SSM perspective
This report concerns a study which has been conducted for the Swedish Radiation Safety Authority, SSM. The conclusions and viewpoints presented in the report are those of the author/authors and do not necessarily coincide with those of the SSM.
Abstract
It is common for nickel-base materials to be used as filler metals in the welding of dissimilar metal welds. Dissimilar metal welds are usually made by first welding a nickel base material onto a ferritic component to form a transition section called buttring. The weld is then heat treated to reduce weld stresses in the ferritic steel and machined prepared. Finally, a component of austenitic stainless steel or nickel-based alloy is welded to the buttring.
High residual stresses are usually generated in the production of dissimilar welds. Residual stresses have a major impact on the initiation and growth of crack-like defects that can occur in mechanical components. Conservative approaches are used when performing fracture mechanics evaluation. Typically, by assuming upper limit data for the residual stress distribution in combination with upper limit data for crack growth for stress corrosion cracking. The conservatisms sometimes result in very high postulated crack growth rates and correspondingly short inspection intervals. However, in-service experiences are different, and there is therefore a need to create more realistic residual stress profiles.
Background
Residual stresses have been modelled and calculated for a number of commonly used butt welds in nickel-based materials. The welds differ in terms of material, buttring design, material thickness, radius, number of weld beads, strain energy and welding process. The results obtained have been compared with well-documented experiments and show good agreement with measurements. Based on the results, best-estimate residual stress profiles have been developed along the weld centre-line and along the heat affected zone and buttring. Sensitivity analyses have been performed on the applied strain energy, which show that changes in applied strain energy have the largest impact on the magnitude of the residual stress for so-called medium thickness pipes.
Results
The project has improved the understanding of the magnitude and distribution of residual stresses for a number of commonly used butt welds in nickel-based materials in Swedish nuclear power reactors. The results from the project are important for developing appropriate damage tolerance analyses for inspection programmes or for assessing safety margins for defects.
Conclusions
The project has developed residual stress profiles based on axi-symmetry. Such an approximation means that local effects from, for example, start and stop positions of the weld bead are not considered. Local effects can be of particular interest when analysing the damage resistance of welds with accumulations of start and stop positions. Two ongoing projects (SSM2018-1621 and SSM2021-3706) are investigating the effects of start and stop positions in welds.