Martensitic structure formed by direct quenching of high-carbon steel are hard and strong, but unfortunately are also brittle. They cannot be plastically deformed and have very little toughness, and although strong they are unable to resist impact loads and are extremely sensitive to stress concentrations. Some of the hardness and strength must be sacrificed to obtain suitable ductility and toughness. This is done by tempering the martensitic steel.
Tempering means giving up a certain amount of hardness but shedding a great deal of brittleness acquired in the process of hardening. It is a trade off between hardness and brittleness, so that hardened component may give useful service without failure. Tempering involves heating the carbon steel part to a temperature varying from 150°-600°C (depending upon how much trade off is required) and cooling the component in an oil or salt bath or even in air.
Thus tempering process is carried out to:
- Increase toughness,
- Decrease hardness,
- Stabilise structure,
- Relieve stress, and
- Change volume.
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