Stability of the nitrogen S-phase in austenitic stainless steel

The stability of nitrogen 'S-phase' has been investigated by means of isothermal annealing and TEM in-situ heating observations. The results show that S-phase is a metastable phase. When thermally annealed at certain temperatures for a long enough time, the nitrogen S-phase decomposes into chromium nitrides enveloped by chromium depleted austenite (γ_N). The incubation time is found to be temperature dependent, ranging from several minutes at 600°C or over to a couple of years at 300°C or below. The decomposition of the S-phase is typical of that for a diffusion limited or thermally activated transformation. A preliminary isothermal transformation (IT) diagram has been constructed, which provides a basic guideline for the application of low temperature plasma nitrided 316 austenitic stainless steel.