Case Hardening

Case hardening is a heat treatment process used to increase the surface hardness of a metal while maintaining its core toughness. It involves adding a hard outer layer to a soft and ductile core, thus improving the wear resistance of the material. Cyaniding and nitriding are two common methods of case hardening.

Cyaniding

Cyaniding is a case hardening process that involves introducing carbon and nitrogen into the surface layer of a metal. The metal is heated in a cyanide bath at temperatures ranging from 800 to 950°C (1472 to 1742°F). The cyanide bath contains sodium cyanide and a source of carbon, such as charcoal or sodium carbonate. The carbon and nitrogen diffuse into the metal's surface, forming a hard and wear-resistant case.

The cyanide bath is usually maintained in a controlled atmosphere to prevent the formation of cyanide gas, which is highly toxic. The process is typically carried out for a few hours, depending on the desired case depth. After cyaniding, the metal is quenched in oil or water to rapidly cool it and lock in the hardened case.

Nitriding

Nitriding is another case hardening process that involves the introduction of nitrogen into the surface layer of a metal. Unlike cyaniding, which introduces both carbon and nitrogen, nitriding focuses only on nitrogen diffusion. The metal is heated in an atmosphere of ammonia gas at temperatures ranging from 450 to 600°C (842 to 1112°F).

During nitriding, the ammonia dissociates into nitrogen and hydrogen. The nitrogen atoms diffuse into the metal's surface, forming nitrides in the microstructure. These nitrides increase the hardness and wear resistance of the material. The process is carried out for an extended period of time, typically ranging from 10 to 100 hours, depending on the desired case depth.

After nitriding, the metal is often cooled slowly to room temperature to relieve residual stresses. This slow cooling process is known as "ageing" or "nitriding age."

Conclusion

In conclusion, cyaniding and nitriding are two methods used in the case hardening process. Cyaniding involves introducing carbon and nitrogen into the metal's surface, while nitriding focuses on nitrogen diffusion. Both methods enhance the wear resistance and hardness of the material, making it suitable for applications where surface durability is essential.