Tempering is used to increase hardness in steel that has been through hardened by heating it to form austenite and then quenching it to form martensite. During the tempering process, the steel is heated to a specific temperature and then cooled slowly. This process allows for the transformation of the martensite structure, which is very hard but brittle, into a structure that is tougher and less prone to cracking under pressure.
The increase in hardness during tempering is due to the formation of fine carbide particles within the steel. These carbide particles act as obstacles to dislocation movement, which is the primary mechanism of plastic deformation in metals. As a result, the steel becomes harder and more resistant to deformation.
Tempering also helps to relieve internal stresses that may have been introduced during the hardening process. When steel is rapidly cooled during quenching, thermal gradients are formed within the material, leading to non-uniform cooling and the development of internal stresses. Tempering allows for the redistribution of these stresses, resulting in a more uniform and stable microstructure.
In addition to increasing hardness, tempering also improves the toughness of the steel. Toughness is the ability of a material to absorb energy and deform plastically before fracturing. During tempering, the steel undergoes a process called temper embrittlement, which involves the precipitation of carbides at grain boundaries. This reduces the cohesion between the grains and makes the material more ductile, thereby improving its toughness.
The tempering temperature and duration can be adjusted to achieve the desired balance between hardness and toughness. Higher tempering temperatures result in greater toughness but lower hardness, while lower tempering temperatures result in higher hardness but lower toughness.
Overall, tempering is an essential step in the heat treatment of steel to achieve the desired mechanical properties. It increases hardness by forming carbide particles, relieves internal stresses, and improves toughness by reducing embrittlement. By carefully controlling the tempering process, engineers and manufacturers can tailor the properties of the steel to meet specific application requirements.
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