Tempering
Tempering is very often, and very incorrectly confused with hardening. Fully
hardened steel (martensitic) 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, creates much stress resulting in the hardeness, but
also the brittleness, of martensite. While this results in high abrasion resistance
and strength, it renders the steel useless for operation requiring high 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 due to the internal stresses 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.
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