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 (sigma) precipitated directly at gamma-austenite grain boundaries (GB) via a common solid-state reaction when carbon and nitrogen contents fell below a critical threshold. Residual delta ferrite was found to be more susceptible to a-phase precipitation; this type of precipitation occurred via two mechanisms that depended on the concentration profiles of delta-ferrite stabilizing elements induced by previous thermomechanical processing: direct a precipitation (delta -> alpha) along the periphery of delta islands followed by a eutectoid decomposition (delta -> alpha + gamma(2)) within these islands. Both simulations and experiments revealed that the a phase at gamma GB contained higher amounts of Mo and Ni, while sigma within delta ferrite possessed higher contents of Fe and Cr. Finally, the simulated time temperature precipitation diagrams for the sigma phase in residual delta ferrite were found to be in very good agreement with the experimental ones and comparable to those observed in duplex stainless steels. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.