| The duplex stainless steel (DSS) is one of high performance structural materials as its excellent corrosion resistance and mechanical properties. Now, more and more DSSs have been applied in chemical, petrochemical, paper and petroleum industries. However, because of the poor hot workability of this steel, which is easy to bring crack during forging, the hot-working plasticity and the correlation study of DSS is focused.The microstructures, tensile properties of room and elevated temperature and hot deformation behavior of 00Cr25Ni7Mo4N super duplex stainless steel (SDSS) were investigated to enrich the hot-working theory of the steel by the use of OM, SEM, TEM, XRD, nanoindentation, M?ssbauer spectroscopy, tensile tests and thermo-mechanical simulator etc.The microstructure of as-cast 00Cr25Ni7Mo4N SDSS usually consists of ferrite and austenite. The sigma phase was observed in SDSS after solution treatment at certain temperature, or continuous cooling after solution treatment as well as in somehow hot deformation process. When SDSS was solution- treated at 800~1000℃, with increasing solution treatment temperature and cooling rate, the volume fraction of sigma phase is decreased. After solution treatment at 1150℃, the critical cooling rate without precipitation of sigma is 0.3℃/s. Hot deformation can also promote the precipitation of sigma phase.The results show that the as-cast SDSS's hot ductility increases with the temperature. Simultaneously, the tensile properties at 600~1300℃for the as-forged SDSS have been studied. The ductility curve of the as-forged SDSS has a peak value and a valley value. The hot ductility increases to maximum value obtained at 1100~1200℃and decrease to minimum value at 800~900℃due to precipitation of the brittle sigma phase.The hot deformation behavior of the as-cast 00Cr25Ni7Mo4N SDSS was investigated using Gleeble-3800 thermo-mechanical simulator, by which a simulated deformation equation was obtained. The effect of peak stress on thetemperature and strain rate was discussed according to the equation. The strain rate sensitivity exponential and efficiency of power dissipation under different hot deformation conditions were also determined, thus the processing maps are established and interpreted on the basis of microstructure observation. The apparent activation energy and stress exponent are about 492kJ/mol and 3.51, respectively. It reveals that the maps obtained at true strain range from 0.1 to 0.9 are essentially similar. With increasing temperature and decreasing strain rate, the efficiency of power dissipation increases gradually. Peak efficiency about 40% appears at the true strain of 0.2 when the deformation carries out at 1200℃with strain rate of 0.1s-1.The SDSS has outstanding mechanical properties at room temperature. Its tensile strength and elongation can reach above 800MPa and 52%, respectively. The nanoindentation results indicate that the mechanical properties of ferrite and austenite are quite close at room temperature, but they are very different at high temperature. This may result from the different dynamic softening mechanism for the two phase, i.e., dynamic recovery is for ferrite and is dynamic recrystallization for austenite.The theoretical foundation for optimizing hot working technics was for build and the steady basis for the application on hot working was set up in the present work. The temperature range of hot-working and the deformation amount of each pass need to be controlled strictly in this research.
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