Tempering

After hardening to a martensitic structure, the steel has a relatively low toughness. Through tempering, i.e. heating to temperatures between 150 and 650 ºC, the toughness can be significantly increased.

Tempering should be performed within a short time after hardening, and in some cases as soon as the steel has cooled to 50 – 75 ºC. Depending on the carbon content and alloy content, some steels are prone to cracking if they are allowed to cool all the way to room temperature in connection with cooling after hardening. The reason is that large tensile stresses can form during cooling due to thermal gradients, phase transformations, differences in cross-sectional thickness, decarburization or other chemical deviations. Since the untempered martensite is usually brittle, such residual tensile stresses can cause cracking.

When tempering at lower temperatures, up to 450 ºC with moderate surface finish requirements, the process usually takes place in a convection oven with an air atmosphere. At higher temperatures, or when the demands on surface finish are high, a protective gas atmosphere such as nitrogen or nitrogen/hydrogen mixtures is required to avoid oxidation.

In many tempering processes, the tempering itself can take place in protective gas with subsequent oxidation that colors the material.

Ref. Steel and heat treatment – A handbook (8.11, 8.11.1)