Growth kinetics of secondary austenite in the welding microstructure of a UNS S32304 duplex stainless steel

A thermodynamic and kinetic assessment was made of secondary austenite (γ2) growth in the welding microstructure of a UNS S32304 duplex stainless steel during reheat cycles, between 900 and 1250 °C. Calphad-based numerical simulation and analytical electron microscopy characterization analyses were performed. A non-monotonic relationship between the proportion of γ2 and reheating temperature was observed, a maximum γ2 fraction being attained at ∼1050 °C. γ2 formed at higher reheating temperatures (above ∼1100 °C) displayed an outer shell rich in chromium and nitrogen and a core region depleted in these alloying elements; in contrast, γ2 formed at lower reheating temperatures (under ∼1000 °C) displayed almost homogeneous chemical composition.