Precipitation of σ phase in superaustenitic stainless steel UHB 904L

Superaustenitic stainless steel UHB 904L with high Mo concentrations is widely used in applications that require high toughness and corrosion resistance. Given certain thermal histories, UHB 904L may be susceptible to the formation of potentially detrimental intermetallic phases, such as the σ (sigma) phase. The formation of the σ phase is promoted by high concentrations of Cr and Mo, while elements such as carbon, nickel and nitrogen retard its formation. Samples of UHB 904L were isothermally annealed within the temperature range between 850 – 1 000 °C, for 8 h each, followed by water quenching. Microstructural analyses using light and electron microscopy showed that the σ phase forms at temperatures up to 1 000 °C. The tensile specimens were solution treated at 1 000 °C, 1 060 °C, 1 100 °C and 1 140 °C for 0,5 h, followed by water quenching. The tensile tests were performed at room temperature.