A process usually following a hardening cycle. It is meant to increase the toughness while typically lowering the hardness of the alloy or component. Tempering is typically done under 1250F, but, that can vary depending upon the specific alloy being processed.
Final Stress Tempering is performed to remove machining stresses before a final machining operation to tight tolerances.
Thin plates and square shafts made from tool steels are very susceptible to warping during the hardening cycles.