Tempering

The martensite which is formed during quenching is too brittle and hence cannot be used in many cases, the residual stresses are also developed during martensite formation hence hardening should be followed by tempering. Tempering consists of heating the hardened steel to a temperature below the lower critical temperature holding it for sometime and then cooling slowly. It is the final operation of heat treatment.

Advantages of Tempering :

1. Residual stresses are relieved.
2. Ductility is improve
3. Toughness is increased

(a) The higher the tempering temperature, the more will be the residual stresses in which hardness is reduced and toughness is increased at high temperature. The work is cooled slowly after tempering. The cooling rate considerably affect the residual stress.
(b) The slower the cooling the lesser will be the stresses. Rapid cooling in water develops new thermal stress.

Tempering can be classified into :

1. High Temperature Tempering (Sorbite) : It is employed at 500 – 650°C. The resulting structure consist of solubility which gives good strength and toughness.
2. Medium Temperature Tempering (Troostite) : This type of tempering is employed at 350 – 500°C the resulting steel structure consists of tempered troostite.

After tempering the work is cooled in water. This increases the endurance limit in case of spring.

3. Low temperature Tempering : The temperature used in this process is 250°C and the holding time is 1.3 hours. This process reduces internal stresses, increases strength and toughness and provide high wear resistance. This method is employed in measuring tools and cutting tools. This method is also employed to the compound which are surface hardened by carburizing and nitriding.