Case hardening (carburising) is a thermochemical treatment that enriches the surface layer of a low-carbon steel with carbon. The part is brought to a high temperature, generally around 900 °C, in a carburising medium, most often gas, sometimes at low pressure. The carbon diffuses from the surface inwards. The part is then quench hardened and tempered: the carbon-rich skin hardens strongly, while the core, which stays low in carbon, keeps its toughness. This combines a wear-resistant surface with a part able to absorb shocks.
The hardened depth is controlled by the duration and temperature of the treatment, from a few tenths of a millimetre to more than a millimetre. It is defined by convention, as the depth at which hardness falls below a given threshold, as described in ISO 2639. Steels known as case-hardening steels are chosen for this, such as 16MnCr5 or 18CrNiMo7-6.
Case hardening is used for gears, pinions, shafts, pins, rollers and bearing tracks: any component subject to both friction and heavy loads. As it is followed by quenching, it distorts parts, so an allowance must be planned and a final grinding is often needed. Areas that must stay soft, such as threads, are protected by masking or have their carburised layer machined off before quenching. On a part that is already hard, nitriding is a less distorting alternative.
In a quote request
Specify the steel grade, the case depth targeted with its method of definition, the surface hardness and, if needed, the core hardness, as well as the areas to keep soft. State the allowance left for finishing or the distortion permitted, and the checks expected: hardness, section, treatment certificate. For transmission components, the mechanical assembly workshop that will fit the parts can specify the functional requirements; the guide on materials for machined parts places these steels.