Swelling effect on stress induced and concentration dependent diffusion of nitrogen in plasma nitrided austenitic stainless steel

The kinetics of plasma nitriding of austenitic stainless steel (ASS) below temperatures of nitrides formation and mechanisms of nitrogen penetration are analyzed by proposed kinetic modeling. By fitting of experimental nitrogen depth profiles it is shown that the main processes are nitrogen concentration gradient and internal stresses induced diffusion with concentration dependant diffusion coefficient and also swelling. Internal stress induced diffusion is responsible for plateau formation in nitrogen depth profile which is characteristic for nitriding process. In order to fit experimental curves of different samples (with different nitriding duration time) with fixed diffusion coefficient it is necessary to include concentration dependant diffusion and swelling process. It is shown that diffusion coefficient depends on concentration according to Einstein–Smoluchowski relation. The swelling thickness is the square root function of nitriding time.