Hardening is a heat treatment consisting of a combined heating and cooling process with the aim of producing a martensitic structure in a steel or cast-iron part. This structure is realized when  a sufficiently high temperature (> 850°C) is reached during heating. The temperature depends on the initial structure and the heating rate. With induction it is possible to achieve a good transformation within  a very short heating time (in some cases < 1s). However, a hardened structure can only be achieved if the cooling ("quenching") is sufficiently fast.

Induction hardening usually only heats the contour of a workpiece. The requirements for the hardened area of the workpiece are described in detail and the workpiece drawings shows the areas in which the hardness, hardness depth and sometimes also a structural requirement are defined (the so-called hardness zone), together with other areas that must not be hardened. Hardening causes residual compressive stresses in the area near the surface and thus stabilizes the workpiece against bending loads.

A distinction is made between total surface hardening (single-shot) and progressive hardening (scanning). With single-shot hardening, the entire hardening zone is heated and then quenched, while with scanning, the hardening zone progresses by a relative movement between the workpiece and the combined inductor and quenching shower. Single-shot hardening has the advantage of a short treatment time, but also requires more power and quenching water volume.