Nitrocarburising

Nitrocarburising is carried out at sub-critical temperatures and involves the diffusion of nitrogen and carbon into the surface of carbon steel to give a somewhat harder case and soft core with a very thin compound layer on the surface.

The compound layer is wear- and corrosion-resistant and yet is not brittle, unlike its counterpart in the nitriding process. Since it provides an essential part of the properties required from the process it must not be removed by subsequent machining. Below the compound layer, the thin case significantly enhances the fatigue resistance of the component.

Although nitrocarburising can be used with most steels that can be nitrided, it is most-commonly applied to mild steel and low alloy steels, the properties of which it improves dramatically.

Salt baths were initially used for nitrocarburising, using a variety of salt mixtures, generally sold under proprietary names. Fluidised beds are nowadays often used when small components require nitrocarburising. They have the benefit of ensuring even treatment throughout the load and across each component.

As with all gaseous processes, control is better than with the salt bath and the quality of the compound layer, in particular its freedom from porosity and evenness, is far superior. Longer treatment times are also possible than with salt baths, since the shortcomings of the compound layer (porosity and spalling problems) do not exist to cause limitations as in the salt processes. Hence gaseous nitrocarburising is applied to a wide range of materials and components.

Nitrocarburising can be used in place of cyaniding and carbonitriding for distortion prone parts e.g. clutch plates, retaining washers etc. Many parts, such as camshafts, crankshafts, torsion bars, benefit from nitrocarburising after hardening and tempering and increases in fatigue life of between 30 and 130% are usual.

All nitrocarburising treatments have the benefit of freedom from component distortion due to the low treatment temperature and the fact that quenching is only necessary if optimum fatigue resistance required. The use of nitrocarburising as an alternative to conventional shallow case nitriding with suitable alloy steels containing chromium or aluminium is also practicable, with great savings in processing time.

See also austenitic nitrocarburising, ferritic nitrocarburising, plasma nitrocarburising, Corr-I-Dur®.