case-hardened

Case-hardened steel has a thin, extremely hard outer layer bonded to a softer, more ductile interior. The surface can reach 58 to 65 HRC (Rockwell hardness), while the core remains at 25 to 40 HRC. This combination gives wear resistance where it matters most, while preserving the material's ability to absorb impact without shattering. The contrasting hardnesses are not surface coating but a genuine metallurgical transformation of the steel itself.

The process works by introducing carbon or nitrogen into the outer layers of low-carbon steel (typically 0.1 to 0.3 percent carbon). Common methods include carburizing (heating the part in a carbon-rich atmosphere), nitriding (exposure to nitrogen at high temperature), and carbonitriding (both elements). The part is then quenched rapidly to lock in the hardness. Depth of the hardened case ranges from 0.5 mm for light work to 2.5 mm for heavily loaded components.

Where it matters and where it fails

Gears, bearing races, cam shafts, and fasteners are case-hardened because they need hardness against wear but cannot be made from brittle fully-hardened steel without fracturing under shock loads. A case-hardened gear tooth resists pitting; the softer core absorbs the bending stress. The problem arises if the case is too thin (inadequate life), too deep (residual stress and distortion), or unevenly applied. Decarburization, where carbon burns away during heating, leaves a soft skin and defeats the purpose.

The term itself reflects the industrial logic: the outer case is hardened while the core is not. It distinguishes this work from through-hardening, where the entire cross-section reaches high hardness. Case hardening is older than the name; armorers carburized sword edges for centuries. Modern manufacturing relies on it wherever small-scale hardness and large-scale toughness must coexist in the same part.