An investigation on the metallurgical process and deformation mechanism of a novel resource-saving duplex stainless steel (DSS)

Abstract

Though most duplex stainless steels are generally water quenched from high temperature, at which the equilibrium phase constitution is close to the desired magnitude. The non-equilibrium phase transformation in duplex stainless steel is less explored. The temperature-time-transformation diagram of duplex stainless steel was established, which shows that duplex stainless steel of equilibrated primary phase content can be obtained through different thermomechanical process. Specifically, we studied the effect of annealing temperature, time, cooling method and plastic deformation on the phase transformation of 15Cr-2Ni-2Al-11Mn duplex stainless steel, in which the consumption of Cr and Ni is reduced by substituting Al and Mn, respectively. The room-temperature duplex structure of duplex stainless steel is always metastable, which suggests that non-equilibrium phase transformation can be further exploited for producing duplex stainless steel having the same chemical compositions and phase constitution but different microstructures. It shows that the duplex stainless steel with about 50% austenite phase can be prepared using different non-equilibrium thermal process, of which the yield strength and elongation vary in the ranges of 307~499 MPa and 20~33%, respectively. The sample, which exhibits the best combination of yield strength (371 MPa) and elongation (33%), is attributed to the bimodal distribution of austenite grain size. The non-equilibrium phase transformation in producing DSS should be more thoroughly explored on the partition coefficient of alloying elements so that to improve the corrosion resistance of duplex stainless steel. The microstructure exhibits impact on the overall corrosion resistance of duplex stainless steel. The annealing temperature and cooling rate affect the partition coefficient of alloying elements associated with the change of the ferrite volume fraction. When the volume fraction of ferrite increases, the partition coefficient becomes more uniform and close to 1. The partition coefficients of Cr and Al increase while those of Ni and Mn decrease with the annealing time, and finally become steady after the sample reaching composition homogenization period. The contrast of corrosion resistance between ferrite and austenite phases varies on the change of the partition coefficient of alloying elements, which is markedly affected by annealing temperature and time. For 1000 °C annealing 15min and 30 min respectively, the first-etched phase for former is ferrite but for latter is austenite. For 1150 °C annealing 15min, the first-etched phase is austenite. The growing kinetics of sigma phase and its precipitation hardening effect on duplex stainless steel were also investigated. Upon aging at 750~900 °C, sigma phase precipitates in the DSS, and the size and amount of sigma phase increase continuously with aging time. TTT diagram of sigma phase precipitation is established, and the kinetics of sigma phase formation reaches peak at 850 °C. Mo increases the formation rate and the mass amount of sigma phase. Aging at 850 °C for 180 minutes leads to about 1.5% volume fraction of sigma phase and a peak strength of duplex stainless steel. In order to gain more understanding of phase evolution in duplex stainless steel, the mean-field theoretical model for austenite reformation is established, which is used to rationalize the exponent of the Avrami equation for fitting the kinetics of phase transformation. The phase field model for the influence of diffusion properties on phase transformation of duplex stainless steel was further established.