슈퍼 2相 스테인리스鋼의 機械的 性質과 耐蝕性에 미치는 合金元素의 影響

Abstract

This study was carried out to investigate the effects of σ phase precipitation and nitrogen addition of 25Cr-7Ni-4Mo-2W super duplex stainless steel, and the effect of partial substitution of tungsten for molybdenum on the mechanical properties and corrosion resistance of 25Cr-7Ni-4Mo-0.2N super duplex stainless steel. The result obtained from this study are as follow: With raising solution annealing temperature in the two phase region of ferrite and austenite, the volume fraction of ferrite increased, which resulted in the increase in tensile strength and the decrease in elongation. N addition raised the volume fraction of austenite. On the other hand, the partial substitution of W for Mo decreased the volume fraction of ferrite. R and χ phase was mainly precipitated at the interface of ferrite/austenite and within ferrite matrix during early stage of aging. R and χ phase began to transform to σ phase with elapsing aging time. However, σ phase began to be precipitated at a mid-term of aging time. The amount of σ phase increased with increasing the volume fraction of austenite and aging time respectively. N addition raised the amount of σ phase. Reversely the partial substitution of W for Mo decreased the amount of σ phase. The partial substitution of W for Mo promoted the precipitation of R and χphase in the early stage of aging. As a result, the amount and the size of R and χ phase increased and became fine. Moreover, transformation retardation effect of ferrite on secondary austenite and σ phase increased the amount of R and χ phase. With elapsing aging time, the amount of σ phase, hardness and tensile strength increased. However, elongation and impact values decreased slowly after showing the fast decrease. For the steels alloyed with N and partially substituted W for Mo, hardness and tensile strength were increased, and elongation and impact value were decreased. R and χ phase precipitated in the early stage of aging were not influenced greatly on hardness and tensile strength. However this phase was affected on elongation and impact value remarkably. The amount of σ phase increased with raising the aging temperature. This caused the high current density. Owing to the decrease in the amount of σ phase, the steel partially substituted W for Mo showed the low current density compared to the steel without substitution. Moreover, the effect of aging temperature on current density was small. Intensive corrosion appeared preferentially in the matrix around the σ phase, this implies that the σ phase deteriorated the corrosion resistance remarkably.