In steels, tempering is reheating hardened steel to some temperature below the lower critical temperature for the purpose of decreasing hardness and increasing toughness.
(The lower critical temperature is the temperature of the austenite-to-pearlite eutectoid transformation in steels- below this temperature austenite does not exist.)
Tempering is also sometimes applied to normalized steels. For the same reasons- decrease hardness and improve toughness.
The chart above shows the colors that are elicited by tempering a 0.95% carbon content steel at the temperatures shown. (Think drill rod.)
I saved this chart back in my youth from a Bethlehem Steel Handbook.
This is what we here at PMPA call “Knowledge Retention” and “Tools You Can Use.”