Case hardening

A method of heat treatment of steel in which the surface of the metal is hardened while its inside stays comparatively soft and tough. Case hardening may be done by causing the steel on the surface to absorb extra carbon or by changing the surface into a very hard chemical compound called aluminum nitride. Case hardening is used on such steel parts as engine crankshafts. Only the surfaces where the bearings ride are made very hard and brittle. The inside of the crankshaft remains tough and strong.