X-ray diffraction characterisation of low temperature plasma nitrided austenitic stainless steels

The nitrided layers produced by low temperature (400-500°C) plasma nitriding on austenitic stainless steels, AlSl 316, 304 and 321, have been characterised by X-ray diffraction, in conjunction with metallographic and chemical composition profile analysis. The min, hard and corrosion resistant layers exhibited similar X-ray diffraction patterns, but the positions of the major diffraction peaks varied with nitriding temperature and nitrogen concentration profile. The low temperature nitrided layers are predominantly composed of a phase with a face centered cubic (fcc) structure, which is named 'S' phase. However, the positions of the diffraction peaks from the 'S' phase deviated in a systematic way from those for an ideal fcc lattice. Detailed analysis of the deviation suggested that very high compressive residual stresses and stacking faults were formed in the layers, resulting in a highly distorted and disordered fcc structure. The lattice parameter of the distorted and disordered 'S' phase was found to increase with increasing nitrogen concentration.