In metallurgy, case-hardening is any process applied to the surface of mild steel to increase its wear resistance and surface hardness. Most commonly, the steel is packed around with powdered charcoal and heated for a certain period, so that the surface regions absorb carbon. The steel is then quenched (rapidly cooled) with water. Other techniques include induction hardening and flame hardening; and nitriding, used for certain suitable steels, where the steel is heated in ammonia gas or molten cyanide salts so that nitrogen is diffused into its surface regions.