Austenitic stainless steel welds are widely used in pressurizer surge-line, weld-overlay cladding and reactor vessel internals of pressurized water reactors (PWRs). Around 5 - 20 vol. % of residual $\delta-ferrite$ formed during solidification process of welding procedure have some advantages on high strength, corrosion resistance and weldability. However, it was known that presence of $\delta-ferrite$ could be susceptible to thermal aging and neutron irradiation due to microstructural evolution. After long-term service at operating temperature and neutron irradiation condition, Cr-rich ($\alpha'$) phase and secondary precipitate like G-phase formed in the $\delta-ferrite$ deteriorate the integrity of austenitic stainless steel weld especially for loss of fracture toughness because of combined effect on thermal aging and neutron irradiation. In order to simulate long-term exposure at 320 ℃ and irradiation dose , the E308 weld was thermally aged at accelerated aging temperature, such as 400 ℃, for 20,000 h prior to irradiation. Proton irradiation at a dose rate of 1.1 × $10^{-5} dpa/s$ was conducted in MIBL at 360 ℃ to 10 dpa. After thermal aging at 400 ℃ for 20,000 h, phase separation into Cr-rich ($\alpha'$) and Fe-rich ($\alpha$) phase induced by spinodal decomposition was observed in the $\delta-ferrite$ while any secondary precipitate have not beed found. After proton irradiation up to 10 dpa, the degree of spinodal decomposition was enhanced and especially secondary G-phase precipitate was formed. Consequently, due to synergism between thermal aging and irradiation, strength of $\delta-ferrite$ was further increased than that of only thermally aged. The result of nano-mechanical tests have shown that irradiation can further accelerate the degradation of mechanical property of $\delta-ferrite$ coming from huge amounts of G-phase formation at pressurized water reactor environment.
오스테나이트계 스테인리스강 용접부는 가압경수로형 원전의 가압기 서지라인 등 일차수 배관 및 내부구조물 접합부로 널리 사용되고 있다. 용접시 잔존하는 약 5 - 20 %의 델타-페라이트는 강도, 부식 저항성 및 용접성 등에서 이점이 있으나, 장기간 가동 온도 및 중성자 조사에 노출 시 열취화 및 조사취화에 취약하다고 알려져 있다. 내부구조물 용접부의 장기 열시효를 모사하기 위하여 E308 용접부에 대하여 약 400 ℃에서 약 20,000 시간 동안 가속열시효를 수행 후 미세구조 변화 및 기계적 특성평가를 수행하였다. 더불어 조사 취화를 포함한 복합손상을 모사하기 위해 열시효 후 양성자 조사를 수행하였으며, 조사량에 따른 미세구조의 변화 및 나노필러압축시험이 수행되었다. 조사량이 10 dpa까지 증가함에 따라 스피노달 분해에 따른 상분리 현상이 가속화되었으며, 2차상인 G phase 석출을 확인하였다. 나노필러 압축시험 결과 양성자 조사로 인해 생성된 다량의 G phase가 용접부의 추가적인 취화에 영향을 주는 것으로 확인되었다.
