The process of hardening the surface of steel whilst leaving the interior unchanged. Both carbon and alloy steels are suitable for case-hardening providing their carbon content is low, usually up to a maximum of 0.2%. Components subject to this process, particularly in the case of alloy steels, have a hard, wear-resistant surface with a tough core.
Adding carbon to the surface of iron-base alloys by absorption through heating the metal at a temperature below its melting point in contact with carbon material. The oldest method of surface hardening wrought iron and other low-carbon steels by packing it in charcoal or some other carbon-based material, then heating it for several hours above 1650.