Experimental Investigation and Analytical Prediction of σ-Phase Precipitation in AISI 316L Austenitic Stainless Steel

The phase precipitation in industrial AISI 316L stainless steel during aging for up to 80,000 hours between 823 K and 1073 K (550 °C and 800 °C) has been studied using transmission electron microscopy, scanning transmission electron microscopy, and carbon replica energy-dispersive X-ray microanalysis. Three phases were identified: Chromium carbides (M 23 C 6 ), Laves phase ( η ), and σ -phase (Fe-Cr). M 23 C 6 carbide precipitation occurred firstly and was followed by the η and σ -phases at grain boundaries when the aging temperature is higher than 873 K (600 °C). Precipitation and growth of M 23 C 6 create chromium depletion zones at the grain boundaries and also retard the σ -phase formation. Thus, the σ -phase is controlled by the kinetic of chromium bulk diffusion and can appear only when the chromium reaches, at grain boundaries and at the M 23 C 6 / γ and M 23 C 6 / η / γ interfaces, content higher than a critical value obtained by self-healing. An analytical model, based on equivalent chromium content, has been established in this study and successfully validated to predict the time–temperature–precipitation diagram of the σ -phase. The obtained diagram is in good agreement with the experimental results.