TEMPERING WITH INDUCTION
Tempering is a heat treatment process that usually follows hardening. Its purpose is to relieve some of the excess hardness and the associated internal stresses after hardening. Tempering generally results in an increase in strength, by avoiding load-related brittle fracture. Tempering is conventionally a furnace process in which workpieces are treated for approx. 1-2h at a temperature of approximately 180-300°C, while quenching and tempering are also performed at temperatures up to 700°C.
In induction hardening, short-time austenitising is used, i.e. brief induction heating to a higher temperature level in order to achieve a result comparable to that of lengthy furnace heating. The same effect is also achieved by tempering at a higher temperature. Treatment times are thereby reduced to a few seconds. Especially if the heating for hardening and tempering can be carried out by the same inductor, the productivity benefits are high.
A variant of tempering is self-annealing or tempering with residual heat. This uses the residual heat still present in the workpiece after a shortened quenching time. This heat is still present at depth in the workpiece despite the quenching, and then distributes itself evenly over the workpiece cross-section.
The selection of the correct tempering behaviour during inductive tempering or residual heat tempering must be precisely adjusted on a workpiece-specific basis.