(introduction...)

The steel becomes hard an brittle by the quenching process after heating. So high stresses can occur in the structure of the material that cracks are produced and the material slivers to pieces like glass.

In order to eliminate those negative effects and give the material the "useful hardness", it is tempered after having been hardened, i.e. it is heated once again. The toughness of the material is increased again at a justifiable decrease of the hardness and strength.

Tempering temperatures relate to the purpose of use of the workpiece.

The higher the tempering temperature, the lower the hardness and the tougher the steel.

When a blank steel is heated, a 0.2 mm thick oxide layer is produced on the surface, this oxide layer becomes discoloured in dependence on the temperature.

Apart from temperature gauges, the temperature can also be estimated by the colour.

Tempering colours for plain carbon steels: