Thermal stability and phase decomposition of nitrided layers on 316L and 310 austenitic stainless steels

AISI 316L and AISI 310 austenitic stainless steels have been plasma nitrided at low temperature (673K) by low glow discharge in 80%H2-20%N2 gas mixture, in order to improve their mechanical and tribological properties while maintaining their starting microstructure. The obtained nitrided layers, under optimum nitriding conditions, have been submitted to isothermal annealing with time at different temperatures, the aim being to study their thermal stability for industrial use. X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) technics have been used to appreciate the emergence of phase transformations and their associated structures. Results showed that the metastable solid solution, supersaturated and disordered in nitrogen, obtained after nitriding under cold plasma of austenitic stainless steels, is decomposed for relatively short times and at temperature ranging from 723K to 823K, in different ways, depending on chromium content in steel. On the other hand, if the annealing is made at low temperature and for fairly long time, the γN phase returns to equilibrium and gives a stable phase, the M4N nitride in this case (were M is compound of iron, chromium and nickel considered steel). •Nitriding treatment of stainless steels at low-temperature•Life expectancy after treatment of nitriding•Nitrided layer decomposition after a heat treatment in relatively high temperatures