Case-hardening or
surface hardening is the process of
hardening the surface of a metal object while allowing the metal deeper underneath to remain soft, thus forming a thin layer of
harder metal (called the "case") at the surface. For iron or steel with low
carbon content, which has poor to no
hardenability of its own, the case-hardening process involves infusing additional carbon into the case. Case-hardening is usually done after the part has been formed into its final shape, but can also be done to increase the hardening element content of
bars to be used in a
pattern welding or similar process. The term
face hardening is also used to describe this technique, when discussing modern
armour.