슈퍼 듀플렉스 스테인리스강 UNS S32750의 FCA 다층 용접부의 용접 후 열처리 영향

This study was carried out to investigate the effect of postweld heat treatment(PWHT, 930, 1080, 1230℃) on the microstructure, phase formation, pitting corrosion and mechanical properties such as hardness, tensile strength and impact values of super duplex stainless steel(UNS S32750) multipass welds. Based on the microstructural examination and X-ray diffraction analysis, it was found that the σ phase was formed in the welds heat treated at 930℃ in which the ferrite content greatly decreased into 5~10% in the welds. The secondary austenite was formed in the reheated zone of welds and redissolved into ferrite with increasing heat treatment temperatures. The tensile strength and impact values of welds heat treated at 930℃ were the lowest and revealed the brittle fracture surface. The weight loss by pitting corrosion increased with test temperatures. It was confirmed that pitting corrosion occurred mainly in secondary austenite of reheated zones. The postweld heat treatment temperature is recommended to be in the range of 1050~1150℃.