amperage and heat for each project, slowing your speed of travel, and pre- and postheating. Welding Defects #9: Warpage If you don t properly control the expansion and contraction of the metals you work with, warpage(an unwanted distortion in a piece of metal s shape) can be the ugly result. Check out an example in Figure. If you weld a piece of metal over and over, the chances of it warping are much higher. You can also cause a piece of metal to warp if you clamp the joints too tightly. (If you allow the pieces of metal that make the joint to move a little, there s less stress on them.) Say you re welding a Tjoint. The vertical part of the Tsometimes pulls itself toward the weld joint. To account for that movement, simply tilt the vertical part out a little before you weld, so that when it tries to pull toward the weld joint, it pulls itself into a nice 90-degree angle! The more heat you use, the more likely you are to end up with warpage, so be sure to use only the amount of heat you need. Don t overdo it. Opting for a slower speed of travel while welding can also help to cut down on warpage. Welding Defects #10: Spatter Spatter(small particles of metal that attach themselves to the surface of the material you re working on.) is a fact of life with most kinds of welding; no matter how hard you try, you ll never be able to cut it out completely. You can see it in all its glory in Figure 11-5 in Chapter 11. You can keep spatter to a minimum by spraying with an anti-spatter compound (available at your welding supply store) or by scraping the spatter off the parent metal surface. Not all weld discontinuities are weld defects, but all weld defects are discontinuities. Understanding the difference will let you know if you need to scrap a part, repair it or simply add more weld. There are many codes depending on what type of product you are welding on. The codes are used as guidelines by manufactures to write their own specifications. Just because a certain Code allows for a certain amount of porosity, a manufacturer may not allow its suppliers to have any.

116 Porosity is one of many weld discontinuities that we must avoid. The list of weld discontinuities below can all be detected visually. Visual inspection is the easiest and least expensive of all non-destructive inspection methods. The tools necessary to carry out a visual inspection are few and not expensive. Tools such as rulers, weld gages and magnifying glass are pretty much all you need. It is key that weld inspection takes place before, during and after welding. A weld discontinuity is a flaw in the weld. Discontinuities, as stated above, are not necessarily weld defects. They become weld defects when they exceed specified maximums of the code or customer specifications. This means that you can have weld discontinuities and still have an acceptable weld. However, we always want welds free of discontinuities whenever possible. Here are the types of weld discontinuities: 1. Porosity Cavities in the weld caused by trapped gas during solidification of the weld metal. Common causes are lack of shielding gas, excessive arc length, or dirty base material. Another no so common cause can be arc blow, where magnetic fields cause an erratic arc. By codes or manufacturer specs porosity may be present but each individual hole should not exceed a certain length and the total length of all holes cannot exceed a certain value per inch of weld. 2. Lack of Fusion Also called lack of penetration or cold lap. Lack of fusion refers to the base material not being fused properly to the other piece or weld metal itself. This is caused by having welding parameters that are too low. Although this cannot be detected, at least not easily, after welding, the welder himself can see this while welding. A trained welder is able to recognize if the arc is digging properly into the base material. It is difficult, but not impossible, to detect lack of fusion while welding. 3. Undercut This is a grove that appears at one of both toes of the weld. This is caused by lack of fill. The lack of fill can be due to excessive voltage or too low wire feed speed. Can also be caused by incorrect welding technique. As with porosity, some undercut may be acceptable. 4. Incorrect Bead Placement As the name states, this discontinuity occurs when the weld bead is not in the right location. It can mean missing the joint completely or not having equal legs in a fillet weld. Incorrect bead placement can be a weld defect if the root is missed or if the smallest leg size does not meet the specification minimum.

117 Undercut is a groove that is melted into the base material at the toe or toes of a weld. 5. Spatter This are small drops of weld metal that escape the arc and land on the adjacent base material fusing themselves to it. Spatter is not a weld defect, but again the maximum allowable is per the customer s specification. Spatter is caused by incorrect welding procedures, including amps, volts, welding speed, travel and work angles, and even shielding gas. Spatter does not decrease weld strength but it may create clearance issues and it looks awful. 6. Incorrect Weld Size This can be either a weld that is too big or too small. Although big welds are preferred over small welds it is still detrimental at times to have a big weld due to excessive heat input, weld stresses and distortion. Weld size is affected by travel speed and welding procedures, specifically wire feed speed. It can be easily measured by the use of weld gages. 7. Slag Inclusions This consists of slag trapped between passes. This is impossible to detect via weld inspection after welding is complete and very hard to detect while welding. Causes are inadequate cleaning of weld surface between passes. It can also occur in single pass welds when slag gets trapped in the root and toes of the weld. 8. Excessive Reinforcement This is a weld that is too big or has too much convexity (too much build up). Usually caused by low travel speeds or incorrect procedures. Excessive reinforcement does not add strength to the weld. 9. Melt Through This occurs when welding procedures and/or technique provide too much penetration and metal comes out of the back of the joint. It will be welding specifications that determine whether any of the above are acceptable and to what degree. However, keep an eye out specially for lack of fusion, slag inclusions and incorrect bead placement. Even in small amounts these have the potential for weld failure.

118 MODULE-III

119 3.1 INTRODUCTION Powder metallurgy is a process of making components from metallic powders. Initially, it was used to replace castings for metals which were difficult to melt because of high melting point. The development of technique made it possible to produce a product economically, and today it occupies an important place in the field of metal process. The numbers of material products made by powder metallurgy is increasing and include tungsten filaments of lamps, contact points. Self lubricating bearings and cemented carbides for cutting tools. 3.2 CHARACTERISTICS OF METAL POWDER The performance of metal powders during processing and the properties of powder metallurgy are dependent upon the characteristics of the metal powders that are used. Following are the important characteristics of metal powders. (a) Particle shape (b) Particle size (c) Particle size distribution(d) Flow rate (e) Compressibility (f) Apparent density (g) Purity (a) Particle Shape: The particle shape depends largely on the method of powder manufacture. The shape may be special nodular, irregular, angular, and dendrite. The particle shape influences the flow characteristics of powders. Special particles have excellent sintering properties. However, irregular shaped particles are good at green strength because they will interlock on computing. (b) Particle Size: The particle size influences the control of porosity, compressibility and amount of shrinkage. It is determined by passing the powder through standard sieves or by microscopic measurement. (c) Particle Size Distribution: It is specified in term of a sieve analysis, the amount of powder passing through 100, 200 etc., mess sieves. Particle size distribution influences the packing of powder and its behaviour during moulding and sintering. (d) Flow Rate: It is the ability of powder to flow readily and confirm to the mould cavity. It determines the rate of production and economy. (e) Compressibility: It is defined as volume of initial powder (powder loosely filled in cavity) to the volume of compact part. It depends on particle size, distribution and shape. (f) Apparent Density: It depends on particle size and is defined as the ratio of volume to weight of loosely filled mixture. (g) Purity: Metal powders should be free from impurities as the impurities reduces the life of dies and effect sintering process. The oxides and the gaseous impurities can be removed from the part during sintering by use of reducing atmosphere. 3.3 BASIC STEPS OF THE PROCESS The manufacturing of parts by powder metallurgy process involves the following steps: (a) Manufacturing of metal powders (b) Blending and mixing of powders (c) Compacting

120 (d) Sintering (e) Finishing operations (a) Manufacturing of Metal Powders There are various methods available for the production of powders, depending upon the type and nature of metal. Some of the important processes are: 1. Atomization 2. Machining 3. Crushing and Milling 4. Reduction 5. Electrolytic Deposition 6. Shotting 7. Condensation 1. Automization: In this method as shown in Fig. 4.1 (a), molten metal is forced through a small orifice and is disintegrated by a powerful jet of compressed air, inert gas or water jet. These small particles are then allowed to solidify. These are generally spherical in shape. Automation is used mostly for low melting point metals/alloy such as brass, bronze, zinc, tin, lead and aluminium powders. 2. Machining: In this method first chips are produced by filing, turning etc. and subsequently pulverised by crushing and milling. The powders produced by this method are coarse in size and irregular in shape. Hence, this method is used for special cases such as production of magnesium powder. 3. Crushing and Milling: These methods are used for brittle materials. Jaw crushers, stamping mills, ball mills are used to breakdown the metals by crushing and impact. See Fig. 4.1 (b) and (c)

121 In earlier stages of powder preparation gyratory crushers (Fig 4.1(b)) are used to crush brittle metals. For fine powder, the metal particles are fractured by impact. A ball mill (Fig. 4.1 (c)) is a horizontal barret shaped container holding a quantity of balls which are free to tumble about as the container rotates, crushes and abrade the powder particles that are introduced into the container. 4. Reduction: Pure metal is obtained by reducing its oxide with a suitable reducing gas at an elevated temperature (below the melting point) in a controlled furnace. The reduced product is then crushed and milled to a powder. Sponge iron powder is produced this way Fe 3 O4 + 4C = 3Fe + 4CO Fe 3 O 4 + 4CO = 3Fe + 4CO 2 Copper powder by Cu 2 O + H 2 = 2Cu + H 2 O Tungsten, Molybdenum, Ni and Cobalt are made by the method. 5. Electrolytic Deposition: This method is commonly used for producing iron and copper powders. This process is similar to electroplating. For making copper powder, copper plates are placed as

122 anodes in the tank of electrolyte, where as the aluminium plates are placed into electrolyte to act as anode. When D. C. current is passed through the electrolyte, the copper gets deposited on cathode. The cathode plates are taken out from electrolyte tank and the deposited powder is scrapped off. The powder is washed, dried and pulverised to produce powder of the desired grain size. The powder is further subjected to heat treatment to remove work hardness effect. The cost of manufacturing is high. 6. Shotting: In this method, the molten metal is poured through a siever or orifice and is cooled by droping into water. This produces spherical particles of large size. This method is commonly used for metals of law melting points. 7. Condensation: In this method, metals are boiled to produce metal vapours and then condensed to obtain metal powders, This process is applied to volatile metals such as zinc, magnesium and cadmium. (b) Blending and Mixing of Powders Powder blending and mixing of the powders are essential for uniformity of the product. Lubricants are added to the blending of powders before mixing. The function of lubricant is to minimise the wear, to reduce friction. Different powder in correct proportions are thoroughly mixed either wet or in a ball mill. (c) Compacting The main purpose of compacting is converting loose powder into a green compact of accurate shape and size. The following methods are adopted for compacting: 1. Pressing 2. Centrifugal compacting 3. Slip casting 4. Extrusion 5. Gravity sintering 6. Rolling 7. Isostatic moulding 8. Explosive moulding 1. Pressing: The metal powders are placed in a die cavity and compressed to form a component shaped to the contour of the die as illustrated in Fig The pressure used for producing green compact of the component vary from 80 Mpa to 1400 Mpa, depending upon the material and the characteristics of the powder used. Mechanical presses are used for compacting objects at low pressure. Hydraulic presses are for compacting objects at high pressure. (See Fig. 4.2)

123 Fig. 4.2 Steps in Pressing Operations 2. Centrifugal Compacting: In this method, the moulder after it is filled with powder is centrifugal to get a compact of high and uniform density at a pressure of 3 Mpa. This method is employed for heavy metals such as tungsten carbide. 3. Slip Casting: In this method, the powder is converted into slurry with water and poured into the mould made of plaster of paris. The liquid in the slurry is gradually absorbed by the mould leaving the solid compact within the mould. The mould may be vibrated to increase the density of the compact. This technique is used for materials that are relatively incompressible by conventional die compaction. The main drawback of this process is relatively slower process because it takes larger time for the fluid to be absorbed by the method. 4. Extrusion: This method is employed to produce the components with high density. Both cold and hot extrusion processes are for compacting specific materials. In cold extrusion, the metal powder is mixed with binder and this mixture is compressed into billet. The binder is removed before or during sintering. The billet is charged into a container and then forced through the die by means of ram. The cross-section of product depends on the opening of the die. Cold extrusion process is used for cemented carbide drills and cutters of ram. The cross-section of products depends on the opening of the die. Cold extrusion process is used for cemented carbide drills and cutters. In the hot extrusion, the powder is compacted into billet and is heated to extruding temperature in nonoxidising atmosphere. The billet is placed in the container and extruded through a die. This method is used for refractive berium and nuclear solid materials. 5. Gravity Sintering: This process is used for making sheets for controlled porosity. In this process. the powder is poured on ceremic tray to form an uniform layer and is then sintered up to 48 hours in ammonia gas at high temperature. The sheets are then rolled to desired thickness. Porous sheet of stainless steel are made by this process and popularly used for fitters. 6. Rolling: This method is used for making continuous strips and rods having controlled porosity with uniform mechanical properties. In this method, the metal powder is fed between two rolls which compress and interlock the powder particles to form a sheet of sufficient strength as shown in Fig It then situated, rerolled and heat treated if necessary. The metals that can be rolled are Cu, Brass, Bronze, Ni, Stainless steel and Monel. 7. Isostatic Moulding: In this method, metal powder is placed in an elastic mould which is subjected to gas pressure in the range of Mpa from all sides. After pressing. the compact is removed from

124 gas chamber. If the fluid is used as press medium then it is called as hydrostatic pressing. The advantages of this method are: uniform strength in all directions, higher green compact strength and low equipment cost. This method is used for tungsten, molybdenum, niobium etc. 8. Explosive Compacting: In this method, the pressure generated by an explosive is used to compact the metal powder. Metal powder is placed in water proof bags which are immersed in water container cylinder of high wall thickness. Due to sudden deterioration of the charge at the end of the cylinder, the pressure of the cylinder increase. This pressure is used to press the metal powder to form green compact. (d) Sintering Sintering involves heating of the green compact at high temperatures in a controlled atmosphere [reducing atmosphere which protects oxidation of metal powders]. Sintering increases the bond between the particles and therefore strengthens the powder metal compact. Sintering temperature and time is usually 0.6 to 0.8 times the melting point of the powder. In case of mixed powders of different melting temperature, the sintering temperature will usually be above the melting point of one of the minor constituent [Ex : cobalt and cemented carbides] and other powders remain in soild state. The important factors governing sintering are temperature, time and atmosphere. (e) Finishing Operations These are secondary operations intended to provide dimensional tolerances, physical and better surface finish. They are: 1. Sizing 5. Infiltration 2. Coining 6. Heat treatment 3. Machining 7. Plating 4. Impregnation 1. Sizing: It is repressing the sintered component in the die to achieve the required accuracy.

125 2. Coining: It is repressing the sintered components in the die to increase density and to give additional strength. 3. Machining: Machining operation is carried out on sintered part to provide under cuts, holes, threads etc. which can not be removed on the part in the powder metallurgy process. 4. Impregnation: It is filling of oil, grease or other lubricants in a sintered component such as bearing. 5. Infiltration: It is filling of pores of sintered product with molten metal to improve physical properties. 6. Heat Treatment: The process of heating and cooling sintered parts are to improve (i) Wear Resistance (ii) Grain Structure (iii) Strength The following heat treatment process are used to the parts made by powder metallurgy: 1. Stress relieving 2. Carburising 3. Nitriding 4. Induction Hardening 7. Plating: Plating is carried out in order to: 1. Import a pleasing appearance (Cr plating) 2. Protect from corrosion (Ni plating) 3. Improve electrical conductivity (Cu and Ag plating) 3.4 DESIGN CONSIDERATIONS FOR POWDER METALLURGY PARTS In designing of powder metallurgy parts, the following are the some of tooling and pressing considerations. 1. Side holes and side ways are very difficult to achieve during pressing and must be made by secondary machining operations. 2. Threads, kurling and other similar shapes should not be formed compacting. They should be produced by machining. 3. Abrupt changes in section thickness and narrow and deeper sections should be avoided as far as practicable. 4. It is recommended that sharp corners be avoided wherever possible. Fillets with generous radii are desirable. 5. Chambers can be made.

126 6. Under cuts that are perpendicular to the pressing direction cannot be made, since they prevent the part ejection. 3.5 ADVANTAGES OF POWDER METALLURGY 1. Although the cost of making powder is high there is no loss of material. The components produced are clean, bright and ready for use. 2. The greatest advantage of this process is the control of the composition of the product. 3. Components can be produced with good surface finish and close tolerance. 4. High production rates. 5. Complex shapes can be produced. 6. Wide range of properties such as density, porosity and particle size can be obtained for particular applications. 7. There is usually no need for subsequent machining or finishing operations. 8. This process facilitates mixing of both metallic and non-metallic powders to give products of special characteristics. 9. Porous parts can be produced that could not be made any other way. 10. Impossible parts (cutting tool bits) can be produced. 11. Highly qualified or skilled labour is not required. 3.6 LIMITATION OF POWDER METALLURGY 1. The metal powders and the equipment used are very costly. 2. Storing of powders offer great difficulties because of possibility of fire and explosion hazards. 3. Parts manufactured by this process have poor ductility. 4. Sintering of low melting point powders like lead, zinc, tin etc., offer serious difficulties. 3.7 APPLICATIONS OF POWDER METALLURGY Powder metallurgy techniques are used for making large number of components. Some of the application are as follows: 1. Self-Lubricating Bearing and Filters: Porous bronze bearings are made by mixing copper and tin powder in correct proportions, cold pressed to the desired shape and then sintered. These bearings soak up considerable quantity of oil. Hence during service, these bearings produce a constant supply of lubricant to the surface due to capillary action. These are used where lubricating is not possible. Porous filters can be manufactured and are used to remove, undesirable materials from liquids and gases. 2. Friction Materials: These are made by powder metallurgy. Clutch liners and Brake bands are the example of friction materials.

127 3. Gears and Pump Rotors: Gears and pump rotor for automobile oil pumps are manufactured by powder metallurgy. Iron powder is mixed with graphite, compacted under a pressure of 40 kg/ cm and sintered in an electric furnace with an atmosphere and hydrocarbon gas. These are impregnated with oil. 4. Refractor Materials: Metals with high melting points are termed as refractory metals. These basically include four metals tungsten, molybdenum, tantalum and niobium. Refractory metals as well as their alloys are manufactured by powder metallurgy. The application are not limited to lamp filaments and heating elements, they also include space technology and the heavy metal used in radioactive shielding. 5. Electrical Contacts and Electrodes: Electrical contacts and resistance welding electrodes are made by powder metallurgy. A combination of copper, silver and a refractory metal like tungsten. molybdenum and nickle provides the required characteristics like wear resistant, refractory and electrical conductivity. 6. Magnet Materials: Soft and permanent magnets are manufactured by this process. Soft magnets are made of iron, iron-silicon and iron-nickle alloys. These are used in D.C. motors, or generators as armatures and in measuring instruments. Permanent magnets known as Alnico which is a mixture of nickle, aluminium, cobalt, copper and iron are manufactured by this technique. 7. Cemented Carbides: These are very important products of powder metallurgy and find wide applications as cutting tools, wire drawing dies and deep drawing dies. These consist of carbides of tungsten, tantalum, titanium and molybdenum. The actual proportions of various carbides depend upon its applications, either cobalt or nickle is used as the bonding agent while sintering. Diamond Impregnated Tools These are made from a mixture of iron powder and diamond dust. Diamond dust acts as a cutting medium and iron powder acts as the bond. These tools are used for cutting porcelain and glass. These bits are welded or brazed to a steel shank. 4.1 Plastic deformation of metals: In metal forming processes, the product shapes are produced by plastic deformation. Hence it is important to know the plastic flow properties of metals and alloys for optimizing the processes. Also the resulting component properties depend upon the intensity and the conditions of plastic deformation during forming. Many forming processes produce raw materials for other processes which in turn produce finished or semi-finished products. For example, steel plants produce sheet metal which is used by automobile industry to manufacture components of automobiles and their bodies. In fact sheet metal is used by a number of manufacturers for producing a large variety of household and industrial products. Similarly billets produced by steel plants are used by re-rolling mills for rolling into products like angles, channels, bars etc. Bars may be further used for manufacturing forgings, wires, bright bars and machined products. Variables in metal forming and their optimization

128 Mechanical properties of the materials means to study the behaviours of the metal under the system of external forces. Elasticity and plasticity are the general mechanical properties considered during the mechanical working of metals. Elasticity- It is the properties of the material by virtue of which the material regains its original shape and size after removal of load. Plasticity- It is the properties of the material by virtue of which the material does not regains its original shape and size after removal of load. During loading if the elastic limit exceeded,the body experience the permanent deformation after removal of load 1.Elastic Behaviour- linear plot of stress vs strain. When stress is applied, strain is instantaneous; i.e., not time dependent. Furthermore, instantaneous recovery ensues upon removal of stress. Some rocks at shallow depths and for short periods of time, approach ideal elastic behaviour during small magnitudes of deformation. Seismic waves are an example of elastic behaviour. Recoverable or Reversible Plastic Behaviour- continuous deformation after some critical stress (σ c )value is achieved and maintained. Many rocks exhibit plastic behaviour

129 Permanent Strain Three Megascale Types of Deformation- Visible effects of strain in rocks are usually of plastic or rupture variety as elastic strain produces little long term features. a.elastic Deformation b.plastic Deformation c.rupture Deformation Elastic Deformation-Occurs when a body is deformed in response to a stress, but returns to its original shape when stress is removed. Stress is totally reversible or recoverable. Viscoelastic (Anelastic) Strain- strain totally recoverable but not instantaneous recovery; time dependent, describe in terms of strain rate. Most rocks have elastic and anelastic properties at small stress magnitudes. Plastic Deformation- Irreversible strain without visible fractures. Stress is applied to a rock body and deformation occurs. When stresses are removed, a portion of the strain remains. That portion of the rock that is deformed has experienced plastic strain. Permanent plastic deformation precludes visible fractures. Material deforms but does not break and produce visible fractures. Microscopic fracturing may occur, however. Plastic strain is not recoverable or reversible. Rupture Deformation- visible fractures form. Irreversible, not recoverable strain. Material loses cohesion. Terms describing Behaviour of Materials during Deformation: Ductile- Rocks experience large amounts of plastic deformation before rupturing. Plastic-flow without macroscopic brittle behaviour Brittle- Rocks that exhibit elastic behaviour followed by rupture. Rupture- loss of cohesion; occurs prior to significant amounts of plastic deformation. Elastic Limit- ductile rocks deform elastically to a point (stress value of which is the yield strength), beyond this point, plastic deformation ensues with increasing stress.

130 Rupture point- (rupture strength) brittle rocks experience elastic deformation until a rupture point is attained, whereat the rock deforms by brittle rupture. Failure- point when a brittle rock loses all resistance to stress and crumbles. Failure is difficult to discern in plastic deformation. Ultimate Strength- maximum stress that a rock can support before failure. Competency- relative term that compares the resistance of rocks to flow. Generalized Stress-Strain Curve for Rocks Brittle Rocks- exhibit elastic behaviour before rupture Ductile Rocks- exhibit elastic-plastic behaviour before rupture True Stress-True Strain The curves of Figs. 1.6 and 1.7 are drawn with the stress defined as load divided by original area of cross-section and the longitudinal strain is defined as δl/l0, where l0 is the original length of test specimen. The curves would look very much different if we use true stress on ordinate and true strain on abscissa. The true stress and true strain are defined below.

131 A typical stress-strain curve for mild steel fig 5 For accurate calculations, the true stress-true strain curve for the metal should be drawn to determine the yield strength. Figure 5 redrawn on true stress-true strain axes would look like the one shown in Fig. 6. Also there are standard specifications for the shape and dimensions of test specimen, which should be adapted in order to obtain meaningful results. Besides, in all above type of tests the following factors should also be noted. (i) Temperature at which the test is conducted. (ii) The strain rate during the test. (iii) Accuracy of load measuring instrument. (iv) Accuracy of instrument which measures elongation.

132 Fig. 6 True stress-true strain curve for tensile test 1.5 FACTORS THAT AFFECT THE YIELD STRENGTH In metal forming, particularly in hot forming many metallurgical processes may take place concurrently. These include strain hardening, recovery, re-crystallization, etc. All these factors affect the yield strength. Therefore, it is important to know the extent of effect of each of these factors. The yield strength of a metal or alloy is affected by following factors. (i) Strain hardening. (ii) Strain rate. (iii) Temperature of metal and microstructure. (iv) Hydrostatic pressure Strain Hardening To understand the effect of strain hardening let us again consider the tension test curve shown below in Fig. 7 In this figure the test piece is loaded beyond the yield point up to a point P. The test piece is then unloaded. The elastic deformation recovers via the unloading curve PR which is more or less parallel to AO. It is generally taken that there is no change in Young s modulus during plastic deformation. The line PR depicts elastic recovery. Out of the total strain OS corresponding to the point P, the part RS is the elastic recovery. The part OR which is not recovered is the plastic strain suffered by the test specimen. Now if we reload the same test piece, it nearly follows the line RP. There is, however, some deviation due to hysteresis which is very small, and the yielding now occurs at the point P. Further loading of the test piece beyond P gives the same stress-strain curve as we would have

133 obtained if there were no unloading. This shows that after suffering a plastic strain represented by OR, the yield strength of metal has increased from point B to point P (or σo1 to σo2). This is called strain hardening or work hardening. Fig. Strain hardening effect Dependence of stress-strain diagram on Strain rate Strain rate:- Strain rate in forming is directly related to speed of deformation v Deformation speed v = velocity of the ram or other movement of the equipment Strain rate is defined: ε=v/h Where ε = true strain rate; and h = instantaneous height of workpiece being deformed.

134 Strain rate/deformation velocity has the following major effects The flow stress of the material increases with strain rate. The temperture of workpiece is increased because of adiabetic heating. If there is improved lubrication at the tool metal interface then the strain rate also increased so long as the lubricant film can be maintain. Dependence of stress strain diagram on temperture When temperture s increased the bonds between the molecules are loosed and therefore the ductility increase due to more deformation takes place at the given stree level.yeild strength and tensile strength are reduced at the elevated temperture Elevated Temp Elevated σ y temp tensile strength strain rate strain rate Effect of temperature on flow stress for a typical metal. 4.3 HOT WORKING AND COLD WORKING Hot Working (a) Properties

135 1. Hot working is done at a temperature above recrystallization but below its melting point. It can therefore be regarded as a simultaneous process of deformation and recovery. 2. Hardening due to plastic deformation is completely eliminated by recovery and recrystallization. 3. Improvement of mechanical properties such as elongation, reduction of area and impact values. 4. No effect on ultimate tensile strength, yield point, fatigue strength and hardness. 5. Poor surface finish due to oxidation and scaling. 6. Refinement of crystals occurs. 7. Due to hot working cracks and blowholes are welded up. 8. No internal or residual stress developed. 9. Force required for deformation is less. 10. Light equipment is used in hot working. 11. Difficult to handle a hot worked metal. 12. Hot working processes are hot forging, hot rolling, hot spinning, hot extrusion, hot drawing, and hot piercing, pipe welding. (b) Advantages of Hot Working 1. Porosity in the metal is largely eliminated. Most ingots contain many small blow holes. These are pressed together and eliminated. 2. Impurities in the form of inclusions are broken up and distributed throughout the metal. 3. Coarse or columnar grains are refined. Since this hot work is in the recrystalline temperature range, it should be continued until the low limit is reached to provide a tine grain structure. 4. Physical properties are generally improved owing principally to grain refinement. Ductility and resistance to impact are improved, strength is increased, and greater homogeneity is developed in the metal. The greatest strength of rolled steel exists in the direction of metal flow. 5. The amount of energy necessary to change the shape of steel in the plastic state is far less than that required when the steel is cold. (c) Disadvantages/Limitations of Hot Working 1. Because of the high temperature of the metal, there is rapid oxidation or scaling of the surface with accompanying poor surface finish. 2. Difficult to achieve close tolerances due to scaling.

136 3. Some metals cannot be hot worked because of their brittleness at high temperatures. 4. Hot working equipment and maintenance costs are high Cold Working (a) Properties l. Cold working is done at temperature below recrystallization temperature. So, no appreciable recovery can take place during deformation. 2. Hardening is not eliminated since working is done below recrystallization temperature. 3. Decreases elongation, reduction of area etc. 4. Increase in ultimate tensile strength, yield point and hardness. 5. Good surface finish is obtained. 6. Crystallization does not occur. Grains are only elongated. 7. Possibility of crack formation and propagation is great. 8. Internal and residual stresses are developed in the metal. 9. Force required for deformation is high. 10. Heavy and powerful equipment is used for cold working. 11. Easier to handle cold parts. 12. Cold working processes are cold rolling, cold extrusion, press work (drawing, squeezing,bending, and shearing). (b) Advantages of Cold Working 1. Cold working increases the strength and hardness of the material due to the strain hardening which would be beneficial in some situations. Further, there is no possibility of decarburisation of the surface. 2. Since the working is done in cold state, hence no oxide formation on the surface and consequently, good surface finish is obtained. 3. Greater dimensional accuracy is achieved. 4. Easier to handle cold parts and also economical for small sizes. 5. Better mechanical properties are achieved. (c) Disadvantages/Limitations of Cold Working 1. Only small sized components can be easily worked as greater forces are required for large sections. Due to large deforming forces, heavy and expensive capital equipment is required.

137 2. The grain structure is not refined and residual stresses have harmful effects on certain properties of metals. 3. Many of the metals have less ductility e.g., carbon steel and certain alloy steels, cannot be cold worked at room temperature. It is therefore, limited to ductile metals and the range of shapes produced is not as wide as can be obtained by machining. 4. Tooling costs are high and as such it is used when large quantities of similar components are required. METAL FORMING Metal forming can be defined as a process in which the desired size and shape are obtained through the deformation of metals plastically under the action of externally applied forces. Metal forming processes like rolling, forging, drawing etc. are gaining ground lately. It is due to the fact that metal forming is the wasteless process which is highly economical. They give high dimensional accuracy, easy formability for complex shapes and good surface finish with desired metallurgical properties. The metal forming is based upon the plastic deformation of metals. For finding out the complete information of the stresses and strains that developed in the metal due to application of loads, comprehensive study and calculations are required. To start with, there are three conditions to be satisfied, while going for stress estimation: 1. There should be equilibrium at all points. 2. The volume should remain same before and after the forming. 3. Stress-strain relationship of material should be maintained. Different Types Of Metal Forming Processes Metal forming processes can be classified under two major groups. Bulk deformation processes and sheet metalworking processes. Bulk deformation is characteristic in that the work formed has a low surface area to volume ratio. In sheet metalworking the metal being processed will have a high surface area to volume ratio. The following is a brief overview of the major metal forming processes.

138 5. 1. ROLLING The process of plastically deforming metal by passing it between rolls is known as "Rolling". In this process the work is subjected to high compressive stresses from the squeezing action of rolls and to surface shear stresses as a result of the friction between the rolls and the metal. Also, the frictional forces help for drawing the metal into the rolls. The initial breakdown of ingots into blooms. and billets is generally done by hot rolling. They are further hot rolled into plate, sheet, rod, bar. rails or structural shapes. By cold rolling sheet. strip and foil with good surface finish and mechanical strength are produced. 5.2.Terminology of Shapes Used in Rolling 1) Ingot - It is the initial product obtained by the casting of molten metal. Ingots are cast in metal molds usually of cast iron, with square sections. Ingots may be also of circular, corrugated and other convenient sections. 2) Bloom - A bloom is the product of the first breakdown of the ingot. It is usually of square section with cross sectional area above 225 cm 2. It is obtained by hot rolling of an ingot. 3) Billet - Hot rolling of bloom yields a billet, with a reduced cross section. The minimum cross section of a billet is about 16 cm 2. But in non ferrous metallurgical terminology, a billet is any ingot which has received hot working by rolling, forging. etc., or the term refers to casting which is suitable for hot working. 4) Slab - A slab refers to hot rolled ingot with a cross sectional area greater than 100 cm 2, and with a width at least twice the thickness. 5) Plate and Sheet - The difference between a plate and a sheet is determined by the thickness of the product. In general, plate has a thickness greater than 6 mm and sheet has thickness less than 6 mm.

139 6) Sheet and Strip - These are rolled products with a thickness less than 6 mm. Strip refers to the rolled product, with a width less than 300 mm, while sheet refers to the product of. width above 300 mm. 5.3Types of Rolling Mills Rolling mills can be conveniently classified with respect to the number and arrangement of the rolls, as follows- 1) Two High Mill. This is the simplest and most common type of rolling mill. These are further classified as reversing and non reversing mills. In non reversing mills, rolls of equal size are rotated only in one direction. The rolled stock is returned to the entrance of the rolls for further reduction (Figure 3-1Oa). In two high reversing mill the work can be passed to and fro through the rolls by reversing their-direction of rotation. Such mill requires less manual work and works faster compared to non-reversing mill (Figure 3-1Ob).

140

141 2) Three High Mill. This consists of three rolls of equal size one above the other. In this the upper and lower rolls are power driven, while the middle roll rotates by friction. In this back and forth operations can be performed simultaneously (Figure 3-1Oc). '3) Four High Mill. This mill consists of two small diameter working rolls and two large diameter backup rolls, placed one above.the other. Such mills require less power for roiling because of lesser friction of contact area. These are, generally used for sheet rolling (Figure 3-10d). 4) Cluster Mill. In this mill each of the work rolls is supported by two backing rolls. This is useful for rolling of thin sheet or foil to close dimensional tolerances (Figure 3-1Oe).The working rolls are power driven. 5) Tandem Mill. In this. a series of rolling mills are installed one after the other to facilitate high production. Each set of rolls IS called a stand. Since a different reduction takes place at each stand. the stnp Will be moving at different velocities at every stage in the mill. The speed of each set of rolls is synchronized so that each successive stand takes the strip at a speed equal to the delivery speed of the preceding stand. The uncoiler and windup reel not only perform feeding and coiling up operations but also supply a back tension and front tension to the strip (Figure 3 11) 6) Steckel Mill. This mill is similar to tandem mill. except for no working roll is power driven. Only the uncoiler and wind up reels are power driven. In thrs mill the amount of reduction per pass is limited, but hard metals can be reduced to thin gages with close tolerances on

142 thickness. 5.4Comparison Between Two high and Four high Rolling Mills 1) Generally two high mills consist of two rollers of same size and diameter, and both perform the rolling action. In four high mills, four rollers, two of which working rolls of smaller. diameter, and two back-up rolls of larger diameter are provided.. 2) Generally two high mills are of reversing type. but four high mills are of non reversing type. 3) Two high mills are used for hot working, and generally for the primary 'breaking down of cast ingots (for cogging to produce blooms), and subsequent reduction to square, rounds, rails, etc., while four high mills are useful for cold rolling of plates, strips and other flat shapes. 4) Friction is more in two high mills because of large diameter rolls, hence require more power, whereas, four high mills use smaller diameter working rolls, due to which friction is less, thus require lesser power. 5) Construction of two high mills is simple, but is to be made robust as it is used for blooming of ingots. while the construction of four high mills is complicated because of four rolls, but need not be so robust as they handle thinner sections. 5.6 HOT ROLLING Hot rolling is the process of rolling a metal above its recrystallization temperature. The first hot working operation for most steel products is done in the blooming mill (also called cogging mill). Characteristics of Hot Rolling Hot working gives the following properties/characteristics to a metal- 1) Hot working brings very little change in the hardness and ductility. Recrystallization is spontaneous, and the resultant fine-grained structure is stranger than the original material. 2) The hot worked metals will have improved properties in the direction of working. This is because the impurities in the basic material segregate into stringers and lie parallel to the direction of metal flow. as the metal is rolled. 3) Metals can be worked to larger reductions in hot working, as recrystallisation and grain growth takes place, and no strain hardening takes place, i.e., the metal regains its ductility continuously while working itself.

143 Advantages of Hot Rolling 1) Hot rolling breaks up brittle film of hard constituents and brings homogeneity in rolled components. 2) Welding of cracks and blow holes takes place. 3) Grain refinement gives optimum mechanical properties to the alloy. 4) Recrystallisation takes place and hence no work hardening and no internal stresses. 5) There is no need for reheating while working since ductility is not lost. 6) Time required to produce a component is less as compared to cold rolling. Disadvantages of Hot Rolling 1) Surface oxidation under high temperature is evident, resulting in the formation of scale. 2) Decarburisation at the surface layers may take place in carbon steels. 3) Chances of scale inclusion in the rolled product exists. 4) The process is more expensive because of the requirement of heating the component to recrystallization temperature. 5) More care is essential in handling the heated part. 5.7COLD ROLLING Cold rolling is the process of rolling metals and alloys below their recrystallization temperatures. Generally they are worked at room temperature. Cold rolling is used to produce sheet and strips with fine surface finish and accuracy. Also, the strength of cold rolled product will be high because of strain hardening. The starting material for cold rolling is pickled hot rolled breakdown coil from the continuous hot-strip mill. High speed 4 high tandem mills with three to five stands are used for the cold rolling of steel sheet, aluminium and copper alloys. The maximum reduction possible by cold rolling varies from about 50 to 90% %. For achieving the maximum reduction, the reduction in each pass should be kept to a minimum. and should be distributed uniformly over various passes. Generally. the

144 lowest percentage reduction is allowed in the last pass to permit better control of flatness, gage and surface finish. Microstructural Changes in Cold Rolling During cold rolling the as cast crystals are distorted as slip takes place and gets work hardened in the process (Figure 3-12b). The capacity for further cold work must then be restored by an annealing process (and during annealing the temperature must be so controlled as to give a grain size that offers optimum mechanical properties. The degree of cold work In the final pass through the rolls is controlled to give the desired combination of work hardening. strength and ductility. in the product. Characteristics of Cold Rolling Cold rolling gives the following properties/characteristics to a metal- 1) Cold rolling mainly brings in greater changes in the properties of hardness and ductility of an alloy. Since, there is no recrystallisation process upon working, the grains remain distorted/deformed with induced intemal stresses. leading to increase in hardness and loss of ductility: 2) Since there is no oxidation and scale formation as in the case of hot rolling. on cold roiling the surface finish improves to a greater extent. 3) Cold rolling to a certain extent improves the tensile strength, beyond which the brittleness increases. 4) Metals and alloys cannot be subjected to larger reductions because of work hardening. Intermittent annealing treatments are essential if further reductions are required. Advantages of Cold Rolling 1) Surface finish is improved. 2) Tensile strength and yield point are increased. 3) Close dimensional tolerances can be achieved. 4) No problem of oxidation and scale formation. 5) Defects like inclusion of scales are not there. Disadvantages of Cold Rolling 1) Internal stresses are induced into the cold worked metal, thus making the metal hard and brittle. 2) For large reductions, intermittent annealing is necessary. 3) Grain structure is distorted and chances of intergranular cracks are more. 4) Ductility is lost to a greater extent. 5) Blow hole and minor surface cracks in the metal continue to exist in the cold worked metal and form large scale defects. 5.8 Rolling defects The rolled parts are more thicker than the required thickness of metal because of the rolls get deflected by high rolling forces apply. The Elastic deformation of mill takes place on the metal.so some of rolling defects are provided in the metal forming.

145 The rolling defects are mainly two types Surface rolling defects Internal structural rolling defects. They are main category of rolling defects. Surface defects: Surface defects are provided from impurities and inclusion in the material surface, roll marks, dirt, rust and other cause related to prior treatment and working of metal. Internal structural defects: The internal structural defects are Wavy edges Zipper cracks in center of strip Edge cracks Alligatoring. The roll part consists of middle position bend or deflection due to compressive load on work rolls resulting more thicker than end of work piece. The thicker of center implies the edges are plastically elongated than center. This reason to induce the residual stress of compression at the edge of part and tension along with the center line of work piece. The uneven distribution of stress in the work piece to provide center line cracking (zipper cracks) warping or wavy edge of final work metal sheet. In the case the roll or cover- cambered together the work piece edge thicker than center and residual stress is provided opposite to the insufficient cambering. The tension at edge of work piece. For the resulting work piece produced in such a manner are splitting, edge cracking Edge cracks: The cracks occurred on edge of hot roll coil due to excess amount of quenching effect. Origion defects are Strip edge are cooled with excess water Un- flatness leading to water carryover.

146 Alligatoring The formed sheet metal will adhere to rolled surface and to follow the path of their respective rolls, sheet will shear in the plane and defect is called as alligatoring. 6.Forging: Forging can be defined as the controlled plastic deformation of metals at elevated temperatures in to a predetermined size or shape using compressive forces exerted, through some type of die, by a hammer, a press or upsetting machine. Forging enhances the mechanical properties of metals and improves the grain flow, which in turn increases the strength and toughness of the forged components. Forgeability: Forgeability can be defined as the tolerance of a metal or alloy for deformation without failure. Thus good forgeability means less resistance to deformation and even adverse effects such as cracking are not there. 6.1Forging Materials: The selection of forging material is made on the basis of certain desirable mechanical properties inherent in the composition of material and some properties can be developed by forging such as strength, resistance to fatigue, good machining characteristics, durability etc. Following is the list of some alloys in ascending order of forgeability 1. Aluminium alloys 2. Magnesium Alloys 3. Copper alloys 4. Plain carbon Steels 5. Low-alloy steels 6.Martensitic stainless Steels 7. Austenitic stainless Steels 8. Nickel alloys 9. Titanium alloys 10. Tantalum alloys 11. Molybdenum alloys 12. Tungsten alloys Advantages of forging I.Strength: Forging reduces the failures. In this process workpiece yields with high strength to weight ratio. Due to this, it can be able to withstand fluctuating stress caused by sudden shock loading. II. Metal conservation: Practically there is no waste of metals.

147 III. Weight saving: Strong thin-walled parts may be produced without damaging important physical requirements. IV. Machining time: Forging can be made to close tolerances, which reduces machining time for finishing operations of the products. V. Speed of production: High rate of production is possible. VI. Incorporation in welded structures: Parts can be welded easily due to fibrous structure. VII. It maintains uniform and same quality all over parts. VIII. Close tolerances. Disadvantages of forging parts I. High tool cost. II. High tool maintenance. III. No cord holes. IV. Limitation in size and shape. V. Heat treatment process increases cost of the product. VI. Brittle materials like cast iron can not be forged. VII. Complex shape can not be produced by forging IX. Smooth surface finish. 6.3 HOT WORKING AND COLD WORKING Hot Working (a) Properties

148 1. Hot working is done at a temperature above recrystallization but below its melting point. It can therefore be regarded as a simultaneous process of deformation and recovery. 2. Hardening due to plastic deformation is completely eliminated by recovery and recrystallization. 3. Improvement of mechanical properties such as elongation, reduction of area and impact values. 4. No effect on ultimate tensile strength, yield point, fatigue strength and hardness. 5. Poor surface finish due to oxidation and scaling. 6. Refinement of crystals occurs. 7. Due to hot working cracks and blowholes are welded up. 8. No internal or residual stress developed. 9. Force required for deformation is less. 10. Light equipment is used in hot working. 11. Difficult to handle a hot worked metal. 12. Hot working processes are hot forging, hot rolling, hot spinning, hot extrusion, hot drawing, and hot piercing, pipe welding. b) Advantages of Hot Working 1. Porosity in the metal is largely eliminated. Most ingots contain many small blow holes. These are pressed together and eliminated. 2. Impurities in the form of inclusions are broken up and distributed throughout the metal. 3. Coarse or columnar grains are refined. Since this hot work is in the recrystalline temperature range, it should be continued until the low limit is reached to provide a tine grain structure. 4. Physical properties are generally improved owing principally to grain refinement. Ductility and resistance to impact are improved, strength is increased, and greater homogeneity is developed in the metal. The greatest strength of rolled steel exists in the direction of metal flow. 5. The amount of energy necessary to change the shape of steel in the plastic state is far less than that required when the steel is cold. (c) Disadvantages/Limitations of Hot Working 1. Because of the high temperature of the metal, there is rapid oxidation or scaling of the surface with accompanying poor surface finish. 2. Difficult to achieve close tolerances due to scaling. 3. Some metals cannot be hot worked because of their brittleness at high temperatures. 4. Hot working equipment and maintenance costs are high.

149 6.3.2 Cold Working (a) Properties l. Cold working is done at temperature below recrystallization temperature. So, no appreciable recovery can take place during deformation. 2. Hardening is not eliminated since working is done below recrystallization temperature. 3. Decreases elongation, reduction of area etc. 4. Increase in ultimate tensile strength, yield point and hardness. 5. Good surface finish is obtained. 6. Crystallization does not occur. Grains are only elongated. 7. Possibility of crack formation and propagation is great. 8. Internal and residual stresses are developed in the metal. 9. Force required for deformation is high. 10. Heavy and powerful equipment is used for cold working. 11. Easier to handle cold parts. 12. Cold working processes are cold rolling, cold extrusion, press work (drawing, squeezing, bending, and shearing). (b) Advantages of Cold Working 1. Cold working increases the strength and hardness of the material due to the strain hardening which would be beneficial in some situations. Further, there is no possibility of decarburisation of the surface. 2. Since the working is done in cold state, hence no oxide formation on the surface and consequently, good surface finish is obtained. 3. Greater dimensional accuracy is achieved. 4. Easier to handle cold parts and also economical for small sizes. 5. Better mechanical properties are achieved. (c) Disadvantages/Limitations of Cold Working 1. Only small sized components can be easily worked as greater forces are required for large sections. Due to large deforming forces, heavy and expensive capital equipment is required. 2. The grain structure is not refined and residual stresses have harmful effects on certain properties of metals.

150 3. Many of the metals have less ductility e.g., carbon steel and certain alloy steels, cannot be cold worked at room temperature. It is therefore, limited to ductile metals and the range of shapes produced is not as wide as can be obtained by machining. 4. Tooling costs are high and as such it is used when large quantities of similar components are required Forging Operations Forging is the oldest metal working process. Because it just requires heating and hammering of metals, man found it easy. The following forging operations are performed. Drawing down or swaging: The process of increasing length and decreasing cross sectional area of the metal is known as drawing. The compressive force (hammering or pressing) are applied perpendicular to the length axis of the metal piece. Upsetting: It is just reverse of drawing. The cross-sectional area of the work piece is increased and length decreases. For it, the compressive forces are applied along the length axis of the metal piece. Coining (closed-die forging): Minting of coins, where the slug is shaped in a completely closed cavity, is an example of closed-die forging. To produce the fine details of a coin, high pressures, and sometimes several operations are needed, while lubricants are not used because they can prevent reproduction of fine die surface details. Heading (open-die forging): Heading is an example of open-die forging. It transforms a rod, usually of circular cross-section, into a shape with a larger cross-section. The heads of bolts, screws, and nails are some examples of heading. The work piece has a tendency to buckle if the length to- diameter ratio is too high. Punching: It is the process of making holes by using punch over or hardy hole on an anvil Hubbing: It is a piercing process where the die cavity produced is used for subsequent forming operations. To generate a cavity by hubbing, a pressure equal to three times the ultimate tensile strength of the material of the workpiece is needed. Cogging: Also called drawing out, successive steps are carried to reduce the thickness of a bar. Forces needed to reduce the thickness of a long bar are moderate, if the contact area is small. Fullering and Edging: It is an intermediate process to distribute the material in certain regions of the workpiece before it undergoes other forging processes that give it the final shape. Roll Forging: A bar is passed through a pair of rolls with grooves of various shapes. This process reduces the cross-sectional area of the bar while changing its shape. This process can be the final forming operation. Examples are tapered shafts, tapered leaf springs, table knives, and numerous tools. Also, it can be a preliminary forming operation, followed by other forging processes. Examples are crankshafts and other automotive components. Skew Forging: It is similar to roll forging but used for making ball bearings. A round wire is fed into the roll gap and spherical blanks are formed continuously by the rotating rolls. Classification of forging process:

151 Mainly forging process classified into two parts. I. Open die forging: a) Hand forging b) Power forging: i. Hammer forging ii. Press forging II. Close die forging: a) Drop forging b) Press forging c) Machine forging 6.5.Forging Methods Open-Die Forging Compression of work part between two flat dies Deformation operation reduces height and increases diameter of work Common names include upsetting or upset forging Open-Die Forging with No Friction If no friction occurs between work and die surfaces, then homogeneous deformation occurs, so that radial flow is uniform throughout work part height

152 Homogeneous deformation of a cylindrical work part start of process with work piece at its original length and diameter, partial compression, 3. final size. Applications Open-die processes can produce: 1. Step shafts, solid shafts (spindles or rotors) whose diameter increases or decreases at multiple locations along the longitudinal axis. 2. Hollow cylindrical shapes, usually with length much greater than the diameter of the part Length, wall thickness, internal and outer diameter can be varied as needed. 3. Ring-like parts can resemble washers or approach hollow cylinders in shape, depending on the height/wall thickness ratio. 4. Contour-formed metal shells like pressure vessels, which may incorporate extruded nozzles and other design features. Open-die forging is further classified as hand forging and power forging: Open-Die Forging with Friction Friction between work and die surfaces constrains lateral flow of work, resulting in barreling effect In hot open-die forging, effect is even more pronounced due to heat transfer at and near die surfaces, which cools the metal and increasess its resistance to deformation.

153 Deformation of a cylindrical work part in open-die forging, showing pronounced barreling start of process, (2) partial deformation, (3) final shape. Impression-Die Forging Compression of work part by dies with inverse of desired part shape Flash is formed by metal that flows beyond die cavity into small gap between die plates Flash serves an important function: As flash forms, friction resists continued metal flow into gap, constraining material to fill die cavity In hot forging, metal flow is further restricted by cooling against die plates Impression-Die Forging (1) just prior to initial contact with raw work piece, (2) partial compression, and (3) final die closure, causing flash to form in gap between die plates. Trimming After Impression-Die Forging Trimming operation (shearing process) to remove the flash after impression-die forging.

154 Forging types These are four types of forging methods Smith forging :-this is the traditional forging operation done openly or in open dies with manual hammering or by power hammers, Drop forging:-this is the operation done in closed impression dies by means of drop hammers.here force for shaping the components is applied in a series of blows. Press forging:-similar to drop forging,press forging is also done in closed impression dies with the exception that the force is a continuous squeezing type applied by the hydraulic presses. Machine forging:-unlike the drop or press forging where the material is drawn out, in machine forging,the material is only upset to get the desired shape. SMITH FORGING The process involves heating the stock in the blacksmith's hearth and then beating it over the anvil. To get the desired shape, the operator has to manipulate the component in between the blows. The types of operations available are fullering, flattening, bending, upsetting and swaging. In fullering, the material cross-section is decreased and length increased. To do this, the bottom fuller is kept in the anvil hole with the heated stock over the fuller. The top fuller is then kept above the stock and then with the sledgehammer, the force is applied on the top fuller. The fullers concentrate the force over a very small area, thus decreasing the cross-section at that point. Metal flows outward and away from the centre of the fullering die. Then the stock is advanced slightly over the fuller and the process repeated, as shown in Fig. After fullering, the stock would have the fullering marks left which are then cleaned by means of flattening. To obtain specific shapes such as round, square, hexagon, etc. open general- hammering or by purpose dies called swages are used. The force for shaping is applied by manual means of the forging hammers, the latter being the industrial practice.

155 Smith forging involves a lot of skill on the part of the operator and also is more time consuming. But since no special dies are used, smith forging is more beneficial in the manufacture of small lots or in trial production, because of the heavy cost of the closed impression dies cannot be justified in these cases. DROP FORGING This method of forging uses a closed impression die to obtain desired shape of the component. The shaping is done by the repeated hammering given to material in the die cavity. The equipment employed for delivering the blows are called drop hammers. The die used in drop forging consists of two halves. The lower half of the die is fixed to the anvil of the machine, while the upper half is fixed to the ram. The heated stock is kept in the lower die of the ram delivers four to five blows on the metal in quick succession so that the metal spreads and completely fills the die cavity. When the two die halves close,the complete cavity is formed. Too complex shapes with internal cavities,deep pockets etc cannot be obtained in drop forging due to the limitation of the withdrawal of the finished forging from the die. In drop forging, the final desired shape cannot be obtained directly from the stock in a single pass. Depending on the shape of the component, and the desired grain flow direction,the material should be manipulated in a number of passes the various passes are, 1. The first two steps are called fullering and edging. Here, the cross-sectional area of the metal is reduced in some areas and gathered in other areas. This also starts the fibrous grain flow. 2. The third step is referred to as blocking. The shape of the part is not pronounced hence, it may take several drops in the blocking cavity of the die. In step three, flash begins to appear. This is a thin (0.04) fin of metal that is squeezed between the dies. 3. The fourth step is called finishing. Here, the final shape of the part is completed. 4. The last step is called trimming. Holes are cleared and the flash is removed from the forging. Drop forging requires machining to obtain dimensional tolerances and good surface finish.

156 Press forging Press forging is variation of drop-hammer forging. Unlike drop-hammer forging, press forges work slowly by applying continuous pressure or force. The amount of time the dies are in contact with the work piece is measured in secondss (as compared to the milliseconds of drop-hammer forges). The main advantage of press forging, as compared to drop-hammer forging, is its ability to deform the complete work piece. Drop-hammer forging usually only deforms the surfaces of the work piece in contact with the hammer and anvil; the interior of the work piece will stay relatively unreformed. There are a few disadvantages to this process, most stemming from the work piece being in contact with the dies for such an extended period of time. The work piece will cool faster because the dies are in contact with workpiece; the dies facilitate drastically more heat transfer than the surrounding atmosphere. As the workpiece cools it becomes stronger and less ductile, which may induce cracking if deformation continues. Thereforee heated dies are usually used to reduce heat loss, promote surface flow, and enable the production of finer details and closer tolerances. The work piece may also need to be reheated. Press forging can be used to perform all types of forging, including open-die and impression-die forging. Impression-die press forging usually requires less draft than drop forging and has better dimensional accuracy. Also, press forgings can often be done in one closing of the dies, allowing for easy automation. Advantages Presses provide a faster rate of production because the die in press forging is filled in a single stoke. Superior quality of the products. Quicker operation comparatively. High output even with unskilled operator. Uniform forging with exact tolerance. Machine forging:

157 This process also known as hot heading or up-setting. This process consist of applying pressure longitudinally on a hot bar clamped or gripped between grooved dies. Forging is done on the end of the bar. Forging is done on various shapes of metals. But most commonly used shape is round shape metals. The equipments used for forging is forging machine or up-setter. Generally it gives forging pressure in a horizontal directions. Fig. Machine Forging These dies are so designed such that complete operation is performed in several stages and gradually final shape is obtained. Operation performed with a die and punch called heading tool. Fig. shows step by step operation done on the round bar. Die is either made hallow to receive the round bar through it or in two parts to open out and receive the bar. Heading tool is advanced in to die. Many strokes of the heading tool may be needed to complete the forging. In this process dies does not have draft as well as flash, so it gives better dimensional accuracy. Advantages: Better quality of forging Since there is no or little draft is needed on forging made by up-setters, therefore, there is saving in materials and also machining expenses. The upsetting rocess can be automated. As copared to drop forging machine forging have higher productivity. Disadvantages High tooling costs It is difficult to produce intricate or non-symmetrical shapes. Not convenient for heavier job.

158 Forging defects Though the forging process gives better quality of products as compared to other manufacturing process still there are some defects likely to developed due to improper care taken during the process, such defects are given bellow: (A): Unfilled Section: In this some section of the die cavity is not completely filled by the flowing metal. The causes of this defects are improper design of the forging die or using forging techniques. (B): Cold Shut: This appears as a small cracks at the corners of the forging. This is caused mainly by the improper design of die. Where in the corner and the fillet radius are small as a result of which metal does not flow properly into the corner and the ends up as a cold shut. (C): Scale Pits: This is seen as irregular depurations on the surface of the forging. This is primarily caused because of improper cleaning of the stock used for forging. The oxide and scale gets embedded into the finish forging surface. When the forging is cleaned by pickling, these are seen as depurations on the forging surface. (D): Die Shift: This is caused by the mis- alignment of the die halve, making the two halve of the forging to be improper shape. (E): Flakes: These are basically internal ruptures caused by the improper cooling of the large forging. Rapid cooling causes the exterior to cool quickly causing internal fractures. This can be remedied by following proper cooling practices. (F): Improper Grain Flow: This is caused by the improper design of the die, which makes the flow of the metal not flowing the final interred direction Difference between drop forging and press forging:

159 Difference between open die forging and close die forging:

160 MODULE-IV

161 7. EXTRUSION Introduction: Extrusion is a metal working process in which cross section of metal is reduced by forcing the metal through a die orifice under high pressure. It is used to produce cylindrical bars, tubes and sections of any regular or irregular types. Forces required to extrude a metal are quite high and hence hot extrusion is most widely done as deformation resistance of metal is low at high temperature. However, cold extrusion is also performed for soft metals like Aluminum, lead etc. Difficult to form metals like stainless steels, nickel based alloys and high temperature metals can also be extruded. History; Originally the principle of extrusion was applied to make lead pipe and lead sheathing of electrical cables. Types of Extrusion 1) Direct Extrusion In this process, the metal billet is placed in a container and compressed and extrudedd through the die by a ram. Some features of direct extrusion: Both the ram and extrusion move in the same direction. A dummy block or pressure plate is in contact with the billet and ram. The relative motion between billet and container wall develops high friction. Hence power required is relatively high. Brittle metals like Tungsten, Titanium alloys are difficult to extrude because they fracture during the process. Fractures occur because of rapid growth of micro cracks due to tensile stresses. 2) Indirect Extrusion

162 A hollow ram compresses metal through a die in a direction opposite to ram motion. Either the ram is moved against a stationery billet or the billet (hence container) is made to move against stationery ram. Some features of indirect extrusion: There is no relative motion between the billet and the wall of the container. Hence friction is lower and power required is relatively less. Limitation; Due to hollow ram, the load that can be applied is limited and only small sections can be extruded. 3) Tube extrusion A mandrel is attached to the end of the ram as shown, which produces a hollow tube. The clearance between the mandrel and die wall determines the thickness of the tube Fig.Tube Extrusion To begin with, either a hollow billet is taken or a solid billet is first pierced through and then extruded. 4) Impact Extrusion In this process a punch moves into the die and squeezes metal around the die cavity. It may have either direct or indirect extrusion arrangement.

163 It is useful to produce short lengths cans. It is usually a cold working process, but the high speed of deformation develops heating. The process is limited to soft metals like lead, tin, aluminum, copper. 5) Hydrostatic Extrusion of hollow shapes like collapsible tooth paste tubes and thin walled In this process the space between the ram plate and billet is filled with water. Hencee billet is subjected to uniform hydrostatic pressure. Fig. Hydrostatic Extrusion Also, there is no direct contact between wall of container and work piece. Hence there is no container- Therefore, large billet friction. As a result, the curve of extrusion pressure v/s ram travel is nearly flat. length to diameter ratios are possible. eg coils of wire. Advantages: i) Lubrication is very effective.

164 ii) Extruded product has good surface finish and dimensional accuracy. iii) It is possible to use dies with very low semi cone angle ( about 20 degrees) because friction is less. iv) This reduces extrusion pressure and improves homogeneity of deformation. v) Redundant deformation is minimized. Limitations: i) Hot working is not possible. ii) Leakages of liquid are frequent due to high pressures involved ( upto 1.7GPa) iii) Liquid used should not solidify at high pressure. iv) Extrusion ratios possible ; 20:1 for mild steel, 200:1 for aluminum. Extrusion Equipment I) Hydraulic Press Types based on direction of ram travel: i) Vertical Press and ii) Horizontal Press i) Vertical Press ( 3 MN to 20 MN) The ram acts vertically on the billet and squeezes it through the die. Advantages: i) Easier alignment between the press ram and tools. ii) Hence closer control on tolerances is possible. iii) High rates of production. iv) Requires less floor space. v) Produces uniform cooling of billet in container. Hence symmetrically uniform deformation of metal occurs. Limitations: i) This requires more head room to accommodate vertical motion of ram ii) Floor pits are needed to accommodate long extrusions.

165 Applications: Thin walled tubing where uniform wall thickness and concentricity are required. II) Horizontal Press (15 MN to 50 MN) Ram moves horizontally and extrudes metal. Advantages: i) The head room required is less compared to vertical press. Limitations: i) Alignment between press ram and tools is difficult. ii) The bottom of the billet is more in contact with the container wall and hencee it is cooled faster compared to the top of the billet. Therefore deformation is non uniform. iii) To overcome above difficulty, the container walls are internally heated to avoid differential cooling of the billet. Ram speed: Higher ram speeds are required for high temperature extrusion to prevent heat loss to container walls. Ram speeds of 0.4n/s to 0.6m/s are used to extrude refractory metals. Dies a d Toolings They must withstand high stress and thermal shocks/ oxidation. From economics point of view, another important requirement of the dies is that easy replacement of damaged parts and reuse of parts after reworking must be possible. Types of extrusion dies Flat Faced Dies Fig. Flat Die Some important features:

166 It is used when metal entering the die forms a dead zone and shears internally to form its own die angle. A parallel land on the exit side of the die strengthens the die. It also allows reworking of the flat face on the entrance side without increasing the exit diameter. Conical Dies Fig. Conical Die Some important features: The entrance side has conical shape and taper. They are used in extrusion with good lubrication. Die angle is decreased and this increases homogeneity of deformation and also reduces extrusion pressure. If the angle is too small, it leads too high friction in die surface. Hence an optimal angle is necessary. Other supporting systems required in extrusion equipment: a) Provision for heating extrusion container b) Billet heating facilities c) Automatic transfer equipment for placing the heated billet in the container d) Hot saw to cut off extrusion product e) Run out table to catch the extrusion f) Straightener to correct minor warpage of the extruded product. Extrusion Variables They affect the extrusion process considerably. They are: 1) Type of extrusion (direct or indirect) In direct extrusion process, metal begins to flow through the die at the maximum value of the pressure called break through pressure. As billet extrudes, the pressure required progressively decreases with decreasing length of the billet in the container (because, the friction between the billet and container decreases). In indirect extrusion, there is no relative motion between billet and wall. Thereforee extrusion pressure is almost constant with increase in ram travel.

167 It represents the stress required to deform the metal through the die. Limited in application by the need of hollow ram, which limits the size of extrusion & pressure. Hence most of the hot extrusion is done by direct extrusion. At the end of the ram stroke, there is a rapid pressure build up & therefore a small discard is left behind in the container, without extruding it. 2. Extrusion Ratio : (R) R= Initial cross area of the billet / final cross section area after extrusion R = Ao / Af Up to = 40 : 1 for hot extrusion of steel Up to = 400 : 1 for Aluminium A small change in the fractional reduction results in large increase in extrusion ratio Velocity of extruded product = ram velocity x R Therefore high sliding velocities exist along the die land. Extrusion Pr. = P = K Ao ln ( Ao / Af ) K = extrusion constant, which accounts for flow stress, friction,and inhomogeneous deformation. 3. Temperature : Hot extrusion decreases flow stress of metal, but increases oxidation of billet & extrusion tools. Other features are: Softens die & tools Difficult to provide lubrication Therefore it is advantageous to use the min. temp. which provides required plasticity to metal. The upper hot working temp. of metal is the temp. at which Hot shortness occurs. Higher plastic deformations involved also lead to internal heating of the metal. Therefore max. working temp. must be safely below the melting point. Typical Values steel billets heated to C to C Toolings : preheated to C. 4. Extrusion pressures range : 800 MPa to 1200 MPa 5. Lubrication : ( Glass ) To be maintained at high temperature & under high pressure. Low strength alloy (Al) does not require lubrication. Metal deformation is non uniform and therefore wide variation in heat treatment response is observed Effect of temperature, pressure & strain rate on the allowable working range or interdependence of extrusion speed & temperature:

168 For a given working pressure & temperature there will be a maximum amount of deformation possible on the work piece. As pre heat temperature of billet increases, the flow stress falls & therefore amount of possible deformation increases As strain rate of deformation increases, more heat is retained in the work & therefore work temperature will have to be reducedd so that final temperature is below hot shortnesss temperature. 6. Ram speed : Increase in ram speed increases the extrusion pressure. Whereas, low ram speeds leads to cooling of the billet and because of billet cooling, flow stress is increased. The higher the temperature of billet, the greater the effect of low extrusion speed on the cooling of the billet. Therefore high extrusion speeds temperatures. At the same time at high extrusion speeds, temperature rise due to deformation is greater. The selection of proper extrusion alloy and billet size. For a given extrusion pressure the extrusion ratio which can be obtained increases with increasing temperature. For a given temperature a large extrusion ratio can be obtained with high pressure. Maximum billet temperature is determined by the temperature at which melting is about to occur. The temperature rise of extrusion is determined by the speed of extrusion & extrusion ratio. Deformation, Lubrication and Defects in Extrusion : Pressure required in extrusion depends on the way the metal flows in the container & extrusion die. The metal flow is mainly determinedd by conditions of lubrication. Deformation in Extrusion Processs The defects in extrusion are related to the way in which metal deforms during extrusion. Homogeneous Deformation: (with less friction) are required with high strength alloys which need high extrusion speed & temperature is best determined by trial & error for each Fig a) indicates homogeneous deformation in direct extrusion.

169 Fig d) shows homogeneous deformation in indirect extrusion. The following conditions are favorable for homogeneous deformation: i) Low container friction ii) Well lubricated billet iii) Hydrostatic extrusion conditions iv) Indirect extrusion process. Fig. Different Grid Patterns During Deformation in Extrusion Characteristics of homogeneous deformation: The deformation is more uniform until close to the die entrance where metal flow is restricted. (2) Deformation with more friction between billet and container wall Fig (b) above indicates increased container wall friction. This is indicated by severe distortion of grid pattern at the corners of the die due to a dead zone. The dead zone consists of stagnant metal which does not undergo any deformation. The grid elements at the centre of the billet undergo pure elongation into the extruded rod. The grid elements near the sides of the billet undergo shear deformation The shear deformation requires additional energy called redundant work. This work is not related to metal working from billet to extruded product. (3) Deformation with very high friction Fig (c) above indicates the condition of high friction at the container billet interface. The metal flow is concentrated towards the centre. An internal shear plane develops due to high friction. This situation also exists when the billet surface is chilled by a cold container. This is because, at low temperature of the billet at the sides, the flow stress increases compared to the flow stress at the central portion of the billet Under such sticking conditions between the billet and container, a shear zone is formed along which the metal separates internally. In this condition, extruded product contains clean new metal and outer surface of billet remains in the container. Lubrication in Extrusion Process The following are the requirements of a lubricant in hot extrusion: It must have a low shear strength It must be stable enough without breaking down at high working temperatures. The most widely used lubricant for steels and nickel based alloys is molten glass. The process using molten glass as lubricant is called Ugine-Sejournet Process. The steps involved are:

170 The billet is heated in inert atmosphere. It is coated with glass powder before it enters the extrusion container. The glass coating serves as a lubricant and also as a thermal insulator, thereby reducing heat loss from billet to container wall and other tools. The thickness of glass film between extrusion and die is about 25 microns. Interaction between optimal lubricant, temperature and ram speed: If ram speed is too low, the lubricant is thick because of low initial extrusion pressure. This exhausts the glass reservoir rapidly. This increases cost of lubricant. If the ram speed is too high, the glass film becomes too thin and friction increases. Important requirements of lubrication: The lubricant film must be complete and continuous to be successful. Any gaps in film develops shear zones in metal which eventually develop into surface cracks. Defects in Extrusion 1) Laminations of glass/ oxide into the interior of extrusion. Cause: Improper lubrication method Remedy: To provide optimum lubrication on the outside of billet and to use optimal ram speed. 2) Extrusion defect: The last 1/3rd of extrusion may have oxides and other impurities in it rendering it unfit for use because of poor mechanical properties. This leads to the formation of annular ring of oxide in the extruded product. Cause: The metal in the middle (2/3rd) is first extruded as it moves faster than the periphery of billet due to friction. This tendency of extrusion defect increases with friction between billet and container wall. Remedy: The last 1/3rd of billet is left out without extruding it. But this is economically not feasible. Instead a follower block is widely used. This block is slightly smaller diameter than the container and it scalps or scrapes the billet, leaving behind the oxide layers in the container. 3) Axial Hole/ Funnel: It is an axial hole in the back end of extrusion. Cause: Rapid radial flow of metal during extrusion of last 1/4th of billet. Remedy: Inclining the face of the ram at an angle to the ram axis. 4) Surface Cracking: It is in the form of rough surface or fir-tree cracking.

171 Cause: i) Longitudinal tensile stresses generated as extrusion passes through the die. ii) Very high ram speed for the given temperature. Remedy: Use of optimal ram speed and billet temperature and heating the container. 5) Center Burst (or Chevron Cracking): Cause: Low extrusion ratios and low friction in the deformation zone at the die. Remedy: Increasing the friction at tool-billet interface to obtain a sound product. 6) Variation in hot worked structure and properties: This is the non-uniform properties of the extruded product, having variation in properties from front to back end. Extrusion of Tubes By piercing and extruding in one step: This method is better compared to the one using a mandrel. The piercing mandrel and the ram are operated by two separate hydraulic systems. This requires a double action extrusion press. Steps: i) First the piercing mandrel is withdrawn and the billet is pressed with the ram. ii) Next, the billet is pierced with the sharp mandrel, ejecting a metal plug through the die. iii) Then the ram advances and extrudes the billet over the mandrel to produce the tube. 8. DRAWING OF RODS. WIRES & TUBES Drawing is a cold working process and involves pulling the material to be drawn through a hole in a hard steel or carbide block called "die" and reduce the diameter of the material.ln drawing, tensile forces are applied by way of pulling the rod at the exit end of the die. Plastic flow is caused by compression forces arising from the reaction of the metal at the die surface. The process of reducing the diameter of a material by successive drawing operations is called "Bar, rod orwire drawing" depending upon the, diameterofthe drawn product. When a hollow tube is drawn through a die without using any mandrel, it is called "tube sinking". If a mandrel/rod is used to support the inside diameter of the tube, it is called "tube drawing" Advantages of Drawing Process 1) Ductile materials can be drawn down to very small diameters and to exact sizes. 2) The surface finish of a cold drawn product is superior to that of a hot rolled or extruded material. 3) Mechanical properties can be easily controlled by controlling the degree of cross sectional reduction in the final cold drawing operation. 4) Operation is simple compared to other processes. 5) There is no problem of surface defects as drawing is a cold working process. ROD DRAWING The process involves cleaning the materials, pointing the rod and actual drawirg in a die. This operation is explained below.

172 Cleaning Before Drawing For steel rods that are previously hot rolled or extruded. surface cleaning IS required. For this. the material is immersed in a dilute solution of hydrochloric or sulphuric acid. This is called as 'Pickling'. This removes any scale, oil. grease etc., present on the material surface. It is then washed with water and dipped in an emulsion of slaked lime and water. This neutralizes the acid and excess lime dries on the material surface and forms a base to absorb the minerai oil or grease, whichever is the lubricant. Soap can be also be used as lubricant, but results in dull surface of the finished product. Pointing or Tagging the Rod and Drawing Since the rod is drawn in straight lengths, the operation carried out on a drawn-bench which must be long enough to accommodate the length of the rod required. A draw-bench consists of a die held rigidly in steel frame and a 'dog' which grips the end of the rod and pulls it through the die. The dog runs on rails to the required length of the rod. To operate the bench the end of the rod must be pointed or 'tagged'. For this purpose. the rod is first forced through the die so that it projects out of the die. This projected part IS rigidly held by the dog (jaw) of the draw bench and pulled by the moving chain. driven either mechanically or hvdraulically. WIRE DRAWING Wire drawing starts with hot rolled 'wire rod'. Similar to rod drawing. the rod IS first cleaned by pickling and washed. It is then coated with lime or plated WIththin layer of copper or tin. The lime neutralizes any acid, and serves as an absorber and carrier of the lubricant during dry drawing. The lubricant may be either grease or soap powder. For wet drawing operation the die itself is immersed in a lubricant. The schematic of wire drawing operation is shown In figure.

173 DIES FOR DRAWING Figure 3-36 shows the important features of a wire drawing die. It consists of a tapered hole, with smooth working surface of high strength and wear resistance. The bell-mounted entrance AB of the die will never be in contact with work, but serves as a reservoir for lubricant carried in by the work. The tapered portion BC is the actual working surface where plastic deformation takes place, and hence must be carefully designed and prepared. The angle of taper is critical, and depends both on the material used for the die and the metal to be drawn. Its surface should be polished to reduce friction to a minimum. The section CO is cylindrical, which must be of adequate length, since the working part of the die wears, and C moves nearer to 0, but the diameter of the wire will remain within specifications. Portion DE is called as 'relief. Its function is to provide reinforcement for the working section of the die and prevent the circular edge from breaking or pulling away. Die Materials Dies can be made from a variety of materials, like chilled cast iron, high carbon steels and alloy steels. Carbon and alloy steel dies have the advantage that they can be forged so that. as the die hole wears to an extent where the resulting wire is over size, the hole can be hammered up and then reamed to the correct size. Chilled cast iron dies are used for the production of low quality materials. Tungsten carbide dies are widely used nowadays because of their long life. As tungsten carbide is expensive and also some-what brittle, only the working part of the die is made from tungsten carbide and this is held in a mild steel block. The schematic arrangement of a carbide die held in a mild steel block is shown in Figure.

174 TUBE DRAWING Tube drawing is similar to wire drawing and uses draw benches and dies. However. To reduce the wall thickness and to control the inside dia, the inside surface of the tube must be supported while drawing. This is accomplished by inserting a mandrel or plug inside the tube positioned at the die throat. The mandrel may have either a cylindrical or tapered cross section. Hence, there are basic methods of tube drawing a) Sinking or drawing without a mandrel b) Plug drawing or drawing with a fixed mandrel c) Drawing with a.floating mandrel d) Drawing with a moving mandrel a} Tube sinking or Drawing withoutt a Mandrel Tubes which have wall thicknesss greater than the diameter can be drawn without a mandrel. Reductions of up to 35% can be obtained in steel tubes by this method, which is known as "tube sinking" (see figure 3-39). One end of the tube is swaged so that can be threaded through the die and drawn similar to a rod. b) Plug Drawing or Drawing with a Fixed Mandrel In this method, the mandrel or plug is short in length, and is held in position in the mouth of the die by means of a tie-rod attached to it and to a fixed support at the opposite end of the draw-bench, similar to wire drawing. The rod is detachable, so that it can be drawn backwards and aside in order that the tube shell can be threaded over it. The tagged end of the tube is then pushed through the die and gripped by the dog while the back end of the mandrel

175 rod is anchored to its support. The tube is then drawn through annulus formed between the die opening and the mandrel as shown in figure. This method can be used to draw longer tubes since the mandrel is fixed. c) Drawing with a Floating Mandrel For tubes of small diameter, drawing with fixed mandrel requires a very fragile mandrel rod Also, the length of the tube that can be drawn is limited by the length of mandrel rod. However. in such cases, a floating mandrel can be used and there Will be no limit to the length of the tube drawn. The contour of the plug is so designed that the plug adjusts itself to the correct position during drawing (Figure ). This method is suitable for the manufacture of small dia tubes in which the wall thickness is greater than the bore. d) Drawing with a Moving Mandrel In this method the mandrel used is made of heat treated alloy steel rod equal in length to the finished tube and having a diameter equal to the bore of the tube. The rod is not fixed to the draw bench, but moves through the die along with the tube as shown in Figure 3. In this method the frictional loss is low, and there is geed lubricatini effect. But in this method there is problem of stripping the mandrel from the drawn tube, and also limited to.tubes below 6 mm diameter. DEFECTS IN DRAWN WIRES AND RODS Defects in drawn wires and reds can be due to.two reasons - one, the defects in the starting material itself, like seams, slivers, pipe, scale, etc.; two, improper deformatien precess like excessive drawing without patenting treatment, without the use of proper lubricant, etc. The most common drawing defect in wires and rods is centre burst or chevro.ncrackinq, which is also. called "cupping". These are the internal cracks developed as a result of secondary tensile stresses which usually occur with large values of h/l ratio (i.e, ratio of mean thickness to the length of the deformation zone). According~ to upper bound analysis, the cupping fracture occurs due to low die angles (.) at low reductlens. The other common defect in drawn products is the surface cracks. Thesee result mainly from heavy die friction in ~xtrusion. This can be due to two reasons - lack of lubricaticn and excessive drawing with the loss of ductility. The defects resulting from oxide layers and seams on the basic materials are the inclusion of these unwanted materials into the surfacess of the drawn products. All these defects can be avoided by the use of proper lubrication, low h/l ratio, frequent patenting and cleaning of the raw material before drawing. 9. SHEET METAL OPERATION

176 In today s practical and cost conscious world, sheet metal parts have already replaced many expensive cast, forged and machined products. The reason is obviously the relative cheapness of stamped, mass produced parts as well as greater control of their technical and aesthetic parameters. That the world slowly turned away from heavy, ornate and complicated shapes and replaced them with functional, simple and logical forms only enhanced this tendency towards sheet metal products. The common sheet metal forming products are metal desks, file cabinets, appliances, car bodies, aircraft fuselages, mechanical toys and beverage cans. Sheet Metal Characteristics In sheet metal forming operations certain characteristics of sheet metal play very important role in getting good quality desirable products. Elongation: Because the material is usually being stretched in sheet forming, high uniform elongation is desirable for good formability. The true strain at which necking begins is numerically equal to the strain-hardening exponent n*, thus a high value of n indicates large uniform elongation. Necking may be localized or it may be diffuse, depending on the strain rate sensitivity m** of the material. The higher the value of m, the more diffuse the neck becomes; diffuseness is desirable in sheet metal operations. In addition to uniform elongation and necking, the total elongation of the specimen is also a significant factor in the formability of sheet metals. Obviously, the total elongation of the material increases with increasing values of both n and m. Yield Point Elongation: Low carbon steels exhibit a behaviour called yield point elongation, one having upper and lower yield points. This behaviour indicates that, after the material yields the sheet stretches farther in certain regions without any increase in the lower yield point, while other regions in the sheet have not yet yielded. Aluminium magnesium alloys also exhibit this behaviour. This behaviour produces Lueder s bands (stretcher strain marks or worms) on the sheet. They are elongated depressions on the surface of the sheet, can be found on the bottom of the cans used for common household products. They may be objectionable in the final product, because coarseness in the surface degrades appearance and causes difficulties in subsequent coating and painting operations. The usual method of avoiding these marks is to eliminate or to reduce yield point elongation, by reducing the thickness of the sheet 0.5 to 1.5 % by cold rolling (temper or skin rolling). Because of strain aging however the yield point elongation reappears after a few days at room temperature or after a few hours at higher temperatures. Anisotropy: An important factor that influences sheet metal forming is anisotropy (directionality) of the sheet. There are two types of anisotropy: crystallographic anisotropy (preferred orientation of the grains) and mechanical fibering (alignment of impurities, inclusions, and voids through out the thickness of the sheet). Grain size: The coarser the grain, the rougher is the surface appearance. An ASTM grain size of 7 or finer is preferred for general sheet metal forming operations. Residual stresses: This is caused by non uniform deformation during forming. It causes part distortion when sectioned and can lead to stress corrosion cracking. This is reduced or eliminated by stress relieving operations.

177 Springback: This is caused by elastic recovery of the plastically deformed sheet after unloading. Due to this distortion of part and loss of dimensional accuracy happened. It can be controlled by techniques such as overbending and bottoming of the punch. Wrinkling: This happened due to circumferential compressive stresses in the plane of the sheet and is controlled by proper tool and die design. Quality of sheared edges: The edges can be rough, not square, and may contain cracks, residual stresses, and a work hardened layer all of which are detrimental to the formability of the sheet. The quality can be improved by control of clearance, tool and die design, fine blanking, shaving, and lubrication. Surface condition of sheet: It depends on rolling practices. This is important in sheet forming as it can cause tearing and poor surface quality. Types of Press Working Operations All sheet metal operations can be grouped into two categories: cutting operations and forming operations. Blanking: It is the operation of cutting a flat shape from sheet metal. The article punched out is called the blank and is the required product of the operation. The hole and the material left behind are discarded as waste. It is usually the first step of series of operations. Punching or Piercing: It is a cutting operation by which various shaped holes are made in sheet metal. Punching is similar to blanking except that in punching the hole is the desired product, the material punched out to form the hole being waste. Notching: This is cutting operation by which metal pieces are cut from the edge of a sheet, strip or blank. Perforating: This is a process by which multiple holes which are very small and close together are cut in flat workpiece material. Trimming: This operation consists of cutting unwanted excess material from the periphery of a previously formed product. Shaving: The edges of a blanked part are generally rough, uneven and unsure. Accurate dimensions of the part are obtained by removing a thin strip of metal along the edges. Slitting: It refers to the operation of making incomplete holes in a work piece. Lancing: This is a cutting operation in which a hole is partially cut and then one side is bent down to form a sort of tab. Since no metal is actually removed, there will be no scrap. Nibbling: This operation is generally substituted for blanking in case of small quantities of components having complex shapes. The part is usually moved and guided by hand as the continuously operating punch cuts away at the edge of the desired contour. Bending: In this forming operation sheet metal is uniformly strained around a linear axis which lies in the neutral plane and perpendicular to the length wise direction of the sheet.

178 Drawing: This is a process of forming a flat workpiece into a hollow shape by means of punch which causes the blank to flow into a die cavity. Squeezing: Under this operation the metal is caused to flow to all portions of a die cavity under the action of compressive forces. Coining: It is a forming operation having two different impressions. in which a slug is deformed such that the two sides of the slug are Embossing: It is also a forming operation in which a sheet is deformed such that an emboss is formed on one side and a corresponding depression on the other side. Principle of Shearing Action In the fabrication of a sheet metal part a suitable intermediate flat shape or blank is first cut from a strip of sheet metal. Shearing operations are conventionally subdivided into (1) shearing, which employs general purpose shearing machines and usually cuts along a straight line, and (2) the die shearing processes, which employs punches and dies of various shapes. The metal is brought to the plastic stage by pressing the sheet between two shearing blades so that fracture is initiated at the cutting points. The fracture on either side of the sheet further progressing towards each other with downward movement of the upper shear, finally result in the separation of the slug from the parent strip. The metal under the upper shear is subjected to both compressive and tensile stresses. In an ideal shearing operation, the upper shear pushes the metal to a depth equal to about one third of its thickness. Because of pushing of the material into the lower shear, the area of cross section of the metal between the cutting edge of the shears decreases and causes the initiation of the fracture. This portion of the metal which is forced into the lower shear is highly burnished and would appear as a bright band around the blank lower portion. The fractures which are initiated at both the cutting points would progress further with the movement of upper shear and if the clearance is sufficient, would meet, thus completing the shearing action.

179 Clearance The die opening must be sufficiently larger than the punch to permit a clean fracture of the metal. This difference in dimensions between the mating members of a die set is called Clearance.When correct clearances are used, a clean break would appear as a result of the extension of the upper and lower fractures towards each other. With an insufficient clearance additional cut bands would appear before the final separation. Ductile materials require smaller clearances (Otherwise soft material will be drawn into the gap) and longer penetration of the punch compared to harder materials. If the clearance is more than the optimum value, then Penetration is more; Work done is more; Burr forms. If the clearance is less than the optimum value, then Peak load is more; Penetration is slightly more; Work done is more; The edge of the product is not smooth.

180 Proper clearance (b) improper clearance Reduction in die clearance reduces the burr, but hastens the blunting of the cutting edges of dies and punches. This results in frequent resharpening of press tools and decreases the tool life and the number of components the tool can produce. Generally, a press tool produces thousands of components per shift. It is uneconomical and impracticable to deburr the millions of components usually produced in mass production runs. So many industries sacrifice tool life to reduce the inconvenient burr on the sheared components. If the shear cutting edges become dull, the shearing force will be spread over a larger area so that more plastic deformation is caused in the metal before the stress reaches the rupture point. Then, even more clearance is necessary, and more energy is required. Sheared edges of work piece may be work-hardened to such an extent that cracking may occur in subsequent working operations. The use of sharp cutting edges and annealing after the shearing will help to prevent possible cracking of sheared edges. The clearance is applied in the following manner: 1. When the hole has to be held to size, i.e., the hole in the sheet metal is to be accurate (Punching Operation) and slug is to discarded, the punch is made to the size of hole and the die opening size is obtained by adding clearance to the punch size. 2. In blanking operation, where the slug or blank is the desired part and has to be held to size, the die opening size equals the blank size and the punch size is obtained by subtracting the clearance from the die opening size. The clearance is a function of type, thickness and temper of work material, harder materials requiring larger clearance than soft materials, the exception being aluminium. The usual clearance per side of the die, for various materials is given in terms of stock thickness.

181 A section through blanking die is given in figure , showing clearance, land, straight and angular clearance. Land: It is the flat usually horizontal surface contiguous to the cutting of a die which is ground and reground to keep the cutting edges of the punch sharp. Straight: It is the surface of a cutting die between its cutting edge and the beginning of the angular clearance. This straight portion gives strength to the cutting edge of the die and also provides for sharpening of the die. This straight portion is usually kept at about 3 mm for all materials less than 2 m thick. For thicker materials, it is taken to be equal to the metal sheet thickness. Angular clearance: This is provided to enable the slug to clear the die. It is placed below the straight portion of the die surface. Its value usually varies from 0.25 to per side but occasionally as high as 20, depending mainly on stock thickness and frequency of sharpening. Punch and die clearance after considering the elastic recovery of the material: Due to springback effect after the release of blanking pressure, the blank expands slightly. The blanked part is thus actually Press Working Terminology A simple cutting die used for punching and blanking operations is shown in figure. The main components of a press tool are described here.

182 Bed: It is the lower part of a press frame that serves as a table to which a bolster plate is mounted. Bolster plate: This is a thick plate secured to the press bed, which is used for locating and supporting the die assembly. It is usually 50 to 125 mm thick. Die set: it is unit assembly which incorporates lower shoe and upper shoe, two or more guideposts and guide posts bushings. Die: The die may be defined as the female part of a complete tool for producing work in a press. It is also referred to a complete tool consisting of a pair of mating members for producing work in a press. Die block: It is a block or a plate which contains a die cavity. Lower shoe: The lower shoe of a die set is generally mounted on the bolster plate of a press. The die block is mounted on the lower shoe. Also the guide posts are placed on it. Punch: This is the male component of the die assembly, which is directly or indirectly moved by and fastened to the press ram or slide. Upper shoe: This is the upper part of the die set which contains guide post bushings. Punch plate: The punch plate or punch retainer fits closely over the body of the punch and holds it in proper relative position. Back up plate or pressure plate: It is so placed that the intensity of pressure does not become excessive on punch holder. The plate distributes the pressure over a wide area and the intensity of pressure on the punch holder is reduced to avoid crushing.

183 Stripper: It is a plate which is used to strip the metal sheet from a cutting or non- cutting punch or die. It may also guide the sheet. Knockout: It is a mechanism, usually connected to and operated by the press ram, for freeing a workpiece from a die. Pitman: It is a connecting rod which is used to transmit motion from the main drive shaft to the press slide. Stroke: The stroke of a press is the distance of ram movement from its up position to its down position. It is equal to twice the crankshaft throw or the eccentricity of the eccentric drive. It is constant for the crankshaft and eccentric drives but is variable on the hydraulic press. Shut height: It is the distance from top of the bed to the bottom of the slide with its stroke down and adjustment up. Types of Dies The dies may be classified according to the type of operation and method of operation. According to the type of operation the dies are classified as cutting dies and forming dies. Cutting dies: These dies are used to cut the metal. The common cutting dies are blanking dies, piercing dies, perforating dies etc. Forming dies: These dies change the appearance of the blank without removing any stock. These dies include bending dies, drawing dies, squeezing dies etc. On the basis of method of operations, the dies may be classified as simple dies, compound dies, progressive dies, combination dies, transfer dies, multiple dies. Simple dies or single action dies: These dies perform single operation for each stroke of the press slide. The operation may be any of the operations under cutting or forming dies. Compound Dies: In these dies two or more operations may be performed at one station. Such dies are considered as cutting tools since only cutting operations are carried out. Figure shows a simple compound die in which a washer is made by one stroke of the press. The washer is produced by simultaneous blanking and piercing operations. Compound dies are more accurate and economical in mass production as compared to single operation dies.

184 Simple die Combination dies: In this die more than one operation may be performed at one station. It differs from compound die in that in this die, a cutting operation is combined with a bending or drawing operation. Figure explains the working of a combination blank and draw die. The die ring which is mounted on the die shoe is counter bored at the bottom to allow the flange of a pad to travel up and down. This pad is held flush with the face of the die by a spring. A drawing punch or required shape is fastened to the die shoe. The blanking punch is secured to the punch holder. A spring stripper strips the skeleton from the blanking punch. A knockout extending through the centre opening and through the punch stem ejects the part on the upward stroke as it comes in contact with the knockout bar on the press. In operation, the blank holding ring descends as the part is blanked, then the drawing punch contacts and forces the blank into the drawing die which is made in the blanking punch. BENDING OPERATIONS Definitions used in Bending In bending, a straight length of metal is worked into a curved length. The common bending operations are the formation of channels. drums, tanks, etc., using dia.in sheets. Figure illustrates the terms used in bending.