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Case hardening (surface hardening) process

The case hardening process is used to harden the outer surface of the material, the inner surface of the material remains soft. hence this process is also called surface hardening.

The type of case hardening process is as follows

  1. Carburizing

  2. Nitriding

  3. Cyaniding

Carburizing

  • In the carburizing process, specimens are heated beyond the upper critical temperature in sealed containers having an atmosphere of carbon (C).

  • Heating is continued for 4 to 10 hours, depending upon the depth of penetration required. Hence, carbon diffuses into the surface layer.

Nitriding

  • The nitriding process is similar to carburizing, but instead of carbon (C), nitrogen (N₂) is used as a hardening agent in nitriding. The nitriding process is carried out in the ammonia atmosphere.

  • The nitriding process requires less temperature compared to carburizing but requires more heating time compared to carburizing.

Cyaniding

  • In the cyaniding process, sodium cyanide (NaCN) is used as a hardening agent. Hence both carbon (C) and nitrogen (N₂) can effectively penetrate the surface.

Comparison

Comparing all 3 described hardening processes, surface hardness increases due to nitrogen (N₂) and is more compared to carbon (C) agent.

Hence, in term of surface hardness achieved,

Nitriding > cyaniding > carburising
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