Tempering is the process of reheating the steel leading to precipitation and spheroidisation of the carbides. The tempering temperature and time are generally controlled to effect the final properties required of the steel. The benefits resulting are the increase in the metal toughness and elongation. The negative effects are the reduction of the martensite (BCT) structure and the progression towards a spheroidal carbide + ferrite matrix structure.
Tempering steel is the process where an already hardened or normalized steel part is heated to a temperature below the lower critical temperature and cooled at a controlled rate to increase the ductility and toughness. Steel is tempered by reheating after hardening to obtain specific mechanical properties and also to relieve quenching stresses and to reduce dimensional instability. Tempering usually follows quenching from above the upper critical temperature; however, tempering is also used to relieve the stresses and reduce the hardness developed during welding and to relieve stresses induced by forming and machining.
The following affecting tempering : Tempering is affects the microstructure and the mechanical properties of a tempered steel
Time at temperature
Cooling rate from the tempering temperature
Steel Chemical Composition, i.e carbon content, alloy content, and other elements
In steel quenched to a microstructure consisting essentially of martensite, the iron lattice is strained by the carbon atoms, producing the high hardness of quenched steels. Upon heating, the carbon atoms diffuse and react in a series of distinct steps that eventually form Fe3C or an alloy carbide in a ferrite matrix of gradually decreasing stress level. The properties of the tempered steel are primarily determined by the size, shape, composition, and distribution of the carbides that form, with a relatively minor contribution from solid-solution hardening of the ferrite. These changes in microstructure usually decrease hardness, tensile strength, and yield strength but increase ductility and toughness.
Under some certain conditions, the hardness may unaffected by tempering or even be increased as a result of tempering. For example, tempering a hardened steel at very low tempering temperatures may cause no change in hardness but may achieve a desired increase in yield strength. Also, those alloy steels that contain one or more of the carbide-forming elements (chromium, molybdenum, vanadium, and tungsten) are capable of secondary hardening; that is, they may become somewhat harder as a result of tempering.