Solution treatment effect on precipitates, microstructure, and properties of S32707 hyper-duplex stainless steel

Abstract

Duplex stainless steel (DSS) has been widely used in some harsh environments, such as flue gas shedding and seawater desalination, because of its high strength and corrosion resistance. These excellent properties rely on a high alloy content (Cr, Mo, N, etc.) and perfect dual-phase equilibrium. The dual-phase equilibrium mainly includes dual-phase proportion balance, properties balance, and absence of clear secondary phase in the solid solution structure. As one of the main developmental directions of DSS, hyper-duplex stainless steel (HDSS) has attracted much attention in recent years. In this paper, the effects of solution treatment on precipitates, microstructure, and properties of S32707 HDSS were studied by Thermo-Calc thermodynamic calculation, OM and FE-SEM observation, mechanical properties, and corrosion property tests. The results showed that σ phase and non-equilibrium nitrides were the main precipitates of solution-treated HDSS. When the solution temperature was lower than 1050 ℃, the σ phase precipitated preferentially along the dual-phase boundaries, which significantly reduced the impact toughness of HDSS. When the solution temperature was higher than 1100 ℃, non-equilibrium nitrides precipitated in ferrite grains, and the number of non-equilibrium nitrides increased with an increase in solution temperature. The reason for the non-equilibrium nitride precipitation was that the nitrogen content in the ferrite increased with an increase in temperature. This led to the supersaturation of nitrogen in the ferrite grains during the rapid cooling process. Under such conditions, the finely dispersed non-equilibrium nitrides precipitated in the ferrite grains. With increasing solution temperature, the content of the ferrite increased, the content of austenite decreased, the strength increased, and the impact toughness decreased. The optimal solution temperature of HDSS was 1080‒1120 ℃. Under this condition, the ratio of duplex was close to 1∶1, and the S32707 hyper duplex stainless steel presented excellent comprehensive mechanical properties and intergranular corrosion resistance.

Keywords

• hyper duplex stainless steel
• microstructure evolution
• σ phase
• non-equilibrium nitrides
• mechanical properties