슈퍼 듀플렉스 스테인리스강의 R상의 석출과 공식에 미치는 W치환의 영향

Abstract

In this study, the Effect of W Substitution on the Pitting Corrosion and R-Phase of Precipitation in 25%Cr-7%Ni-4%Mo and 25%Cr-7%Ni-2%Mo-4%W super duplex stainless steel are investigated. The results obtained from the study are summarized below. The R phase in super duplex stainless steel was mainly precipitated from the interface of ferrite/austenite phases, and it was also precipitated from the inside of the ferrite phases during the initial stage of aging. The R-phase has transformed into the σ–phase with an increase of aging time. The ferrite phase has decomposed into a new austenite phase and σ–phase. The R phase was an intermetallic compound, which has a lower Ni concentration, a higher Mo concentration than the matrix, and a higher Mo and Cr concentration than the σ phase. The pitting potential Ep has slowly increased as the R-phase precipitated with an increase of aging time. Then it has rapidly decreased as the σ-phase precipitated. The R-phase has decreased by the pitting potential, but it had smaller effect than σ-phase. With the substitution of W for Mo in super duplex stainless steel, the volume fraction of σ-phase has increased. However, the pitting corrosion only had a slightly improvement from the substitution effect of W for Mo and the degradation of σ–phase precipitaties.