슈퍼 2상 스테인리스강에서 석출물의 석출거동과 공식에 미치는 W치환의 영향

Abstract

This study is carried out to investigate the effect of W substitution on the precipitation behavior of the precipitate and pitting corrosion in super duplex stainless steels. The result obtained from this study are as follow: The χ phase is precipitated at the interface of the ferrite / austenite phases and inside of the ferrite phase at the initial stage of aging in the both steels. With the increase of the aging time, the volume fraction of the χ phase has been increased, and then decreased by the transformation from the χ phase to the σ phase. The σ phase has been precipitated later than the χ phase, and the volume fraction of the σ phase has been increased with the increase of the aging time. The volume fraction of the χ phase has been increased. On the other hand, the σ phase has been decreased by W substitution. The σ and χ phases has lower nickel concentration than the matrix. On the other hand, they have higher chromium, molybdenum and tungsten concentration. There is no big difference between the amount of chromium and nickel in the χ and σ phase, but the χ phase has higher concentration of molybdenum and tungsten than the σ phase. Not only the σ phase but also χ phase which is precipitated by aging treatment has influence on the pitting corrosion. In the 25Cr-7Ni-4Mo and 25Cr-7Ni-2Mo-4W super duplex stainless steels, their pitting corrosion resistance get lower as the aging time gets longer. By substituted W for the part of Mo, the pitting corrosion resistance gets lower compared to the specimen which is not substituted for W.