Introduction to Basic Manufacturing Processes and Workshop Technology 8 CHAPTER

130 Introduction to Basic Manufacturing Processes and Workshop Technology 8 CHAPTER HEAT TREATMENT 8.1 INTRODUCTION Heat treatment is a heating and cooling process of a metal or an alloy in the solid state with the purpose of changing their properties. It can also be said as a process of heating and cooling of ferrous metals especially various kinds of steels in which some special properties like softness, hardness, tensile-strength, toughness etc, are induced in these metals for achieving the special function objective. It consists of three main phases namely (i) heating of the metal (ii) soaking of the metal and (iii) cooling of the metal. The theory of heat treatment is based on the fact that a change takes place in the internal structure of metal by heating and cooling which induces desired properties in it. The rate of cooling is the major controlling factor. Rapid cooling the metal from above the critical range, results in hard structure. Whereas very slow cooling produces the opposite affect i.e. soft structure. In any heat treatment operation, the rate of heating and cooling is important. A hard material is difficult to shape by cutting, forming, etc. During machining in machine shop, one requires machineable properties in job piece hence the properties of the job piece may requires heat treatment such as annealing for inducing softness and machineability property in workpiece. Many types of furnaces are used for heating heat treatment purposes. The classification of such heat treatment furnaces is given as under. 8.2 HEAT TREATMENT FURNACES 8.2.1 Hearth Furnaces These furnaces are heated by fuel which may be coke, coal, gas (town, blast or natural) and fuel oil. They can also be operated electrically. They are generally of two types. (a) Stationary type It consists of four types (1) Direct fuel fired furnace (2) Indirect fuel fired furnace (3) Multiple furnace (4) Re-circulation furnace 130 Heat Treatment 131 (b) Movable type It consists of two types (1) The car bottom type (2) The rotary type 8.2.2 Bath Furnaces In bath type furnaces, heating may be done using by gas, oil or electricity. These furnaces are further classified as: (1) Liquid bath type (2) Salt bath type (3) Lead bath type (4) Oil bath type 8.3 CONSTITUENTS OF IRON AND STEEL Fig. 8.1 shows micro structure of mild steel (0.2-0.3% C). White constituent in this figure is very pure iron or having very low free carbon in iron in form of ferrite and dark patches contain carbon in iron is chemically combined form known as carbide (Cementite). Cementite is very hard and brittle. Now if the dark patches of the above figure are further observed, a substance built up of alternate layer of light and dark patches is reflected in Fig. 8.2. These layers are alternatively of ferrite and cementite. This substance is called as pearlite and is made up of 87% ferrite and 13% cementite. But with increase of carbon content in steel portion of pearlite increases up to 0.8% C. The structure of steel at 0.8% C is entirely of pearlite. However if carbon content in steel is further increased as free constituent up to 1.5% C, such steel will be called as high carbon steel. The micro structure of high carbon steel is depicted in Fig. 8.3. Ferrite Cementite Crystals Areas Fig. 8.1 Micro structure of mild steel Fig. 8.2 Micro structure of Fig. 8.3 Micro structure of pearlitic eutectoid steel of high carbon steel 8.4 ALLOTROPY OF IRON In actual practice it is very difficult to trace the cooling of iron from 1600°C to ambient temperature because particular cooling rate is not known. Particular curve can be traced from temperature, time and transformation (TTT) curve. However allotropic changes observed during cooling of pure iron are depicted in Fig. 8.4. When iron is cooled from molten condition up to the solid state, the major allotropic changing occurs which are: 1539-1600°C Molten-Fe (Liquid state of iron) 1400-1539°C Delta-Fe (Body centered) 132 Introduction to Basic Manufacturing Processes and Workshop Technology 910-1400°C Gamma-Fe (FCC atomic arrangement and austenite structure) 770- 910°C Beta-Fe (Body centered-nonmagnetic) Up to 770°C Alpha-Fe (BCC atomic arrangement and ferrite structure) 1700 1539°C 1500 Delta Body Centered Iron 1404°C A4 1300 Gamma Face Centered Iron 1100 910°C A3 900 Temperature °C Beta Iron 768°C A2 700 Body Centered Alpha Iron 500 Time Fig. 8.4 Allotroic changes during cooling of pure iron (i) First changing occurs at l539°C at which formation of delta iron starts. (ii) Second changing takes place at 1404°C and where delta iron starts changes into gamma iron or austenite (FCC structure). (iii) Third changing occurs at 910°C and where gamma iron (FCC structure) starts changes into beta iron (BCC structure) in form of ferrite, leadaburite and austenite. (iv) Fourth changing takes place at 768°C and where beta iron (BCC structure) starts changes into alpha iron in form of ferrite, pearlite and cementite. Therefore, the temperature points at which such changing takes place into allotropic forms are called critical points. The critical points obtained during cooling are slightly lower than those obtained in heating. The most marked of these range commonly called the point of recalescence and point of decalescence. 8.5 TRANSFORMATION DURING HEATING AND COOLING OF STEEL When a steel specimen is heated, its temperature rises unless there is change of state or a change in structure. Fig. 8.5 shows heating and cooling curve of steel bearing different structures. Similarly, if heat is extracted, the temperature falls unless there is change in state or a change in structure. This change of structure does not occur at a constant temperature. It takes a sufficient time a range of temperature is required for the transformation. This range is known as transformation range. For example, the portion between the lower critical temperature line and the upper critical temperature line with hypo and hyper eutectoid Heat Treatment 133 steels, in iron carbon equilibrium diagram. This range is also known as critical range. Over heating for too long at a high temperature may lead to excessive oxidation or decarburization of the surface. Oxidation may manifest itself in the form of piece of scale which may be driven into the surface at the work piece if it is going to be forged. If steel is heated, well above the upper critical temperature, large austenite grains form. In other words steel develops undesirable coarse grains structure if cooled slowly to room temperature and it lacks both in ductility and resistance to shock. Molten Iron Austenite Austenite 0.5% C 1505 E E 1340 D D Austenite Ferrite Pearlite Ferrite Temperature °C Temperature 800 C C 721 A A B B Time Time Fig. 8.5 Heating and cooling curve of steel 8.6 IRON-CARBON EQUILIBRIUM DIAGRAM Fig. 8.6 shows, the Fe-C equilibrium diagram in which various structure (obtained during heating and cooling), phases and microscopic constituents of various kinds of steel and cast iron are depicted. The main structures, significance of various lines and critical points are discussed as under. 8.6.1 Structures in Fe-C-diagram The main microscopic constituents of iron and steel are as follows: 1. Austenite 2. Ferrite 3. Cementite 4. Pearlite 134 Introduction to Basic Manufacturing Processes and Workshop Technology δ Fe + liquid 1600 D δ iron A t1 1500 B Liquid H J t2 γ Solid Solution 1400 δ-Iron crystals + Austenite t Liquidus 1300 3 Liquid + (Austenite) 1200 Solidus Cementite C t4 E F 1130° 1100 Austenite Eutectic γ-iron Solidus 1000 Fe+Austenite Austenite Cementite G (αγ + ) + + 900 Acm Ledeburite Ledeburite A 3 800 A 1 P S 723 0.025% K 700 Austenite Temperature°C Carbon α-Iron Eutectoid + Fe 600 Cementite 500 400 Cementite Cementite + + 300 Fe + Pearlite Pearlite Ledeburite Pear- + + 200 lite Cementite Ledeburite 100 Q0.8 0 12344.3566.7 H ypo- H yper- eutectoid eutectoid Steel Cast Iron Carbon Percentage Fig. 8.6 Fe-C equilibrium diagram 8.6.1.1 Austenite Austenite is a solid solution of free carbon (ferrite) and iron in gamma iron. On heating the steel, after upper critical temperature, the formation of structure completes into austenite which is hard, ductile and non-magnetic. It is able to dissolve large amount of carbon. It is in between the critical or transfer ranges during heating and cooling of steel. It is formed when steel contains carbon up to 1.8% at 1130°C. On cooling below 723°C, it starts transforming into pearlite and ferrite. Austenitic steels cannot be hardened by usual heat treatment methods and are non-magnetic. Heat Treatment 135 8.6.1.2 Ferrite Ferrite contains very little or no carbon in iron. It is the name given to pure iron crystals which are soft and ductile. The slow cooling of low carbon steel below the critical temperature produces ferrite structure. Ferrite does not harden when cooled rapidly. It is very soft and highly magnetic. 8.6.1.3 Cementite Cementite is a chemical compound of carbon with iron and is known as iron carbide (Fe3C). Cast iron having 6.67% carbon is possessing complete structure of cementite. Free cementite is found in all steel containing more than 0.83% carbon. It increases with increase in carbon % as reflected in Fe-C Equilibrium diagram. It is extremely hard. The hardness and brittleness of cast iron is believed to be due to the presence of the cementite.

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