Which of the following heat treatment processes is/are used for surface hardening of steels?

The correct options/answers are Carburizing, Cyaniding, and Carbonitriding.

Here is a step-by-step explanation of why these heat treatment processes are used for surface hardening, while the remaining process is not:

1. Understanding Surface Hardening (Case Hardening):
Surface hardening is a heat treatment process used to increase the wear resistance, hardness, and fatigue life of the outer layer (case) of a steel component, while maintaining a tough, ductile, and impact-resistant inner core. This is typically achieved by diffusing carbon, nitrogen, or both into the surface layer of the steel at elevated temperatures.

2. Analyzing the Options:

• Carburizing: This process involves heating low-carbon steel in a carbon-rich environment (solid, liquid, or gas) to a temperature where carbon diffuses into the outer surface. This increases the surface carbon content, allowing it to be hardened by subsequent quenching and tempering. Hence, it is a surface hardening process.


• Cyaniding: This is a liquid bath method where the steel component is heated in a molten cyanide salt bath containing sodium cyanide ($\mathrm{NaCN}$). During this process, both carbon and nitrogen diffuse simultaneously into the surface of the steel, forming a hard, wear-resistant outer layer. Hence, it is a surface hardening process.


• Carbonitriding: Similar to cyaniding, carbonitriding is a gas-phase process where both carbon and nitrogen are diffused into the surface of the steel by heating it in a gaseous atmosphere consisting of hydrocarbons (like methane or propane) and ammonia (${\mathrm{NH}}_3$). This produces a hard wear-resistant case. Hence, it is a surface hardening process.


• Annealing: In contrast to the processes above, annealing is a full-body heat treatment process. It involves heating the steel to a high temperature, holding it at that temperature, and then cooling it very slowly (typically inside the furnace). The primary objective of annealing is to soften the steel, relieve internal stresses, improve ductility, and refine the grain structure, rather than hardening the surface.

Conclusion:
Therefore, among the given options, Carburizing, Cyaniding, and Carbonitriding are surface hardening methods, whereas Annealing is a softening process.