SHRI GURU GOBIND SINGHJI INSTITUTE OF ENGG & TECHNOLOGY DEPARTMENT OF PRODUCTION ENGINEERING SUBJECT:MECHANICAL WORKING OF METALS EXPERIMENT NO: 3 AIM: STUDY OF FORGING EQUIPMENT AIM: Study of forging equipment. OBJECTIVES: Forging is the metalworking processes, in which the material is deformed to the various required shape and size. The various parts produced by this processes varies from automotive applications to the various structural parts in different structures like bridges, etc. After completing the experiment, the students will be able to: 1. Classify and understand different Forging Equipment s. THEORY: 1) FORGING EQUIPMENTS: a) FORGING: Forging is the Oldest of the metal forming operations. It is a Deformation process in which work is compressed between two dies. The basic metals industries use forging to establish basic shape of large parts that are subsequently machined to final geometry and size.forging is a manufacturing process involving the shaping of metal using localized compressive forces. Forging is often classified according to the temperature at which it is performed: "cold", "warm", or "hot" forging. Forged parts can range in weight from less than a kilogram to 580 metric tons. Forged parts usually require further processing to achieve a finished part.most forging operations are carried out hot, although certain metals may be cold-forged. Forging can produce a piece that is stronger than an equivalent cast or machined part. As the metal is shaped during the forging process, its internal grain deforms to follow the general shape of the part. As a result, the grain is continuous throughout the part, giving rise to a piece with improved strength characteristics. Other advantages include less noise, heat and vibration. It also produces a distinctly different flow pattern.the different Products of forging are engine crankshafts, connecting rods, gears, aircraft structural components, jet engine turbine parts, etc. Depending upon the application of the forging operation and the required temperature of the operation, the forging can be grouped into two categories: i) Hot Forging. ii) Cold Forging.
i) HOT FORGING: The hot-forging is the operation of forging product or components above the recrystallization temperature for that product. The hot forging results in a reduction in strength and increase in ductility of work metal, due to refining of the grains of the material. The main reason behind the use of hot-forging is the capability for substantial plastic deformation of the metal is far more than as possible with cold working. The other reasons that favours the application of hot-forging are: Strength coefficient is substantially less than at room temperature. Strain hardening exponent is zero (theoretically). Ductility is significantly increased. The various advantages of hot forging are as follows: 1. Lower forces and power requirement than cold working. 2. More intricate work geometries are possible to process. 3. Need for annealing may be reduced or eliminated. In spite of the various advantages, there are also some disadvantages associated with the hot forging, as follows: 1. Lower dimensional accuracy. 2. Higher total energy required (due to the thermal energy to heat the workpiece). 3. Work surface oxidation (scale), resulting in a poorer surface finish. 4. Shorter tool life. ii) COLD FORGING: The cold-forging is the operation of forging product or components at the room temperature or above the room temperature, but far below than the recrystallization temperature of the product. The cold forging results in an increase in strength and reduction in ductility of work metal.the various advantages of cold forging are as follows: 1. Higher dimensional accuracy 2. Lower total energy required. 3. There is no work surface oxidation or scale, resulting in high surface finish 4. There is a longer tool life. The various disadvantages of cold forging are as follows: 1. It requires larger forces and power than hot forging. 2. The cold forging cannot be used for complex and intricate shape work parts. 3. There is a need of annealing or any other stress relieving process after cold-forging, in order to relieve stresses in the component, which are induced due to cold-forging.
b) TYPES OF FORGING: i) OPEN-DIE FORGING: Open-die forging is carried out between flat dies or dies of very simple shape. The process is used for mostly large objects or when the number of parts produced is small. Open-die forging is often used to preform the work piece for closed-die forging.open die forging involves the shaping of heated metal parts between a top die attached to a ram and a bottom die attached to a hammer anvil or press bed. Metal parts are worked above their recrystallization temperatures-ranging from 1900 F to 2400 F for steel-and gradually shaped into the desired configuration through the skill-full hammering or pressing of the work piece. Although the open die forging process is often associated with larger, simpler-shaped parts such as bars, blanks, rings, hollows or spindles, in fact it can be considered the ultimate option in "custom-designed" metal components. High-strength, long-life parts optimized in terms of both mechanical properties and structural integrity are today produced in sizes that range from a few pounds to hundreds of tons in weight. In addition, advanced forge shops now offer shapes that were never before thought capable of being produced by the open die forging process. FIG.1. OPEN-DIE FORGING ii) CLOSED DIE FORGING: The work piece is deformed between two die halves which carry the impressions of the desired final shape. The work piece is deformed under high pressure in a closed cavity. The process provide precision forging with close dimensional tolerance. Impression or closed die forging confines the metal in dies, open die forging is distinguished by the fact that the metal is never completely confined or restrained in the dies. Most open die forgings are produced on flat dies. However, round swaging dies, V-dies, mandrels, pins and loose tools are also used depending on the desired part configuration and its size.closed die forging is expensive than open-die forging.
FIG.2. CLOSED DIE FORGING iii) IMPRESSION-DIE FORGING: In impression-die forging, the work piece acquires the shape of the die-cavities or impression, while being forged between two shaped dies. Also, there are some materials that flows outwards and forms a flash. The flash plays significant role in the flow of material in impression-die forging. The thin flash cools rapidly and because of its frictional resistance, it subjects the material in the die cavity to high pressures, thereby encouraging the filling of the die cavity. The blank to be forged is prepared by different means, such as, Cutting or cropping from an extruded or drawn bar stock, Powder metallurgy. Casting, Preform blank in a prior forging operation. FIG.3. IMPRESSION-DIE FORGING As shown in the figure, the blank is placed on the lower die and the upper die begins to descend, the blank s shape gradually changes, followed by the creation of the flash between the die cavities.one variation of impression-die forging is called flashless forging, or true closed-die forging. In this type of forging, the die cavities are completely closed, which keeps the workpiece from forming flash. The majoradvantage to this process is that less metal is lost to flash. Flash can account for 20 to 45% of the startingmaterial. The disadvantages of this process include additional cost due to a more complex die design and theneed for better lubrication and workpiece placement.
iv) UPSET FORGING: Forging of the ring and rod types with all kinds of heads and shoulders, such as bolts, nuts, washers, collars, pinion gear blanks, etc. can be conveniently produced by the upset forging. The upset forging increases the diameter of the work piece by compressing its length. Based on number of pieces produced, this is the most widely used forging process. A example of parts produced by using the upset forging process are engine valves, couplings, bolts, screws, and other fasteners.. FIG.4. UPSET FORGING Upset forging is usually done in special high-speed machines, i.e. crank presses, but upsetting can also be done in a vertical crank press or a hydraulic press. The machines are usually set up to work in horizontal plane, to facilitate quick exchange of work pieces from one station to the next. The standard upsetting machine employs split dies that contain multiple cavities. The dies open enough to allow the work piece to move from one cavity to the next, the dies then close and the heading tool or ram, then moves longitudinally against the bar, upsetting it into the cavity. If all of the cavities are utilized on every cycle, then a finished part will be produced with every cycle, which makes this process advantageous for mass production. The various rules that must be followed when designing parts to be upset forged are as follows: 1. The length of unsupported metal that can be upset in one blow without injurious buckling should belimited to three times the diameter of the bar. 2. Lengths of stock greater than three times the diameter may be upset successfully, provided that thediameter of the upset is not more than 1.5 times the diameter of the stock. 3. In an upset requiring stock length greater than three times the diameter of the stock, and where thediameter of the cavity is not more than 1.5 times the diameter of the stock, the length of unsupportedmetal beyond the face of the die must not exceed the diameter of the bar.
v) PRESS FORGING: Press forging works by slowly applying a continuous pressure or force, which differs from the nearinstantaneous impact of drop-hammer forging. The amount of time the dies are in contact with theworkpiece is measured in seconds (as compared to the milliseconds of drop-hammer forges). The press forging operation can be done either cold or hot.the main advantage of press forging, as compared to drop-hammer forging, is its ability to deform the completeworkpiece. Drop-hammer forging usually only deforms the surfaces of the workpiece in contact with thehammer and anvil; the interior of the workpiece will stay relatively undeformed. Another advantage to theprocess includes the knowledge of the new part's strain rate. We specifically know what kind of strain can beput on the part, because the compression rate of the press forging operation is controlled. FIG.5. PRESS FORGING There are a few disadvantages to this process, most stemming from the workpiece being in contact with the dies for such an extended period of time. The operation is a time-consuming process due to the amount and length ofsteps. The workpiece will cool faster because the dies are in contact with workpiece; the dies facilitatedrastically more heat transfer than the surrounding atmosphere. As the workpiece cools it becomes stronger andless ductile, which may induce cracking if deformation continues. Therefore heated dies are usually used toreduce heat loss, promote surface flow, and enable the production of finer details and closer tolerances. Thework piece may also need to be reheated. When done in high productivity, press forging is more economical than hammer forging. The operation alsocreates closer tolerances. In hammer forging a lot of the work is absorbed by the machinery, when in pressforging, the greater percentage of work is used in the work piece. Another advantage is that the operation can beused to create any size part because there is no limit to the size of the press forging machine. By the
constraint ofoxidation to the outer layers of the part, reduced levels of micro-cracking occur in the finished part.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. c) FORGING EQUIPMENTS: The most common type of forging equipment is the hammer and anvil. The choice of forging equipment depends on a number of factors, including part size and complexity, material, and the quality of the parts to be produced. Hammers are often preferred for small to medium batches because of quicker tool. This forging equipment can be divided into two basic types: i. WORK-RESTRICTED MACHINES: In work-restricted machines the amount of deformation that can be achieved during each stroke or blow of the machine is limited by the energy or maximum force available. If the energy or force capacity is less than is required to deform the part, then more than one stroke or blow is needed. Machines that fall into this category are hammers, friction screw presses, and hydraulic presses. 1. HAMMERS: Hammers are the most common types of machine used. They are often preferred for small to medium batches because of quicker tool setups and lower overheads. They are also used for elongated and branch-type forgings because die areas can be provided for the larger number of preform dies required for such shapes. The various types of hammers used are as follows: a. GRAVITY DROP HAMMERS: Gravity drop hammers are the oldest type of forging equipment available. The principle of operation is that the moving die block is raised by a lifting mechanism and then released, so that it falls onto the fixed die attached to the anvil. The amount of deformation that can be carried out is determined by the potential energy of the moving die block at its maximum height. This potential energy is converted into kinetic energy as the die block falls and is then dissipated in deformation of the work piece. Various lifting mechanisms are used, including frictional means with boards, band brakes or belts, or a lifting cylinder employing steam, compressed air, or hydraulic fluid, as shown in figure These machines are available in a range of blow energies from 0.6kNm (60kg-m) to 400 kn-m (40,000 kg-m).
FIG.4. DIFFERENT TYPES OF DROP HAMMERS b. DOUBLE ACTING OR POWER HAMMERS: These machines are similar to gravity hammers in that a lifting cylinder raises the moving tup, but power is also applied to the downward-moving tup to increase the energy capacity. Energy ratings for similar tup weights are considerably more than for gravity hammers, and the die closing speeds are higher also. Power comes from double-acting steam, compressed air, or hydraulic cylinders. Double-acting hammers are manufactured in a range of energy ratings from 3 kn-m (300 kg-m) to 825 kn-m (82,500 kg-m). c. VERTICAL COUNTERBLOW HAMMERS: In these machines two tups with nearly equal masses are driven by double-acting cylinders toward each other and impact in the center of the machine. More energy is dissipated in the work piece than in the foundations and subsoil compared to single-acting hammers. Very high energy capacities are available in the largest machines, with ranges from 30 kn-m (3 0,000 kg-m) to 2000 kn-m (200,000 kg-m). d. HORIZONTAL COUNTERBLOW HAMMERS: These machines are also called impacters and two rams are actuated by double acting cylinders. Heated stock is positioned vertically between the dies by an automatic transfer mechanism. Energy ranges from 4 kn-m (400 kg-m) to 54 kn-m (5400 kg-m) are typical.
FIG.5. COUNTERBLOW HAMMERS 2. SCREW PRESSES: In screw presses, the upper ram and die are connected to a large vertical screw that can be rotated by a flywheel, so that the ram can move up and down relative to the fixed die in the bed of the machine. The ram has a limited amount of energy for each stroke, thus multiple blows are usually employed similar to hammers. Screw presses are available in ratings from 0.63 MN to 63 MN (63-6300 tons). FIG.6. SCREW PRESS
3. HYDRAULIC PRESSES: Hydraulic presses are available in a wide range of sizes up to the largest at 50,000 tons or more capacity. The moving die is attached to a ram actuated by a large hydraulic cylinder (Fig. 14.18d). Various strokes, forces, and closing speeds can be obtained on hydraulic presses. In some cases hydraulic presses are fitted with auxiliary horizontally moving rams, and these enable side depressions to be forged into some parts, although this is not done to a great extent. FIG.7. HYDRAULIC PRESS ii. STROKE RESTRICTED MACHINES: In stroke-restricted machines the amount of deformation that can be done is fixed by the stroke of the machine. If sufficient force or energy to carry out the operation is not available, then the machine will stall and a larger machine should be used. Mechanical presses fall into this category, as a crank or eccentric determines the amount of ram movement. 1. MECHANICAL PRESSES: Mechanical presses belong to a class of machine tools that encompass a wide range of different machine types. Primarily, the mechanical press transforms the rotational force of a motor into a translational force vector that performs the pressing action. Therefore, the energy in a mechanical press comes from the motor. These types of presses are generally faster than hydraulic or screw presses, (actually the screw press may also be classified as a mechanical press). Unlike some presses, in a mechanical press, the application of force varies in both speed and magnitude throughout the distance of the stroke. When performing a manufacturing operation using a mechanical press, the correct range of the stroke is essential. In mechanical presses, a crank, knuckle joint, scotch yoke, or moving-wedge mechanism is used to apply a vertical squeezing motion between the upper moving die and a lower fixed die, as shown in fig.
FIG.8. CRANK PRESS FIG.9. KNUCKLE JOINT PRESS FIG.10. ECCENTRIC PRESS FIG.11. RACK & PINION PRESS vi) FORGING DEFECTS: The different types of defects, occurring in the forging operations are as follows: Incomplete die filling. Die misalignment. Forging laps. Incomplete forging penetration- should forge on the press. Microstructural differences resulting in pronounced property variation. Hot shortness, due to high sulphurconcentration in steel and nickel. Pitted surface, due to oxide scales occurringat high temperature stickon the dies. Buckling, in upsetting forging, due tohigh compressive stress. Surface cracking, due to temperaturedifferential between surface and center, or excessive working of the surfaceat too low temperature. Micro cracking, due to residual stress.
CONCLUSION: In this way, we have studied the different Forging equipment s. REVIEW QUESTIONS: 1. What is forging? 2. What are the different products produced by forging? Enlist. 3. What is the significance of hammer in forging? 4. What are the different types of Hammers? 5. What are the different Mechanical Press?