The nitriding kinetics of iron-chromium alloys; the role of excess nitrogen: Experiments and modelling

To investigate the morphology and the growth kinetics of nitrided layers of Fe-Cr alloys, nitriding experiments were performed for alloys with 4.3, 7.7, 14.0, and 21.5 at. pct Cr. The precipitation morphology of the nitrided samples was investigated with light optical microscopy and scanning electron microscopy (SEM). The elemental compositional variation was determined with electron probe microanalysis. To describe the evolution of the thickness of the nitrided layers, a numerical model was developed that has as important (fit) parameters: the surface nitrogen content, the solubility product of chromium and nitrogen dissolved in the ferrite matrix, and a parameter defining the composition of the precipitated chromium nitride. Fitting of the model to the experimental data demonstrated for the first time that mobile and immobile excess nitrogen is present in the nitrided layers and that the mobile excess nitrogen considerably influences the nitriding rate.