Tempering is very often, and very incorrectly, confused with hardening. Fully hardened steel has its normal bcc
atomic configuration distorted into a tetragonal state due to the carbon atoms that were trapped in the interstitial spaces by the rapid cooling of the quench. This "unnatural" state, known as alpha martensite, is under much stress from the strain induced hardeness, also making it brittle. While this results in high abrasion resistance and strength, it renders the steel useless for operation requiring impact strength or toughness. To correct this, a "compromise" must be made through tempering. In the tempering process the martensitic steel is heated just enough to release some of the trapped carbon atoms to a desired degree, reducing stress and increasing toughness as it is transformed to beta martensite. Not only does this increase toughness and ductility but it reduces the chance of distortion or cracking from hardening. This is why tempering should be done as soon as possible after the martensitic transformation has completed. So, very contrary to the popular misuse of the word, tempering can actually be called the opposite of hardening. Different steels, due to varying alloy compositions, have different temperature ranges to achieve the desired results. In tempering the end result is effected by two factors, time and temperature. Of these, temperature has the most immediate and profound affect. Time at temperature has a more subtle effect, relieving stresses and increasing toughness with less loss of hardness.
Click on the links below for heat treating information on the steel types listed: