텅스텐 함유 이상스테인리스강의 초소성에 미치는 σ상의 영향

Abstract

The superplasticity of Fe-25Cr-7Ni-3Mo, Fe-25Cr-7Ni-1Mo-2W and Fe-22Cr-5Ni-1Mo-2W duplex stainless steels was investigated by tensile test at 850℃ with initial strain rate ranged from 10^(-3)s^(-1) to 10^(-1)s^(-1). The constituent phases in the duplex stainless were found to change following a sequence of α→α+γ→α+γ+σ→γ+σ through phase transformation with increasing time at 850℃. The substitution of W for Mo in the duplex stainless steels retarded the formation of σ phase due to the lower diffusivity of W compared to Mo. The tensile elongation of Fe-25Cr-7Ni-3Mo and Fe-25Cr-7Ni-2W alloys were larger than that of Fe-22Cr-5Ni-1Mo-ZW alloy due to the larger volume fraction of fine grained σ phase without remaining untransformed coarse α grains at 850℃. Significant strain-enhanced grain growth of γ and σ phase was observed maintaining equiaxed grain shape with low dislocation density during tens e deformation. These results indicate the grain boundary sliding is dominant mechanism of superplasticity in γ/σ duplex stainless steels at 850℃.