The tempering and Stress Relieving process takes place after the steel is hardened, but is no less important in the heat treatment of the component. The transformation to martensite through quenching creates a hard structure. Untempered martensite is often considered too brittle for commercial use, exposing low toughness and ductility and retaining a significant amount of residual stresses.
Appreciable surface-to-core temperature gradients during the heating and quenching
stages of induction hardening as well as the presence of phase transformations result in
transient stresses of substantial magnitude and the buildup of three-dimensional internal
residual stresses. The magnitudes and distributions of those stresses might not always be
desirable and need to be adjusted by applying tempering and stress relieving.
Similar to induction hardening, tempering by means of electromagnetic induction has
been found to be a viable commercial process, replacing conventional furnace/oven operations in many high-volume production applications.
Tempering is a form of subcritical heat treatment producing an attractive combination of
microstructures and mechanical properties. Some of the main purposes of tempering are
to provide a desirable combination of strength, toughness, and ductility and to relieve to
some degree internal residual stresses.