Case hardening

Case hardening is a method of hardening low carbon steels.  Low carbon steels have poor hardenability. 

In case hardening, a "case" of hard material is created around the relatively soft body. 

The case hardening process involves heating the component to a high temperature .  The hot steel component is immersed in a case hardening compound.  The case hardening compound  is high in carbon. 

The component is again heated and then quenched in water.  The process can be repeated till the desired depth of hardness is obtained.  In case hardening, the outer surface of the metal is hardened while the inner surface is kept soft.