Oxidation of the surface during tempering of steel leads to occurrence of tempering colors, which are used for assessment of the tempering temperature (Fig. 1).

However, assessment of the tempering temperature on the basis of the tempering color is only an approximate method. The final color is affected by alloy elements and the tempering duration. For example, corrosion-free steel types are less prone to oxidation and thus require higher temperatures for achieving a specific tempering color.