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During heating or soaking in various hot-working processes, the oxidizing atmosphere causes all or part of the carbon at the steel surface to be lost. This phenomenon is called decarburization. The decarburized layer depth is the distance from the surface of the decarburized layer to the position where the metallographic structure can no longer be distinguished from the base metal.
Decarburization of the steel surface greatly reduces surface hardness, tensile strength, wear resistance and fatigue limit. For this reason, standards for tool steels, bearing steels and spring steels all specify limits on the decarburized layer. Critical mechanical components must not contain decarburization defects, so the decarburized layer must be completely removed during machining.
Decarburization is a surface defect of steel. For most industrial steels, especially high-carbon tool steels, chromium ball-bearing steels, spring steels and certain medium-carbon structural steels for critical applications, the depth of the decarburized layer is strictly limited. Steel standards require inspection of decarburization and specify the permitted decarburized layer depth.