Evaluation on effects of the thermal aging and irradiation of Austenitis stainless steel welds in PWRs

Abstract

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.