Influence of gas nitriding pressure on the surface properties of ASTM F138 stainless steel

The present work investigates the influence of gas nitriding pressure on the modification of surface properties of ASTM F138 stainless steel. Nitrided surfaces were characterized by X-ray diffraction, Conversion Electron Mössbauer Spectroscopy, Scanning Electron Microscopy and Vickers microhardness. Plasma nitriding was carried out in a medium of 80% H2 and 20% N2, at 400 °C, for 4 h, pressures between 3 and 7 torr. A modified layer, whose depth ranging from 5.4 to 6.6 μm, was observed for all the nitrided samples, regardless of the nitriding pressure. The sample treated at 7 torr showed the best result of hardness.Besides γN phase, the hexagonal ε phase (Fe2 + xN) was also observed on the Mössbauer spectra up to 0.4 μm depth of the nitrided samples. X-ray diffraction spectra were analyzed by full profile fitting, using the triclinic structure for the γN phase. The results indicated that nitrogen concentration reaches a maximum for a pressure of 4 torr.