Thermochemical treatment theories - Nitriding - Nitrocarburizing- Iron-nitrogen binary systems and iron-nitrogen ternary systems - Compound layer

The aim of nitriding and nitrocarburizing treatments is to transfer nitrogen atoms (nitriding) or nitrogen and carbon atoms (nitrocarburizing) from a heterogeneous surface reaction to the solid metal alloy. Although these treatments are not confined to ferrous alloys, they are unquestionably the most commonly used on many steel grades. Thanks to the formation of the phases that accompany the transfer of nitrogen into the steel, a hardness gradient and a compressive stress gradient are obtained from the surface, improving wear and fatigue resistance in particular.

Except in the case of austenitic stainless steels, treatments are carried out in the ferritic phase, i.e. at temperatures between 380 and 580°C, the latter temperature being around ten degrees lower than that of the eutectoid transformation in the iron-nitrogen binary.

The theoretical approach to nitriding and nitrocarburizing treatments is presented for treatments involving heterogeneous gas-solid reactions (gas nitriding with ammonia-containing mixtures), for which we have some knowledge:

• thermodynamics of iron-nitrogen and iron-nitrogen-carbon systems ;

• on material transfer mechanisms, particularly at the gas-solid interface.

Here, we explain the formation of iron nitrides and carbonitrides (combination layers) from iron-nitrogen and iron-nitrogen-carbon systems.

In the second article, we look at diffusion/precipitation mechanisms in alloy steels containing up to 5% chromium (mass fraction), with a view to explaining the formation of nanometric precipitates that give 38CrMoV13 and 32crMoV5 steels, for example, remarkable surface mechanical properties.