Case hardening is a heat-treatment involving diffusion of carbon into the steel surface. This results in a part having a hard and wear resistant outer layer with a softer tougher core, a favourable combination for many applications, e.g. gears. Steels for case hardening have low carbon content and are supplied in a soft-annealed condition which means that they machine easily. Usually, a part is first machined and then case-hardened before finally being finished by grinding.

Even regular structural steels can be case hardened but for best properties, especially when parts are larger, it is necessary to alloy most often with different combinations of chromium, nickel and molybdenum.