Heat treatment: Types of heat treatment processes

 Heat treatment

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.

Objectives of Heat Treatment

The major objectives of heat treatment are given as under

1. It relieves internal stresses induced during hot or cold working.

2. It changes or refines grain size.

3. It increases resistance to heat and corrosion.

4. It improves mechanical properties such as ductility, strength, hardness, toughness, etc.

5. It helps to improve machinability.

6. It increases wear resistance

7. It removes gases.

8. It improves electrical and magnetic properties.

9. It changes the chemical composition.

10. It helps to improve shock resistance.

11. It improves weldability.

The above objectives of heat treatment may be served by one or more of the following heat treatment processes: 

Types of Heat treatment process

1. Normalizing

2. Anne aling.

3. Hardening.

4. Tempering

5.  Case hardening

(a) Carburizing

(b) Cyaniding

(c) Nitriding

6. Surface hardening

(a)  Induction hardening,

(b)  Flame hardening.

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