Abstract
There have been a great number of occurrences of stress corrosion cracking (SCC) in the structural materials of boiling water reactors (BWRs) in recent decades. Stress corrosion cracking consists of three main factors: susceptible materials, caustic environments and tensile stress. To address this crucial issue, researches have been investigating the effect of these three factors for decades. Reducing the susceptibility of the materials by heat treatments, applying hydrogen water chemistry (HWC) and eliminating residual stress are several feasible methods. The water environments became less oxidizing after applying HWC, which lowered the electrochemical corrosion potential (ECP) and thus mitigated stress corrosion cracking. In this study, 316L SS and alloy 52 were used in the dissimilar metal weld (DMW). Hardness and residual stress measurements on the surface of the DMW as well as heat treatments were performed after tungsten inert gas (TIG) welding. The effect of welding on the microstructure of the weld, heat affected zone and unaffected base metal was also evaluated. The influence of heat treatments with different temperatures on SCC was also investigated. In this study, slow strain rate testing (SSRT) was implemented in simulated BWR water environments. The mechanical properties of the materials were determined from the stress-strain curve, and the fracture surfaces were characterized via scanning electron microscope (SEM). From the above steps, we evaluated the most effective way to mitigate SCC. The results showed that the difference in microstructure between heat affected and unaffected zone was insignificant. All samples were fractured at the base metal 316L SS, which was distant from the heat affected zone. In dissolved oxygen and dissolved hydrogen environments, the materials exhibited higher mechanical ductility after solution annealing. But the materials tended to be susceptible to stress corrosion cracking after post weld heat treatment. On the whole, solution annealing treatment and HWC could remarkably lower the susceptibility of the materials. As-welded samples exhibited lower susceptibility to SCC than samples with one post weld heat treatment.