After hardening to a martensitic microstructure, steel has relatively low toughness. But this can be greatly improved through tempering, which is to say heating to between 160 and 650 ºC.

Tempering must be applied shortly after hardening and in some cases when the steel has been quenched or has cooled to 50 – 75 ºC, Depending on the carbon content and alloy content, some steels are susceptible to cracking if they are permitted to ool to as low as room temperature during cooling after hardening. This is because of the extreme tensile residual stresses that can form during quenching as a result of thermal gradients, phase transformation, differences in cross-section dimensions, decarburization or other chmical deviations. Since the untempered martensite is often brittle, these tensile stresses can result in cract formation.

Tempering at lower temperatures, up to 450 ºC, on parts with average surface finish requirements is normally carried out in convections furnaces and air atmospheres. At higher temeratures, and with higher demands on surface finish, a protective atmosphere such as nitrogen or a itrogen-hydrogen mixture is requried to prevent oxidation. In the cae of local tempering, the heat can be applied using a gas flame or be made inductively. When inductively heating thin sections very short tempering times are needed, 5 – 10 seconds, but at a considerably higher temperature than conventionally.

In many tempering processes the tempering itself can take place under protective atmosphere followed by oxidation which colors the material.

When steam tempering, water (vapor) or some other oxidizing medium is added to achieve a thick or thin oxide layer.

Ref. Steel and its Heat Treatment – A Handbook (8.11, 8.11.1)