Early stages of γ′-Fe4N1−x nucleation and growth in a post-discharge nitriding reactor

Early stages during post-discharge nitriding were investigated and related to a diffusion based model that describes the evolution of nitrogen surface concentration. Microstructural observations during the nucleation of nitrides in post-discharge nitriding have provided valuable kinetics information for the development of the model. Long pure iron samples were nitrided for short periods to develop an incipient structure that varied along the sample. The products were examined by optical and scanning electron microscopy and X-ray diffraction. A nitriding gradient was developed along the sample surface and based on this a nitriding nucleation time could be established. Both observations were incorporated into the model to obtain the surface concentration and correlate to the spatial distribution of active species in post-discharge nitriding.