Case Hardening (Carburising)

What is case hardening?

Case hardening means carburising, hardening and tempering the surface layer of steel. During carburising, the surface of the part is enriched to 0.5–0.85% carbon in a controlled, carbon-rich gas atmosphere at about 900–970 °C, after which the surface layer is ready for hardening. Hardening, for example in an oil bath, is applied before tempering at around 200 °C.

Areas that should not be carburised are coated with a heat-resistant protective compound. Case hardening provides a wear-resistant 45–65 HRC surface with a tough core. Typical case-hardened parts include transmission shafts and gear shafts.

Effect on fatigue strength

During hardening, the carburised martensite in the outer layer expands much more than the core, creating residual compressive stresses at the surface. This improves the fatigue strength of the parts.

Residual compressive stresses counteract tensile stresses under bending and torsion, so cracks can only start at high stress levels and existing cracks grow slowly.

Which steels and parts?

Case hardening is applied to low-carbon steels (about 0.10–0.25% carbon): case-hardening steels such as C15, 16MnCr5, 20MnCr5, 20NiCrMo2-2 (8620) and 18CrNiMo7-6, and low-carbon free-cutting steels. Gears, pinions, gearbox and gear shafts, cam and piston pins and sprockets are typical parts. Distortion after heat treatment is corrected by straightening, and precise dimensions are finished by grinding.

For a quote

State the steel grade, the required surface hardness and effective case depth (e.g. 0.8–1.0 mm), and mark the areas that must not be carburised on the drawing when you fill in the quote form.