Nitriding and nitrocarburizing - Industrial Processes and Practices

Nitriding and nitrocarburizing, whose mechanisms are described in [M 1 224] and [M 1 223] , are thermochemical treatments involving the addition of nitrogen by combination-diffusion in the case of nitriding, and the addition of nitrogen and carbon in the case of nitrocarburizing. These processes cover a family of treatments, which differ according to the conditions under which they are applied and the composition of the medium, which can be enriched with additional elements such as sulfur and oxygen. For temperatures below 590°C, the combination and diffusion reactions take place in the ferritic range of non-austenitic iron-carbon alloys, and the processes are classified as such; above 590°C, starting from the austenite range in the iron-nitrogen system, nitriding is said to be austenitic or ferrito-austenitic.

Hardening of the treated layer is achieved by in situ transformation during diffusion, so no further treatment is required. The nitriding process involves two mechanisms: a combination (or conversion) mechanism, which has been and still is the subject of numerous fundamental studies [M 1 224] , followed by a diffusion and precipitation hardening mechanism in the substrate, most often fed by the combination layer [M 1 223] .

This first article describes the various industrial processes and their industrial practices. In a second article [M 1 228] , the application to different steel grades and the different properties of nitrided or nitrocarburized parts are examined.