슈퍼 2상 스테인리스강의 내식성에 미치는 석출물의 영향

Abstract

This study was carried out to investigate the precipitation behavior of χ-phase and the corrosion resistance in 25%Cr-7%Ni-4%Mo and in 25%Cr-7%Ni-2%Mo-4%W super duplex stainless steel. The χ-phase was precipitated at the interface of ferrite / austenite phase and inside of the ferrite phase at the initial stage of aging, and it was transformed to σ-phase with an increase of aging time. The ferrite phase was decomposed into new austenite(γ2) phase and σ-phase by aging treatment. The decomposition of the ferrite phase into new austenite phase and σ-phase was retarded by tungsten substitution for molybdenum. Thus, the number of χ-phase increased. It was proved that the χ-phase was a intermetallic compound, which represents the higher molybdenum concentration and significantly higher tungsten concentration than the matrix phase. With increasing of the volume fraction of χ-phase, the corrosion resistance decreased,and, χ-phase precipitation, thus, affected the corrosion resistance. However, the corrosion resistance was affected remarkably by the precipitation of σ-phase. With the substitution of tungsten for molybdenum in super duplex stainless steel, the volume fraction of χ-phase increased. On the other hand, the corrosion resistance was improved slightly from the substitution effect of tungsten for molybdenum and the degradation of σ-phase precipitates.