Effect of Silicon on Intergranular Corrosion Resistance of Ti-stabilized 11 wt% Cr Ferritic Stainless Steels

Abstract

Ti-stabilized 11 wt% Cr ferritic stainless steels (FSSs) for automotive exhaust systems have been experienced intergranular corrosion (IC) in some heat-affected zone (HAZ). The effects of sensitizing heat-treatment and silicon on IC were studied. Time-Temperature-Sensitization (TTS) curves showed that sensitization to IC was observed at the steels heat-treated at the temperature lower than 650oC and that silicon improved IC resistance. The sensitization was explained by chromium depletion theory, where chromium is depleted by precipitation of chromium carbide during sensitizing heat-treatment. It was confirmed with the results from the analysis of precipitates as well as the thermodynamical prediction of stable phases. In addition, the role of silicon on IC was explained with the stabilization of grain boundary. In other words, silicon promoted the formation of the grain boundaries with low energy where precipitation was suppressed and consequently, the formation of Cr-depleted zone was retarded. The effect of silicon on the formation of grain boundaries with low energy was proved by the analysis of coincidence site lattice (CSL) grain boundary, which is a typical grain boundary with low energy.

Intergranular Corrosion of 316L Stainless Steel by Aging and UNSM (Ultrasonic Nano-crystal Surface Modification) treatment

“…대부 분의 결정입계공학의 기구는 많은 쌍정의 생성과 관계되어 있으며, 이러한 쌍정은 Σ3 경계 중의 하나로서 이는 작은 적층결함에너지를 갖는 금속 및 합금에서 나타난다. 작은 Σ 경계 (이를 CSL(Coincidence Site Lattice 또는 special boundaries라 명명하고 있음))의 비율이 높을수록 특 성 개선이 이루어지는 것으로 보고되고 있다 [23][24][25][26][27][28][29][30][31][32][33] . 또한 입 계부식에 미치는 잔류응력의 영향에 대한 연구를 통하여 압축잔류응력이 존재하게 되면 부식을 억제시킬 수 있다고 보고하고 있으며 34) , 또한 최근의 결정립계 구조에 대한 연 구결과에 따르면 24) , 결정립계 현상(결정립계 확산 35) , 석출 36,37) , 부식 38,39) .…”