Precipitation behavior and phase transformation of hyper duplex stainless steel UNS S32707 at nose temperature

Abstract

In this work, time temperature transformation (TTT) diagram, thermodynamic calculations and microstructure observations were performed to investigate the precipitation behavior of hyper duplex stainless steel UNS S32707 aged at nose temperature for various aging time up to 48h. The corresponding secondary phase transformation mechanisms were proposed. The results demonstrate that the precipitation kinetics of σ phase in UNS S32707 is very fast due to its higher Cr and Mo contents at nose temperature of 950°C. At the initial stage of aging, σ phase preferentially formed along the α/α and α/γ phase boundaries and then penetrated into α phase, resulting from the eutectoid reaction α→γ2+σ. Meanwhile, a few σ phases nucleated at the γ/γ phase boundaries. With the aging time prolonging, some σ phases nucleated within γ phase due to the decomposition of the supersaturated γ phase. In addition, a cellular structure consisting of lamellar σ and γ2 phase was detected during short term aging, and it gradually grew with aging time via cell boundary migration. After 30min and 6h aging, the lamellar σ phases in the cellular structure broke into small lamellas or into bending bars and evolved into blocky shapes. Most of rounded Cr2N particles formed at the σ/γ2 interfaces and were surrounded by σ and γ2 phase. A few rod-like Cr2N particles nucleate at γ/γ phase boundaries due to the low energy interfaces between Cr2N and γ phase. No Cr2N was detected in α phase, which was attributed to the fact that σ phase has much faster precipitation kinetics than that of Cr2N.