Carbonitriding

Carbonitriding is the absorption and diffusion of carbon and nitrogen into the surface of steel to give a hard surface and softer core after hardening by quenching. Carbonitriding is a surface heat treatment, a form of case hardening, for plain low carbon and low alloy steels and cast irons, which provides wear resistance and moderate load bearing capability.s

It is found with plain carbon steels, that the use of gas carburising is restricted to small section sizes if the case is to be fully hardened by oil quenching. The addition of nitrogen (provided by adding ammonia as well as propane to the furnace atmosphere in a sealed quench furnace), increases surface hardenability by allowing both carbon and nitrogen diffusion to occur. Carbonitriding can therefore be considered as a gaseous equivalent to cyanide salt bath hardening. The normal range of temperature employed is 820/910°C, with 870°C being the optimum temperature for best case hardening conditions with the majority of suitable steels. Single quench treatments are generally employed and the process is mainly used for case depths up to 0.75mm (0.030″). For deeper cases in plain carbon steels it is useful to carburise only at 930/950°C and then reduce the furnace temperature to 870°C and complete the process by carbonitriding followed by oil quenching.

Fluid bed furnaces may also be employed to provide carbonitriding heat treatment. This method is particularly suited to the treatment of small components and those whose geometry would be prone to masking and attendant uneven hardening, were the sealed quench method to be used. Cyanide salt bath treatments have now largely been superseded by fluid bed treatments which do not have the health and safety and environmental risks associated with the operational and disposal aspects of cyanide salt baths.

As with all hardening processes, it is good practice to finish with a tempering treatment to reduce brittleness and impart optimum strength. Irrespective of which carbonitriding method is used, a tempering temperature of 150°C is generally suitable.

Carbonitriding should not be confused with its lower temperature partner, nitrocarburising.