Friday, 26 February 2016

Case hardening

Case hardening :
          As mentioned previously, only those carbon steels can be hardened whose carbon content is about 0.25% or more.
How do we harden dead mild steel? The answer is by case hardening. In this process, the work piece is packed in charcoal and heated as in annealing. It is kept at that high temperature for a few hours. The result is that carbon enters into the surface of the work piece to the depth of a mm or two depending upon the heating time. The work piece now has a case where carbon percentage is as per requirement for hardening. It is then heated and quenched in the usual manner. The result is a component whose surface acquires hardness, but core remains soft and tough.
The Objects of  case hardening are:
  1. To obtain a hard and wear resistance surface on machine parts with enrichment of the surface layer with carbon to concentration of 0.75 to 1.2%.
  2. To obtain a tough core.
  3. To obtain close tolerance in machine parts.
  4. To obtain a higher fatigue limit and high mechanical properties in the core.
          Case hardening consists in heating a steel in the presence of a solid, liquid or gas, rich in carbon in order to enable the surface to be hardened, while retaining a tough ductile core.
          There are three methods of adding carbon to the surface of the metal:
1. Pack hardening               2. Liquid Carburizing               3. Gas carburizing.

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