tempering

Fresh martensite is like glass: very hard but liable to shatter. Tempering is gently reheating that quench-hardened steel to a moderate temperature (well below the transformation temperature) and holding, so it relaxes just enough. You give up a little hardness to buy back a lot of toughness. It is the shock-it-hard-then-ease-it-back second step of heat treatment.

Reheating (typically 200 to 650 degrees C) lets the carbon trapped in the BCT martensite finally diffuse out and form tiny, well-dispersed cementite particles in a ferrite matrix (tempered martensite). Higher tempering temperature and longer time mean coarser carbides, lower hardness, and higher toughness. So the engineer dials the balance: temper low (say 200 degrees C) for a hard, wear-resistant edge that keeps most of the hardness; temper high (say 600 degrees C) for a tough, shock-resistant part like a gear or shaft.

Tempered martensite gives the best all-round strength-plus-toughness combination available in steel, which is why quench-and-temper is the backbone of structural steel heat treatment. One caveat: some steels suffer temper embrittlement if held in or slow-cooled through certain ranges, so tempering schedules are not arbitrary; the temperature window matters.