Abstract Scope |
Ductility-dip cracking (DDC) in face-centered cubic (FCC) alloys is a challenge faced by nuclear power generation. Aging reactors need to be repaired via large, multipass weld overlays designed to mitigate underlying damage from primary water stress corrosion cracking (PWSCC). DDC often occurs in the first few layers of these overlays, and in the nuclear industry, even a small amount of DDC is subject to costly repair and rework. The prevailing theory describing DDC is based on observations of grain boundary (GB) sliding, microvoid formation, and the effect of GB tortuosity. However, the micro-mechanistic details need to be studied and there is disagreement over the precise role impurities play. Specifically, sulfur has been shown to exacerbate DDC in laboratory testing, but this behavior is not consistently observed in industry settings. Four variants of high-chromium nickel-based alloy 690 were prepared with varying sulfur levels. Bead-on-plate welds were completed on the material to generate a weld metal microstructure. Samples were extracted and examined with electron backscatter diffraction (EBSD) in the as-welded condition to establish starting conditions. Unrestrained thermal cycling was used with variation in the number of cycles applied to each sample in each variant of 690. Subsequent EBSD was completed on samples after thermal cycling. The results are compared with the as-welded condition, specifically regarding the properties of GB character, grain size, grain orientation, GB migration, and GB faceting. Finally, samples are subject to highly restrained simulated strain ratcheting (SSR) and destructively analyzed to evaluate the severity of cracking. It is predicted that high sulfur, and high thermal cycle samples will contain more severe cracking, and possibly experience complete separation failure. The objective of the study is to evaluate the effect of sulfur in a way which may provide insight into the previously identified inconsistency in cracking behavior relative to sulfur content. |