The process of giving the requisite degree of hardness or softness to a substance, as iron and steel; especially, the process of giving to steel the degree of hardness required for various purposes, consisting usually in first plunging the article, when heated to redness, in cold water or other liquid, to give an excess of hardness, and then reheating it gradually until the hardness is reduced or drawn down to the degree required, as indicated by the color produced on a polished portion, or by the burning of oil.
A heat treatment process where a steel that may have been heat treated to harden it is reheated to modify it's structure. See also: Colour Temperatures, Hardening. Search for steel treatment books on Amazon.co.uk
Tempering (aka quench tempering) is the process for making material harder. This method has been known for hundreds of years but was only perfected in the last century. The metal is heated to a specific temperature and rapidly cooled (quenched) in a bath of water, brine, oil, or air to increase its hardness. See also annealing, normalizing, stress relieving