Case hardening is the process of hardening the surface of steel whilst leaving the interior unchanged. The idea behind case hardening is to have two different types of steel in the same item. This allows a relatively soft, tough core of a component to be combined with a hard (but potentially brittle) surface.

Both carbon and alloy steels are suitable for case-hardening providing their carbon content is low, usually up to a maximum of 0.2%. Case hardened steel is usually formed by diffusing carbon and/or nitrogen into the outer layer of the steel at high temperature.