In which of the following process does mild steel absorb carbon and nitrogen to obtain a hard surface ?

Option 3 : Cyaniding

Concept:

Case hardening is a method used to harden the outer surface of low-carbon steel while leaving the center or core soft and ductile. Case hardening involves heating the metal to its critical temperature in some carbonaceous material. The following methods are commonly used:

1. Pack method
2. Cyaniding
3. Nitriding
4. Induction Hardening
5. Flame hardening

Cyaniding:

• In this process of surface hardening, both carbon and nitrogen are added to the surface layer of steel (ferrous material, usually low carbon grade).
• The process is based on the decomposition of cyanide compounds that easily release the cyan group (CN). Cyaniding involves heating the steel in a liquid or solid medium.
• The steel is heated in a molten cyanide salt bath maintained at 950°C, followed by water or oil quenching.
• Salt bath compositions may vary according to the temperature of the salt, the thickness of the case to be obtained, type of steel to be heat-treated, and period of operation.
• Case thickness from 0.075 – 1.5 mm can be obtained in the process.

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Heat treatment is an operation involving heating and cooling of a metal or alloy so as to obtain certain desirable properties. A few important heat treatment processes are:

1. Annealing
2. Normalizing
3. Hardening
4. Tempering