Hardening heat treatments increase the strength and wear resistance of carbon steel, alloy steel, and cast iron and steel parts.  After stamping, forming, and machining, many steel parts are hardened to produce the necessary strength for the final application.  These parts typically include castings, forgings, stampings, fasteners, bearings, tools, gears, and springs.  Hardening is preformed in internal oil quench furnaces with carbon controlled protective atmospheres.  Hardening heat treatments are typically performed in both batch and continuous furnaces, depending upon the size and geometry of the parts.

Hardening is also called quench and temper.  This refers to the oil quenching of steel parts, which produces the high hardness microstructure, and the accompanying tempering process, which removes some of the brittleness of the quenched part.  Some high alloy tool steels are hardened by air quenching, rather than oil quenching, in order to avoid some of the distortion associated with rapid quenching in oil and to reduce the risk of cracking high hardness tool steel parts.