Thermal ageing and mechanical performance of narrow-gap dissimilar metal welds

NIWEL (Nickel-Base Alloy Welding Forum) project was primarily addressed to theneeds of the nuclear energy industry, and it was the fourth consecutive cooperative project on the dissimilar metal weld (DMW) topics. The main focus of the NIWEL project was to assess the effects of thermal aging on the performance of DMWs. Cooperation with PARENT (Program to Assess the Reliability of Emerging Nondestructive Techniques) program was also a part of the NIWEL project, and PARENT results are shortly presented in this report. The long-term behaviour of the DMWs is of particular interest in Finland, because the nuclear power plants are at critical stages of their lifetime: either beginning their operation or reaching lifetime extensions. In both cases, understanding of the aging phenomena is of utmost importance in order to ensure safe operation for lifetimes that can reach 60 years or even beyond that. With the exception of the fusion boundary region, the obtained microstructural characterization results on the weld metal and the base material did not show any clear indications of metallurgical changes. The fusion boundary region exhibits zones that are very narrow, some tens of micrometres or even less in width. The nanoindentation results show that the high local hardness mismatch between the soft carbon-depleted zone (CDZ) and very hard carbon-enriched zone in the Alloy 52 weld metal side of the fusion boundary, caused by PWHT, reduces due to thermalageing. Narrow-gap Alloy 52 DMW sustains its high performance, regarding ductile and brittle fracture properties adjacent to the fusion boundary between SA 508/Alloy 52 after thermal aging (10000 h at 400 °C) representative of 60 years of plant operation. Ductile-to-brittle transition temperature (T _42J ) measured after 10000 h aging is 12 °C, which gives a ΔT _42J of 47 °C as compared to the as-received material. Fracture toughness T₀ was not significantly affected by thermal aging. Thus, it is assumed that thermal aging may promote brittle crack propagation and degrade material's capability to arrest the growing cracks at low temperatures, whereas the brittle crack initiation tendency of the material is not markedly affected. However, further investigations are needed to clarify the mechanisms explaining the differences. CDZ is the preferred crack path in all types of fracture mechanical tests applied in this study. However, crack deflections to the Alloy 52 weld metal side ofthe fusion boundary occur due to the small weld defects observed in the weld metal in tests, where the fracture mode is ductile.