Carburizing and carbonitriding - Processes

Cementation and carbonitriding (or carbon carburizing and carbon-nitrogen carburizing) are two related thermochemical treatments. In both cases, the main diffusion element is carbon, while nitrogen is used in carbonitriding to promote carbon hardening (improved carbon transfer and diffusion, higher hardenability of the case-hardened layer thanks to nitrogen enrichment), to provide a higher interstitial content and to contribute to hardening and residual stress levels.

Carbonitriding only concerns the shallowest diffusion depths (< approx. 0.6 mm).

A distinction should be made between carbonitriding and treatments activated and/or enhanced by ammonia injection, either in the first phase as a depassivation element, or in the last phase ("flash nitrogen" process) to improve hardening.

Hardening is achieved by quenching, and post-diffusion heat treatment cycles are carried out according to the same principles.

The steels used belong to the same families.

The applications are comparable, and the choice of one or other of the processes, at equivalent depths, is based on considerations including the implementation of the process and the choice of steel. A second article [M 1 226] deals specifically with implementations.

Until the advent of low-pressure processes, carbonitriding supplanted carburizing for shallow depths. In recent years, low-pressure carburizing has taken over some of carbonitriding's field of application.

And low-pressure carbonitriding is only just beginning to be considered.