Surface Hardening of Steels

Surface hardening is a heat treatment process used to improve the hardness and wear resistance of the surface layer of steel components. It involves altering the composition and microstructure of the surface layer while maintaining the toughness of the core.

The following heat treatment processes are commonly used for surface hardening of steels:

1. Carburizing:
Carburizing is a process in which carbon is diffused into the surface layer of steel. It involves heating the steel in a carbon-rich atmosphere at high temperatures for an extended period. The carbon atoms diffuse into the surface, forming a high-carbon layer known as the case. This case provides enhanced hardness and wear resistance, while the core remains tough and ductile.

2. Cyaniding:
Cyaniding is a case-hardening process that involves introducing carbon and nitrogen into the surface layer of steel using a molten cyanide salt bath. The steel is heated in the bath at temperatures between 760°C and 870°C for a short duration. Carbon and nitrogen atoms diffuse into the surface, forming a hard and wear-resistant compound layer known as the case. Cyaniding is particularly effective for low carbon steels.

3. Annealing:
Annealing is a heat treatment process used to soften and refine the grain structure of steel. It involves heating the steel to a specific temperature and holding it at that temperature for a certain time, followed by slow cooling. Annealing helps relieve internal stresses, improve machinability, and enhance ductility. However, it does not contribute to surface hardening.

4. Carbonitriding:
Carbonitriding is a case-hardening process that introduces both carbon and nitrogen into the surface layer of steel. It is similar to carburizing but involves the use of a nitrogen-rich atmosphere. The steel is heated at temperatures between 760°C and 930°C for a specified time. Carbon and nitrogen atoms diffuse into the surface, forming a compound layer that is harder and more wear-resistant than the core.

Therefore, the correct answer is options A, B, and D - Carburizing, Cyaniding, and Carbonitriding are the heat treatment processes used for surface hardening of steels.