Tempering

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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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For more information on annealing common steels follow the links below:
1080
1084
1095
5160
52100
O1
L6
W1
W2

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