2.10.4 Tempering
Martensitic structures formed by direct quenching of high carbon steel are hard and
strong but also brittle. They contain internal stresses which are severe and unequally
distributed to cause cracks or even fracture of hardened steel. The tempering is carried
out to obtain one or more of the following objectives:
(1) To reduce internal stresses produced during heat treatment operations.
(2) To stabilize the structure of metal.
(3) To make steel tough to resist shock and fatigue.
(4) To reduce hardness and improve ductility.
Thus, tempering consists of heating quenched hardened steel in martensitic condition
to a temperature below lower critical temperature, holding it at that temperature for
sufficient time and then cooling it slowly down to room temperature. Tempering is
classified into the following three types:
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