As a prerequisite for the predictability of properties obtained by a nitriding treatment of iron based workpieces, the relation between the process parameters and the composition and structure of the surface layer produced 111ustbe known. At present, even the description of thermodynamic equilibrium of pure Fe–N phases has not been fully achieved. It is shown that taking into account the ordering of nitrogen in the ε and γ'iron nitride phases leads to an improved understanding of the Fe–N phase diagran1. Although consideration of thermodynamics indicates the state the system strives f01∼ the nitriding result is determined largely by kinetics. Nitriding kinetics are shown to be characterised by localnear equilibria and stationary states at surfaces and intelfaces, and the diffusion coefficient of nitrogen in the various phases, for which new data are presented. The necessary background for process control of gaseous nitriding by monitoring the partial pressure of oxygen in the furnace using a solid state electrolyte is provided.

The response of various austenitic and duplex stainless steel grades to low temperature gaseous nitriding and carburising was investigated. Gaseous nitriding was performed in ammonia/hydrogen mixtures at temperatures <723 K; gaseous carburising was carried out in carbon monoxide/hydrogen mixtures for temperatures ≤783 K. The case developed by thermochemical treatment was examined using reflected light microscopy, X-ray diffraction analysis and microhardness testing. Both nitriding and carburising led to the development of expanded austenite in the surface adjacent zone, irrespective of the phase constitution of the substrate. A two step process, consisting of carburising followed by nitriding, provides great flexibility with regard to adjusting the hardness–depth profile.