Dissimilar metal welds (DMWs) are commonly used to join austenitic and ferritic components in the reactor coolant pressure boundary, RCPB, often using a nickel-based filler metal. DMWs can become potential concerns regarding the structural integrity of the nuclear power systems, structures and components (SSC). The Stress Corrosion Cracking behaviour of nickel based weld metals have been extensively researched, whereas other features necessary for structural integrity assessments regarding fracture mechanical properties are partially missing. 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. The amount of information available in the open literature on the fracture mechanical and microstructural changes occurring at the fusion boundary after buttering, welding, PWHT and long-term ageing is relative limited and often very case dependent.
The ageing and degradation behaviour and effects on the structural integrity have been studied over an extended period by the Nordic country stakeholders. Current ongoing effort is exemplified by the FEMMA project. The objective of FEMMA (Forum for the Effect of Thermal Ageing and Microstructure on Mechanical and EAC Behaviour of Ni-based Alloy Dissimilar Metal Welds) is to study the microstructural properties and fracture mechanical performance of Alloy 52 narrow-gap (NG) DMW provided by TVO and Alloy 52 DMW mock-up consist of Alloy 52 buttering on both sides provided by Ringhals.
So far, a 12 °C shift in the ductile to brittle transition temperature was observed after 10000 hours ageing at 400 °C, representative of 60 years of plant operation. In addition to this, it was observed that the maximum hardness at the fusion line between weld- and base metal decreased due to ageing. Three types of fusion boundaries were identified, i.e., narrow fusion line, tempered martensitic transition region and wide partially mixed zone caused by the local heat flow and complex elementary diffusion, which were found to significantly influence the hardness mismatch across the local DMW interface.
In the current work scope, detailed microstructure and chemical composition mapping at the different types of fusion boundaries will be performed with scanning and transmission electron microscopy (TEM). Hardness mismatch of the SA508/nickel-based Alloy 52 fusion interface will be investigated by nanoindentation. Atom probe tomography (APT) will be used to identify the elemental segregation (especially phosphorous) at the boundaries upon ageing, primarily by comparing the phosphorous content of interiors of the grains. The fracture mechanical tests include fracture toughness J-R testing at both soft and hard zones (i.e., weld metal and LAS next to the fusion line), using different specimen types and crack lengths and master curve transition temperature T0 testing at slightly varying locations in the LAS carbon-depleted zone and in a bit deeper into the LAS matrix will be performed.
The work will be executed by two Ph. D. students Noora Hytönen (microstructural investigations) and Laura Sirkiä (Fracture mechanical studies) at VTT. Three young scientists Kristina Lindgren (Chalmers), Zaiqing Que (VTT) and Sebastian Linqvist (VTT) who successfully defended their theses in 2018-2020 are as the main working/supervision group. Additionally, young researchers Pentti Arffman and Jari Lydman at VTT will be integral part of the work.
A seminar, summarising the gained knowledge and lessons learned from the earlier DMW projects will be arranged at KTH with the main objective to facilitate spreading of the gained knowledge to the industry, authorities, research community, and especially the young generation.