Case hardening steel or surface hardening steel is the steel 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 steel case hardening process involves infusing additional carbon into the case. Steel 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.

Case hardening steel is done through a simple method of hardening steel. It is less complex than hardening and tempering. This techniques is used for steels with a low carbon content. Carbon is added to the outer surface of the steel, to a depth of approximately 0.03mm. One advantage of this method of hardening steel is that the inner core is left untouched and so still processes properties such as flexibility and is still relatively soft.

STAGE ONE:
The case hardening steel is heated to red heat. It may only be necessary to harden one part of the steel and so heat can be concentrated in this area.

STAGE TWO:
The case harden steel material is removed from the brazing hearth with blacksmiths tongs and plunged into case hardening compound and allowed to cool a little. The case hardening compound is high in carbon.

STAGE THREE:
The case harden steel material is heated again to a red colour, removed from the brazing hearth and plunged into cold, clean water.

Because hardened metal is usually more brittle than softer metal, through-hardening (that is, hardening the metal uniformly throughout the piece) is not always a suitable choice for uses where the metal part is subject to certain kinds of stress. In such circumstances, case-hardening can provide a part that will not fracture (because of the soft core that can absorb stresses without cracking) but also provides adequate wear resistance on the surface.