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1 Chapter Name of the Topic Marks 02 2 HEAT TREATMENT Specific Objectives: Study various methods of Heat treatment processes as applied to automobile components. Understand ironcarbon phase equilibrium diagram. Contents: 2.1 Introduction: Concept of phase and phase transformations Iron-Iron carbide phase (Fe-Fe3C) equilibrium diagram. 2.2 Common heat treatment processes and their applications: Annealing, Normalizing, Hardening, Tempering. Surface hardening processes:- Case carburizing, Nitriding, Cyaniding, Induction and Flame hardening. 12 Rohan Desai, Auto. Engg. Dept.NPK. Page 1

2 2.1 INTRODUCTION Phase: Atoms are similar to a bunch of balls, and these balls can be arranged in many different ways. A simple example is that the billiard balls are arranged in a hexagonal close-packed fashion to start with, whereas the keyboards you are using are arranged in another way. Although both of these examples are two dimensional, atoms can certainly also be arranged in many different ways three dimensionally. The reason that atoms have a regular or homogeneous way to stack themselves is because such arrangement results in a stable or low energy configuration. The homogeneous arranged portion of atoms is called a phase. A phase may be defined as a homogeneous portion of a system that has uniform physical and chemical characteristics. Every pure material is considered to be a phase; so also is every solid, liquid, and gaseous solution. The graph below is the phase diagram for pure H2O. Parameters plotted are external pressure (vertical axis, scaled logarithmically) versus temperature. In a sense this diagram is a map wherein regions for the three familiar phases solid (ice), liquid (water), and vapor (steam) are described. The three curves represent phase boundaries that define the regions. A photograph located in each region shows an example of its phase ice cubes, liquid water being poured into a glass, and steam that is spewing forth from a kettle. Rohan Desai, Auto. Engg. Dept.NPK. Page 2

3 One other interesting example is the different forms of carbon: graphite is one; diamond is another. Diamond can only be formed at extremely high temperature and pressure, but it stays almost `unbreakable' once it is formed. Therefore, it is possible to transform graphite into diamond as long as enough high temperature and pressure are exerted. You may also imagine that not too large of a diamond can be formed, because the high pressure is required throughout the whole diamond. If the graphite body is big, it is difficult to transfer the applied pressure to the inside of that graphite body. Phase Transformation In the history, metals stand for a very special class of materials, because metals are stable enough to remain solid at room temperature, but it is also unstable enough so that we can burn coals to melt them and to shape them into different tools. On the contrary, it is almost impossible to melt rocks or sands, and these are categorized as ceramics. Therefore, metals have long been used to understand the nature of materials. The science and technology of metals are defined as metallurgy. Part of the advantages for metals in scientific research is due to some of their crystal structures. For example, pure iron has body center cubic (bcc) structure at room temperature. Bcc structure has eight atoms sitting on the corners of a cube and one atom in the center of this cube. Other metals that have this structure include Chromium (Cr), Tungsten (W), etc. The other important family of structure is face center cubic (fcc). Fcc structure also has eight atoms on the corners of the cube, but it has six other atoms sitting at the centers of the cube faces. Fcc metals are famous for their ductility. Many ductile metals, such as Gold (Au), Silver (Ag), Copper (Cu), Aluminum (Al), Nickel (Ni), etc., all have this structure. It should be noted that although pure iron has a bcc structure at room temperature, iron does not melt but changes from the bcc structure into fcc structure at temperatures above 910 degree centigrade. Such crystal structure change between phases is phase transformation. Rohan Desai, Auto. Engg. Dept.NPK. Page 3

4 Heat treatment: It is defined as an operation involving heating and cooling of metals or alloys in its solid state with the purpose of changing the properties of the material. The physical and mechanical properties of the materials depend upon the size, shape and form of the micro-constituents present. The micro-constituents generally present in steel are ferrite, troostite, sorbite, austenite and cementite. Steel possesses many properties like strength, cheapness and workability in addition to toughness, stiffness creep resistance, fatigue resistance, impact strength, etc. Proper heat treatment of steel plays an important part in engineering. Heat treatment of all components, whether cast forged or rolled, is necessary before actual use. Factors in Heat Treatment Processes: Before carrying out any heat treatment operation the following factors need consideration: 1. Chemical composition of the material. 2. Mode of manufacture of the material, i.e. cast, ingot, rolled or forged, etc. 3. Whether any previous heat treatment operation has been carried out on the material and what is its structure. 4. Heat treatment operations to be performed and properties and structure required. Objectives of Heat Treatment: When steel is subjected to the heat treatment operations, it undergoes many structural changes due to which the properties of the material change. In studying the effects of heat treatment therefore, the following points need consideration. 1. Structural condition of the object before heat treatment. 2. Structural changes occurring during the heat treatment. 3. Structural conditions permanently retained. In addition to the above, the chemical composition of the material plays a vital role in controlling the properties of the materials. Rohan Desai, Auto. Engg. Dept.NPK. Page 4

5 The following are the main objects of the heat treatment of steel. 1. To soften the steel that has been hardened by the previous heat treatment or mechanical working. 2. To harden the steel and increase its strength. 3. To adjust its other mechanical and physical properties like ductility, malleability, permeability corrosion resistance, etc. 4. To stabilize the dimensions of the steel instruments so that they do not expand or contract with time. 5. To refine the grain size of the steel. 6. To reduce the internal stresses. To eliminate gases. 7. To produce a hard surface on a ductile interior. 8. To improve electrical and magnetic properties. COOLING CURVE OF PURE IRON Fig 2.1: Cooling curve of pure iron Rohan Desai, Auto. Engg. Dept.NPK. Page 5

6 Fe-C PHASE TRANSFORMATION DIAGRAM (IRON-IRON CARBIDE EQUILIBRIUM DIAGRAM) The various phases existing in the diagram are as below: (i) α (Ferrite): Ferrite is a solid solution of carbon in low temperature B.C.C. α iron. It is almost pure iron and the name ferrite comes from the Latin word ferrum which means iron. It is a relatively soft and ductile phase (ii) γ (Austenite): Austenite is a solid solution of carbon in F.C.C. γ - iron. It can dissolve upto 2.0% carbon at 1147 C. The phase is stable only above 727 C. It is a soft, ductile, malleable and non-magnetic (paramagnetic) phase (iii) δ (δ - ferrite): It is a solid solution of carbon in high temperature B.C.C. δ- iron. It is similar to α-ferrite except its occurrence at high temperature. (iv) Fe 3 C (Cementite): It is an intermetallic compound of iron and carbon with a fixed carbon content of 6.67% by weight. It is extremely hard and brittle phase. It is also called Iron Carbide. Rohan Desai, Auto. Engg. Dept.NPK. Page 6

7 TTT DIAGRAM / ISOTHERMAL TRANSFORMATION DIAGRAM Time Temperature Transformation diagrams or Isothermal diagrams are also called S curve or C curve due to their shape. For each steel composition, different IT diagram is obtained. Fig 2.3 shows TTT diagram of eutectoid steel (i.e. steel containing 0.8% C). Austenite is stable above eutectoid temperature 727 C. When steel is cooled to temperature below this eutectoid temperature, austenite is transformed into its transformation product. TTT diagram relates transformation of austenite to time and temperature conditions. Thus, TTT diagram indicates transformation product according to temperature and also time required for complete transformation. Curve 1 is transformation begin curve while curve 2 is transformation end curve. The region to the left of curve 1 corresponds to austenite (A ). The region to the right of curve 2 represents complete transformation of austenite (F+C). The interval between these two curves indicates partial decomposition of austenite into ferrite and Cementite (A +F+C). Rohan Desai, Auto. Engg. Dept.NPK. Page 7

8 Fig 2.3: TTT diagram of eutectoid steel At temperatures just below eutectoid temperature, austenite decomposes into pearlite; at lower temperatures (600 C) sorbite is formed and at C troostites is formed. If temperature is lowered from 500 C to 220 C acicular troostite or bainite is formed. In eutectoid steels, the martensite transformation begins at M S (240 C) and ends at M F (50 C). The change in the hardness of the structures is shown in Rockwell units (R C ) at the right hand side of the diagram. Rohan Desai, Auto. Engg. Dept.NPK. Page 8

9 2.2 COMMON HEAT TREATMENT PROCESSES Common heat treatment processes can be classified as follows: 1. Annealing (i) Full annealing (ii) Process annealing (iii) Isothermal annealing (iv) Spheroidize annealing (v) Homogenizing 2. Normalizing 3. Hardening 4. Tempering 5. Surface hardening Rohan Desai, Auto. Engg. Dept.NPK. Page 9

10 ANNEALING The annealing operation is carried out mainly to obtain the following properties. 1. To soften the steels. 2. To improve machinability. 3. To relieve internal stress induced by some previous treatment (rolling, forging, extrusion, uneven cooling). 4. To remove coarseness of grains. 5. To produce a completely stable structure. Annealing treatment is applied to castings, forgings, cold worked sheets and wires. The operation consists of (i) heating the steel to- a certain predetermined temperature (ii) soaking at a constant temperature for a sufficient time to allow the necessary changes to occur and (iii) cooling at a predetermined very slow rate. 1. Full Annealing: Purpose: (i) To reduce internal stresses produced due to cold working, welding etc. (ii) To reduce hardness and increase ductility. (iii) To refine the grain size. (iv) To increase machinability. (v) To make the steel suitable for further heat treatment. Process: Hypoeutectoid steel (steel containing less than 0.8 % C) is heated to C above the upper critical temperature and hypereutectoid steel (steel containing more than 0.8 % C) is heated to 50 C above the lower critical temperature. The steel is soaked at the annealing temperature (soaking time depend upon the thickness of steel parts). Then these steel parts are slowly cooled at the rate of 20 to 40 C per hour. The cooling is carried out in the furnace. Rohan Desai, Auto. Engg. Dept.NPK. Page 10

11 2. Process Annealing: It is also known as sub- critical annealing or recrystalization. Purpose: (i) To soften the component to restore the ductility. (ii) To remove the internal stresses produced in the casting by welding or by previous heat treatment. Process: Steel is heated to a temperature from 600 to 650 C, holding at that temperature, and then cooling in air or in furnace. By this process, high degree of softening takes place due to removal of stress from pearlite. No phase change takes place and the ferrite & pearlite simply rearrange themselves to induce softening in materials. 3. Isothermal Annealing: This process is suitable for small rolled and forged components and not for large components. It is faster than full annealing and saves much time. Purpose: (i) To obtain stable structure (ii) To save the time required for heat treatment Process: The process is similar to ordinary annealing but it is first cooled rapidly in air or by blast in furnace to temperature C. The steel is held isothermally at this temperature for certain duration then it is rapidly cooled in air. 4. Spheroidize Annealing: The process of producing a structure of globular pearlite is known as Spheroidizing or spheroidizes annealing. Purpose: (i) To improve machinability of the steel (ii) To reduce hardness (iii) To prevent chances of cracking during cold working. Rohan Desai, Auto. Engg. Dept.NPK. Page 11

12 Process: This operation is generally applied to the hypereutectoid steels. Steel is heated just above the lower critical temperature (740 to C), held for the required time and cooled very slowly upto C in furnace. Further cooling is conducted in still air. The cooling rate varies from 20 to 25 0 C per hour. It should be noted that heating much above A cm will produce lamellar pearlite instead of granular cementite. 5. Homogenizing: It is also known as diffusion annealing. Purpose: (i) To remove non uniformity of castings this is caused by coring. Coring means variation in the composition from centre to surface of a grain. (ii) To improve the structure of steel. Process: The steel is heated as rapidly as possible up to 1150 C and is held at this temperature for sufficient time so that diffusion takes place. It is then cooled in 6 to 8 hours to a temperature of 800 to 850 C and then further cooled in air. After homogenizing, the full annealing is done to refine the grain structure. NORMALIZING Purpose: 1. To eliminate coarse-grained structure. 2. To remove internal stresses that may have been caused by working. 3. To improve the mechanical properties of the steel. 4. To increase the strength of medium carbon steels to a certain extent (in comparison with annealed steels) 5. To improve the machinability of low carbon steels 6. To improve the structure of welds Normalizing is frequently applied as a final heat treatment for items which are to operate at relatively high stresses. Rohan Desai, Auto. Engg. Dept.NPK. Page 12

13 Process: 1. Heating the metal to temperatures within the normalizing range usually 40 C to 50 C above Ac3 (for Hypoeutectoid steels) and Acm (for hypereutectoid steels) 2. Holding at this temperature for a short time (about 15 minutes). 3. Cooling in air. Normalized steels have a higher yield points, tensile strength and impact strength than if they were annealed, but ductility and machinability obtained by normalizing will be somewhat lower. Difference between annealing and normalizing Annealing Normalizing Less hardness, toughness. Slightly more hardness, For plain carbon steel the toughness. microstructure shows pearlite. Microstructure shows more Pearlite is coarse and usually pearlite. gets resolved by the optical Pearlite is fine and appears microscope. unresolved with optical Grain size distribution is.more microscope. uniform. Grain size distribution is slightly Internal stresses are least. less uniform. Internal stresses are slightly more HARDENING AND QUENCHING Hardening: Objectives: (i) To improve mechanical properties, like elasticity, strength, ductility, toughness, etc. (ii) To enable the metal to cut other metals, (iii) To develop desired hardness. Rohan Desai, Auto. Engg. Dept.NPK. Page 13

14 Process: The process consists of heating the metal to a temperature above critical point. The metal is held at this temperature for a considerable time and then it is rapidly cooled. The cooling media used varies between water, oil or molten salt. Hardening is applied to tools and machine parts to perform the operations more efficiently. Quenching: The rapid cooling of a metal in a bath of liquid during heat treatment is known as quenching, e.g. Steel is heated above its critical temperature and plunged into water to cool it, an extremely hard, needle shaped structure known as martensite is formed. The rapidity with which heat is absorbed by the quenching bath has different effects on the hardness of the metal. Cold clean water is used as quenching media, while addition of salt increases the hardness considerably. Oil gives the best balance between hardness, toughness and distortion. Special soluble oils are used as quenching media. The parts which are subjected to hardening have good tensile strength, but poor ductility, toughness and impact strength. TEMPERING Objectives: (i) To reduce internal stresses developed during previous heating, (ii) To reduce the hardness developed during hardening, (iii) To give the metal a right structural condition (To stabilize the structure). Why tempering is done after hardening? After hardening, when a metal is removed from the quenching media, it is very hard and brittle and there are several other inequalities in the structure of the metal. Tempering is done to restore ductility and reduce hardness. The process involves re-heating of the metal below critical point, then holding it for a considerable time and then slowly cooling it. Tempering Rohan Desai, Auto. Engg. Dept.NPK. Page 14

15 should be done immediately after quenching in order to relieve hardening strains. The temperature at which tempering is done varies with the carbon content of the metal and mechanical properties desired in the finished article. Lathe tools, chisels in which only the cutting ends need hardening may be hardened and tempered in one operation only. The whole tool is heated to the hardening temperature and the cutting end is quenched. When the cold end is rubbed bright and the heat from unquenched portion causes tempering, when the colour is satisfactory, the whole tool is quenched. Three types of tempering processes are classified as: (i) Low temperature tempering: This type of tempering is done in the range of C. At this range, hardness changes to a very small extent. Tensile strength is increased. Internal stresses are reduced comparatively. (ii) Medium temperature tempering: Tempering done in this case at a range of 350 to 450 C. In this case, the properties of the structure are improved, mostly employed for coil and laminated springs. Highest elastic limit and toughness are achieved. (iii) High temperature tempering: This tempering is performed in the range of 550 C to 600 C. Eliminates the internal stresses completely. Comparatively high strength and toughness are achieved CASE HARDENING OF STEELS A large number of industrial components like cams, change-over switch shafts, drive worms brake drums, gears, etc. require a hard wear resistant surface (also called case) and a soft core, so that it is tough and shock resistant too. No plain carbon steel and even alloy steels possess both the requirements, i.e. hard surface and tough core to resist shock. It is noticed that steel containing 0.1% carbon is tough whereas the steel containing 0.8 %C is very hard and brittle. Both these properties are obtained by the case hardening process. The heat treatment process of producing a hard wearresistant carbon rich case (surface layers) on a tough and soft core of steel part is known as case hardening. Low carbon steel is used for the case hardening processes except in induction, hardening, where medium carbon Rohan Desai, Auto. Engg. Dept.NPK. Page 15

16 steel or high carbon steel is used. The processes generally employed for case hardening are as follows. 1. Carburising 2. Cyaniding 3. Nitriding 4. Carbonitriding 5. Flame hardening 6. Induction hardening. 1. Carburising Process: Curburising is a method of enriching in carbon the surface layer of low carbon steel in order to produce a hard case. Carburising is also known as cementation. Roughly, the machined parts of the low carbon steel are packed with carburising mixture in a steel box as shown in Fig. The carburising mixture contains 50 to 70% charcoal, 5 to 15% barium carbonate, 2 to 15% calcium carbonate and 3 to 13% sodium carbonate. A layer of the carburising mixture of nearly 25 mm thickness is placed at the bottom. Then the components are so placed that no component touches one another or even the sides of the box. The box is covered and the lid tightly sealed with fireclay to avoid the entry or escape of gases. Fig: Packing components for solid carburising The portion which is not to be case hardened is protected by electroplating on the surface which does not absorb carbon. The boxes are placed in a furnace Rohan Desai, Auto. Engg. Dept.NPK. Page 16

17 and heated to a temperature of 900 to 980 C for 6 to 8 hours. Temperature and time of heating depends upon the depth of the case required. After heating, the box is allowed to cool along with components inside the furnace. Carbon percentage increases on the surface as the austenite has a tendency to absorb carbon at high temperatures. Depth of the case obtained in this case varies from 1 mm to 1.5 mm with the carbon content on the outer surface at 1.1 to 1.2%. 2. Cyaniding The process of providing a hard wear resistant case with a tough core to the low carbon steels by liquid cyanide bath is called cyaniding. Process: The cyanide mixture (20 to 50 % Sodium cyanide and 40% Sodium carbonate) is heated to a temperature of 870 to 930 C, and the work pieces contained in a wire basket are immersed in the molten bath of cyanide. The soaking period varies from component to component depending on the depth of the case, but generally, it varies from-10 minutes to 3 hours. Nitrogen produced in atomic form also dissolves on the surface and increase in hardness takes place due to the formation of nitrides. In nitriding, a portion of the surface to the parts to be kept soft is coated with such materials which are not affected by the bath. Careful handling of cyanides is needed as these salts are very poisonous. 3. Nitriding The heat treatment process which produces a hard-wear resistant layer of nitrides on a tough core of low carbon steel is known as nitriding. Process: The process is suitable for the steels containing 1% aluminium, 1.5% chromium and 0.2 per cent molybdenum. The percentage of carbon in these steels varies from 0.2 to 0.5. The process consists of heating machined and heat treated components to a temperature of 500 C for 40 to 90 hours in a gas tight box through which ammonia gas is circulated. The essential requirement of the operation is close Rohan Desai, Auto. Engg. Dept.NPK. Page 17

18 adherence to a temperature of 500 C. The component is allowed to cool in the furnace after switching of the supply of ammonia. When ammonia vapours come in contact with the steel, they get dissociated NH3 = 3H +N and nascent nitrogen so produced diffuses into the surface of the workpiece forming hard nitrides. Rohan Desai, Auto. Engg. Dept.NPK. Page 18

19 Difference between Carburizing and Nitriding. Carburizing Nitriding 1) Carburizing is a method of heat 1) Nitriding is a case hardening treatment by which carbon content at process by which nitrogen content at the surface of a ferrous material is the surface of steel is increased. increased. 2) High temperature (930 C). 2) Temperature employed =600 C. Quenching is done. Quenching is not required. 3) Hardening and tempering is 3) No need of hardening and needed. tempering. 4) This process is very simple and 4) This process is complex and inexpensive. expensive. 5) Grain refinement is not necessary. 5) Before nitriding, grain refinement is necessary. 4. Carbonitriding The process of producing a hard case by the addition of carbon and nitrogen on the surface of the steel. Process: Hydrocarbons, carbon monoxide and ammonia gases are used for Carbonitriding. Carbonitriding is carried out at a temperature of 800 to 875 C for 6 to 10 hours and the case depth obtained is 0.5 mm. Carbonitriding is applied to the low carbon steels (steels used for carburising). Nitrogen in the surface layer of the steel increases its hardenability and permits hardening in oil quenching. Thus, chances of distortion and cracking are eliminated. The portion of components which is not to be carbonitrided is protected by copper plating. SURFACE HARDENING Surface hardening involves the following two methods. 1. Flame hardening: The process of heating the metal with a flame of an oxyacetylene torch and is then almost immediately quenched is called as flame hardening. Rohan Desai, Auto. Engg. Dept.NPK. Page 19

20 Fig: Principle of flame hardening Process: The surface to be case hardened is heated by means of an oxyacetylene torch for sufficient time and Quenching is achieved by sprays of water which are integrally connected with the heating device. The heating is generally accomplished for sufficient time so as to raise the temperature of the surface of the specimen above the critical temperature. As the temperature desired is achieved immediately, spraying of water is started. In mass production work, progressive surface hardening is carried out where it is arranged to have the flame in progress along with quenching. Advantages: Selective surface can be hardened even on very large components. There is less distortion than in ordinary methods. Disadvantages: Temperature can not be precisely controlled. Hardening is restricted to parts which are affected by wear. 2. Induction hardening: The process of the surface hardening by inductive heating is known as induction hardening. Process: A high frequency current is passed through the inductor blocks which surround the surface to be hardened without actually touching it. The inductor block current induces current in the surface of the metal which the block surrounds. The induced eddy current and hysterisis losses in Rohan Desai, Auto. Engg. Dept.NPK. Page 20

21 surface material effect the heat required. When the surface, to be hardened, is heated upto a proper length of time, the circuit is opened and water is sprayed immediately on the surface for quenching. It is extensively used for hardening of crank shaft, cam shafts, axles and gears. Advantages: (i) Time required for this process is less (ii) Deformation is reduced. (iii) Hardening can be controlled by controlling the current (iv) Depth of hardening can be controlled. Disadvantages: (i) High equipment cost (ii) High maintenance cost (iii) Method is suitable only for large scale production. Rohan Desai, Auto. Engg. Dept.NPK. Page 21

22 Difference between Flame and Induction hardening Flame Hardening Material is heated with oxyacetylene flame at a required temperature, and then it is followed by water spraying. Holding time is required. Oxidation and decarburization are minimum. Irregular shape parts can be flame hardened. Flame hardening requires more care in control of temperature. Induction Hardening Material is heated by using high frequency induced current and then it is followed by water spraying. Due to very fast heating, no holding time is required. No scaling and decarburization. Irregular shape parts are not suitable for induction hardening. Easy control of temperature by control of frequency of supply voltage. 2.4 SELECTION AND APPLICATION OF HEAT TREATMENT PROCESS TYPES OF HEAT TREATMENT PURPOSE COMPONENT Annealing Isothermal annealing Normalizing Spheroidizing Hardening Tempering Carburizing Softening, and removing residual stress for post processes Transformation control hardness and micro-structure for machining. Control microstructure and hardness for machining Control microstructure and hardness for cold forming Applications demanding high strength wear resistance Optimize hardness for strength and toughness For fatigue strength and wear resistance through diffusion of Carbon and or Forged blanks for gearing and misc. parts Machinability control Reduce hardness for machining Reduce hardness Heavy gears, heavy duty crankshafts, automotive ball studs. Fasteners, Rods and Arms For fatigue and wear resistancegears and shafts, Same to above carbon Rohan Desai, Auto. Engg. Dept.NPK. Page 22

23 Nitriding Induction hardening Nitrogen at the surface of components and quench for case hardening Diffuse Nitrogen depending to impart wear and corrosion resistant nitride layer at surface and to get a deeper diffusion layer to improve fatigue strength Heat up by inductive power and quench to get hard case locally. steels Cam shafts, oil pump gears, valves, Brake pad liner plates, A/T gears Cam shafts, Drive shafts, steering knuckles Rohan Desai, Auto. Engg. Dept.NPK. Page 23