Forum for the Effect of Thermal Aging and Microstructure on Mechanical and EAC Behaviour of Ni-base Alloy Dissimilar Metal Welds (FEMMA)

Dissimilar metal welds (DMWs) are commonly used to join austenitic and ferritic components in the reactor coolant pressure boundary and can become potential concerns regarding the structural integrity of the nuclear power systems, structures and components. In particular, the knowledge on the local strength mismatch at the low alloy steel (LAS)/nickel-based alloy weld metal interface upon post-weld heat treatment (PWHT) and during long-term ageing is lacking. Understanding the fracture mechanical and microstructural changes occurring at the fusion boundary after buttering, welding, PWHT and long-term ageing is crucial for both the improvement of nuclear component integrity and to ensure safe long-term operation. In this study, materials representative of actual power plant components are studied, including a Ringhals SA508/Alloy52 DMW mockup consisting of Alloy 52 buttering on both sides and a TVO narrow gap DMW mock-up. The project deals with the structural integrity, long-term operation, and ageing management. The technical results provide a basis for assessment of long-term operation for the Finnish and Swedish nuclear power plants for both the operators and the regulatory perspectives.