NITRIDATION OF AUSTENITIC STAINLESS STEEL IN A NITROGEN PLASMA

The effectiveness of the austenitic stainless steel nitriding can be significantly increased using an ultra high atomic nitrogen flux produced by a plasma torch at atmospheric pressure. It is shown, employing Glow Discharge Optical Spectroscopy (GDOS) profiling technique and cross-sectional Scanning Electron Microscope (SEM) analysis that an efficient nitriding proceeds at temperature above 350-400°C. The thickness of the nitrided layer is up to 150-200 µm after 25 min of processing. The possible nitriding mechanism is proposed with the emphasis on the interaction between the free surfaces, grain boundaries and bulk. The high-flux external irradiation increases the surface chemical potential and creates the compressive stress in the near surface layer. Stress relaxation initiates inelastic processes in grains, mainly dislocation glide, and related mass-transport of matrix atoms including nitrogen, supplied through the grain (subgrain) boundaries of crystallites and dislocations.