Modeling of plasma nitriding and thermal annealing processes of austenitic stainless steel

In plasma nitriding of austenite stainless steel experiments, nitrogen penetration depth profiles typically show a characteristic plateau. This is associated with the phase transition to expanded austenite and non-Fickian diffusion of nitrogen involving the trapping of nitrogen by Cr sites, baro-diffusion and concentration dependent diffusion coefficient mechanisms. However, recent studies on thermally annealed post-nitrided austenitic samples have revealed the presence of two plateaus in the nitrogen depth profile curves. This phenomenon is new and has never been considered, and it is of particular interest as it suggests the presence of additional processes distinct from those responsible for the formation of the first plateau during nitriding. This work aims to analyze the appearance, mechanisms and processes responsible for the formation of the second plateau in the nitrogen depth profile during post-nitriding thermal annealing. The primary hypothesis is that the second plateau forms due to nitride formation, which is supported by XRD spectra showing chromium nitride after thermal annealing. The new mathematical model was developed for nitriding with non-Fickian diffusion mechanisms and post-nitrided annealing. The simulations performed with the developed model demonstrated that, under specific conditions, the second plateau emerges, thereby confirming the model's capability to reproduce nitrogen depth profiles exhibiting two plateaus. In the paper, these conditions are analysed in detail, and the influence of various parameters (diffusion coefficient, internal lattice stresses, processing time, etc.) is quantitatively investigated and it reveals that the appearance of the second plateau is very sensitive to these parameters not only during simulations but also in experiments.