Heat treatment. 4/22/20221

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. 4/22/20222

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. A hard material is difficult to shape by cutting, forming, etc. 4/22/20223

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 machinability property in work piece. Many types of furnaces are used for heating heat treatment purposes. 4/22/20224

5

Heat treatment process consists of three main phases namely: heating of the metal soaking of the metal cooling of the metal 4/22/20226

1. Heating stage: The primary objective in the heating stage is to maintain uniform temperatures. If uneven heating occurs, one section of a part can expand faster than another and result in distortion or cracking. Uniform temperatures are attained by slow heating. 2. Soaking stage: After the metal is heated to the proper temperature, it is held at that temperature until the desired internal structural changes take place. The length of time held at the proper temperature is called the soaking period. 4/22/20227

3.Cooling stage; After a metal has been soaked, it must be returned to room temperature to complete the heat-treating process. To cool the metal, you can place it in direct contact with a cooling medium composed of a gas, liquid, solid, or combination of these. The rate at which the metal is cooled depends on the metal and the properties desired. The rate of cooling depends on the medium; therefore, the choice of a cooling medium has an important influence on the properties desired. 4/22/20228

Objective of heat treatment The major objectives of heat treatment are given as: 1. It relieves internal stresses induced during hot or cold working. 2. It changes or refines grain size. 3. It improves mechanical properties such as ductility, strength, hardness, toughness, etc. 4. It helps to improve machinability. 5. It increases wear resistance 6. It removes gases. 7. It improves electrical and magnetic properties 8. It changes the chemical composition 9. It helps to improve shock resistance 10. It improves weldability 4/22/20229

Types of heat treatment Annealing: It is a softening process in which iron base alloys are heated above the transformation range held there for proper time and then cool slowly (at the rate of 30 to 150°C per hour) below the transformation range in the furnace itself. Heating is carried out 20°C above upper critical temperature point of steel in case of hypo eutectoid steel and the same degree above the lower critical temperature point in case of type eutectoid steel. 4/22/202210

Objectives of Annealing: The purpose of annealing is to achieve the following 1. Soften the steel. 2. Relieve internal stresses 3. Reduce or eliminate structural in-homogeneity. 4. Refine grain size. 5. Improve machinability. 6. Increase or restore ductility and toughness. 4/22/202211

1. Process annealing: In process annealing, ductility is increased with somewhat decrease in internal stresses. In this, metal is heated to temperature some below or close to the lower critical temperature generally it is heated 550°c to 650°c holding at this temperature and it is slowly cooled. This causes completely recrystallization in steel. 4/22/202212

2. Full annealing: The main purpose of full annealing of steel is to soften it and to refine its grain structure. In this, the hypo-eutectoid steel is heated to a temperature approximately 20°c to 30°c above the higher critical temperature and for hypereutectoid steel and tool steel is heated to a temperature 20 to 30°C above the lower critical temperature and this temperature is maintained for a definite time and then slowly cooled very slow1y in the furnace itself. 4/22/202213

3. Spheroidization: It is lowest temperature range of annealing process in which iron base alloys are heated 20 to 40°C below the lower critical temperature, held there for a considerable period of time e.g. for 2.5 cm diameter piece the time recommended is four-hours. It is then allowed to cool very slowly at room temperature in the furnace itself. This treatment is carried out on steels having 0.6 to 1.4% carbon. The objectives of spheroidising are given as: 1. To reduce tensile strength 2. To increase ductility 3. To ease machining 4. To impart structure for subsequent hardening process 4/22/202214

Normalizing: Normalizing is defined as softening process in which iron base alloys are heated 30 to 50°c above the upper-critical limit for both hypo and hyper eutectoid steels and held there for a specified period and followed by cooling in still air up to room temperature. Objectives: To 1. Soften metals 2. Refine grain structure 3. Improve machinability after forging and rolling 4. Improve grain size 5. Improve structure of weld 6. Prepare steel for sub heat treatment 4/22/202215

Comparison between Annealing and Normalizing S.No. Annealing Normalising 1 In this hypo-eutectoid steel is heated to a temperature approximately 20 to 30°C above the higher critical temperature and for hypereutectoid steel is heated 20 to 30°C above the lower critical temperature. this metal is heated 30 to 50°C above higher critical temperature. 2 It gives good results for low and medium carbon steels It also gives good results for low and medium carbon steels 3 It gives high ductility It induces gives higher ultimate strength, yield point and impact strength in ferrous material. 4 It is basically required to soften the metal, to improve machinability, to increase ductility improve, to refine grain size. It is basically required to refine grain size, improve structure of weld, to relieve internal stresses 4/22/202216

Hardening: The hardening treatment for most steels consists of heating the steel to a set temperature and then cooling it rapidly by plunging it into oil, water, or brine. Most steels require rapid cooling (quenching) for hardening but a few can be air-cooled with the same results. Hardening increases the hardness and strength of the steel, but makes it less ductile. Generally, the harder the steel, the more brittle it becomes. To remove some of the brittleness, you should temper the steel after hardening. 4/22/202217

Steel is hardened by heating 20-30°C above the upper critical point for hypo eutectoid steel and 20-30°C above the lower critical point for hyper eutectoid steel and held at this temperature for some time and then quenched in water or oil or molten salt bath. 4/22/202218

Tempering: After the hardening treatment is applied, steel is often harder than needed and is too brittle for most practical uses. Also, severe internal stresses are set up during the rapid cooling from the hardening temperature. To relieve the internal stresses and reduce brittleness, you should temper the steel after it is hardened. The purpose of tempering is to reduce the brittleness imparted by hardening and to produce definite physical properties within the steel. 4/22/202219

Low Temperature Tempering: Hardened steel parts requiring tempering are heated up to 200°c and then quenched in oil. Tempering is used to retain hard micro-structure of martensite which increases brittleness. Medium Temperature Tempering: Hardened steel parts requiring tempering are heated in the temperature range of °c. This process gives troosite structure. Troosite structure is another constituent of steel obtained by quenching tempering martensite. High Temperature Tempering; Hardened steel parts requiring tempering are heated in the temperature range of °c. 4/22/202220

Case hardening: Case hardening produces a hard, wear-resistant surface or case over a strong, tough core. The principal forms of casehardening are carburizing, cyaniding, and Nitriding. Only ferrous metals are case-hardened. (i) Carburizing : Carburizing is a case-hardening process by which carbon is added to the surface of low-carbon steel. This results in carburized steel that has a high-carbon surface and a low-carbon interior. (ii) Cyaniding : This process is a type of case hardening that is fast and efficient. Preheated steel is dipped into a heated cyanide bath and allowed to soak. Upon removal, it is quenched and then rinsed to remove any residual cyanide. 4/22/202221

Application: Cyaniding is generally applied to the low carbon steel parts of automobiles (sleeves, brake cam, speed box gears, drive worm screws, oil pump gears etc), motor cycle parts (gears, shaft, pins etc.) and agriculture machinery. iii) Nitriding : This case-hardening method produces the hardest surface of any of the hardening processes. It differs from the other methods in that the individual parts have been heat-treated and tempered before nitriding. The parts are then heated in a furnace that has an ammonia gas atmosphere. No quenching is required so there is no worry about warping or other types of distortion. 4/22/202222

Flame hardening: It consists of moving an oxyacetylene flame, over the part where hardening is required. Immediately after this, the heated portion is quenched by means of water spray or air passing over it. Induction hardening: Induction hardening is accomplished by placing the part in a high frequency alternating magnetic field. It differs from surface hardening in the way that hardness of surface is not due to the increase in carbon content but due to rapid heating followed by controlled quenching. 4/22/202223

Advantages: Induction hardening is comparatively quicker. A minimum distortion or oxidation is encountered because of the short cycle time. The operation is very fast and comparatively large parts can be processed in a minimum time. Application: Induction hardening is widely used for hardening surfaces of crankshafts, cam shafts, gear automobile components, spline shafts, spindles, brake drums etc. It is also used for producing hard surfaces on cam, axles, shafts and gears. 4/22/202224

Difference between Flame and Induction Hardening: Flame hardening and induction hardening methods have the same purpose of obtaining hard and wear surface whilst the core remains soft. The main difference between them is in the manner or the mode of heating. In the induction hardening heating by high frequency current is accomplished. This way 750°C to 800°C temperature is obtained in the metal. Flame hardening method is cheaper as initial investment in this process is less in comparison to induction hardening method. However, same equipment's can be used for all sizes of specimen in induction hardening process. 4/22/202225

In induction hardening, the hardness depth is controlled very accurately by using different frequencies and a method is very clean and quick in comparison of flame hardening method. Induction hardening method is generally used for crank shaft shafts, gears, pinions and a wide range of automobile and tractor components. Flame hardening method is generally used for local hardening of components such as hardening of gear wheel teeth only. 4/22/202226