Heat Treatment of Steel:
It may be defined as the process of carefully heating the steel to a high temperature followed by cooling to room temperature under controlled conditions. Heat treatment of steel is done for two purposes.
i. To develop certain special properties like hardness, strength, ductility, etc. without changing chemical composition.
ii. To remove some undesirable properties or gases like entrapped gases, internal stresses, and strain.
1. Annealing:
It is a process of heating steel to reduce followed by slow cooling. Annealing means softening. It increases machinability and also removes imprisoned gases and internal stresses.
2. Hardening or Quenching:
It is a process of heating steel to redness followed by sudden cooling by plunging the red hot steel into water or oil. It produces steel of greater hardness.
3. Tempering:
It is a process of heating the already hardened steel to a temperature much below redness (200 -350°C) followed by slow cooling. Tempering removes any stress and strain that might have developed during quenching. By tempering, the brittleness and also some hardness is reduced but toughness and ductility are simultaneously increased. Cutting tools like blades, chisels, cutters, etc. always require the tempering of steel.
4. Nitriding:
It is a process of heating steel at ~ 700°C in an atmosphere of NH3. This process imparts of hard coating of iron nitride (Fe4N) on the surface of the steel. It is a process of getting a super-hard surface and can only be employed effectively for alloy steel and is used for cylinder bores.