Microstructure and microchemistry of laser welds of irradiated austenitic steels

Abstract

This article investigates the integrity of laser welds on neutron irradiated, He-containing steels. Life extension of the current fleet of light water reactors could necessitate repair of cracks on irreplaceable internal components, but heat input of weld repairs exacerbates the problem by initiating He-induced cracking. Laser welding is a promising low-heat-input technology thought to limit the extent of He-induced cracking. In this study, we produce laser welds in a hot cell on AISI 304L stainless steel plates previously irradiated in the Experimental Breeder Reactor (EBR)-II. We select a systematic set of three specimens spanning fluences ~1–28 displacements per atom (dpa) at ~415–430 °C and He concentrations ~0.2–8 atomic parts per million (appm) amounting to ~0.2–2.8% swelling. He-induced cracking is observed only in specimens containing ≥3 appm He. Laser welding nearly eliminates all irradiation-induced cavities and reduces the dislocation loop number density, similar to conventional post-irradiation annealing. Microchemically, laser welding induces Cr-rich precipitation and suppresses grain boundary radiation-induced segregation. The mechanism of He-induced cracking is discussed in the context of these microchemical changes. The weld heat input is calculated and suggests that further refinement of laser welding parameters may improve the cracking resistance for higher dose and He conditions.