Increasing the mechanical resistance of metal materials by specifically changing and transforming the structure by means of heat treatment. Hardening is mainly classified by precipitation hardening (curing) and hardening by transformation processes (transformation hardening), e.g. by quenching (quench hardening).

The hardening process of iron alloys and steel is quench hardening. The precipitation of ferrite in the hypoeutectoid range or of cementite in the hypereutectoid range and perlite formation if the eutectoidequilibrium temperature is not reached is partly or completely suppressed by quick cooling.

Martensite with a typical needle structure is formed which imparts a high degree of hardness to the structure The cooling rate at which martensite is initially formed is referred to as lower critical cooling rate whereas the cooling rate at which exclusively martensite is formed is referred to as upper critical cooling rate (s. a. Martensite point).

The most important hardening methods are: Flame hardening, Case hardening, Interrupted quenching, Induction hardening, Nitriding, Immersion hardening and Hot-bath hardening. Hardening with subsequent tempering for the purpose of achieving adequate toughness at a certain tensile strength is referred to as Hardening and tempering.