Understanding sigma-phase precipitation in a stabilized austenitic stainless steel (316Nb) through complementary CALPHAD-based and experimental investigations

Sigma-phase precipitation in a 316Nb “stabilized” austenitic stainless steel was studied through complementary CALPHAD-based and dedicated experimental investigations. Thermokinetic calculations performed using Thermo-Calc (with the DICTRA module) and MatCalc software showed that the sigma phase (σ) precipitated directly at γ-austenite grain boundaries (GB) via a common solid-state reaction when carbon and nitrogen contents fell below a critical threshold. Residual δ ferrite was found to be more susceptible to σ-phase precipitation; this type of precipitation occurred via two mechanisms that depended on the concentration profiles of δ-ferrite stabilizing elements induced by previous thermomechanical processing: direct σ precipitation (δ → σ) along the periphery of δ islands followed by a eutectoid decomposition (δ → σ + γ2) within these islands. Both simulations and experiments revealed that the σ phase at γ GB contained higher amounts of Mo and Ni, while σ within δ ferrite possessed higher contents of Fe and Cr. Finally, the simulated time-temperature-precipitation diagrams for the σ phase in residual δ ferrite were found to be in very good agreement with the experimental ones and comparable to those observed in duplex stainless steels.