Tempering:
Hardened steels possess many undesired properties like internal stresses and high degree of brittleness. These are not desirable for certain service conditions. It is essential to achieve a relatively stable condition in such cases. Such a stable state can be achieved by providing thermal energy to the hardened steel. This process is called tempering. In this process the hardened steel is heated to a particular temperature below LCT followed by cooling the steel in air at any desired rate.
The process results in marginal reduction of hardness, strength and wear resistance. Hence the very purpose of hardening is challenged. But this is adequately compensated by the following factors.
- Reduction of internal stresses,
- Restoration of ductility and toughness and
- Transformation of restored austenite.
The higher the tempering temperature, the more is the restored ductility and tougher the steel. The tempering temperature should be selected in such a way that optimum combinations of these properties are achieved. Hardening followed by tempering is he only conventional process to improve elastic limit of steel.
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