Repair welding of irradiated materials - Modeling of helium bubble distributions for determining crack-free welding procedures

In this paper, a computational simulation study is presented on the prediction of helium bubble evolution during repair welding of irradiated 304 stainless steel. Realistic spatial and temporal temperature and stress evolution during welding were obtained from simulation of the repair welding operation using the finite element model approach. The helium bubble evolution model by Kawano et al. was adopted as a user subroutine in the finite element model to predict the spatial distribution and temporal evolution of the helium bubble size and density in the heat-affected zone (HAZ) of partial penetration welds. Comparisons with experimental results available in open literature show that the predicted average helium bubble sizes were consistent with those observed experimentally under similar conditions. In addition, the computer simulation revealed strong spatial variation of helium bubble size due to the differences in combined thermal and stress conditions experienced in different locations in the HAZ. The predicted location of the maximum helium bubble agreed well with the observed helium-induced cracking site. The effect of welding heat input and welding speed was also investigated numerically. The modeling approach adopted in this study could be used as a cost-effective tool to quantitatively correlate the welding condition, radiation damage, and the likelihood of cracking, under the influence of welding-induced thermal and stress cycles. The model will also be useful in studying the degradation of properties from helium bubble formation of post-welded structures, even if a successful weld is made.