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

• Published in: Acta materialia Vol. 79; pp. 16 - 29
• Main Authors: Perron, A., Toffolon-Masclet, C., Ledoux, X., Buy, F., Guilbert, T., Urvoy, S., Bosonnet, S., Marini, B., Cortial, F., Texier, G., Harder, C., Vignal, V., Petit, Ph, Farré, J., Suzon, E.
• Format: Journal Article
• Language: English
• Published: Kidlington Elsevier Ltd 15-10-2014; Elsevier
• Subjects: Applied sciences; Austenitic stainless steels; CALPHAD; Carbon; Chromium; Eutectoid decomposition; Exact sciences and technology; Ferrite; Intermetallic phases; Islands; Metals. Metallurgy; Phase transformations; Precipitation; Precipitation kinetics; Simulation; Intermetallic phases; CALPHAD; Precipitation kinetics; Phase transformations; Austenitic stainless steels; Austenitic stainless steel; Intermetallic compound; Phase transformation; Precipitation; Austenitic steel; Stainless steel; Kinetics; Microstructure
• ISSN: 1359-6454; 1873-2453
• DOI: 10.1016/j.actamat.2014.06.066

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.