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:
5. Case hardening
6. Surface hardening
(a) Induction hardening,
(b) Flame hardening.
Fig. 1 shows the heating temperature ranges for various heat treatment processes.