Disputation - Rickard Shen
Rickard Shen defended his PhD thesis in solid mechanics, “On the low primary water stress corrosion cracking susceptibility of weld deformed Alloy 690”, on March 29, 2017. Faculty opponent was Professor Grace Burke from the Materials Performance Center of University of Manchester, UK. Adj Professor Pål Efsing has been Rickard’s supervisor.
Rickard has investigated factors influencing stress corrosion cracking, SCC, susceptibility in structural materials used in Nuclear Power Plants. In recent years it has been shown that the nickel-base Alloy 690 can become susceptible to SCC in the primary water of nuclear power plants with pressurized water reactors if it has been sufficiently deformed at room temperature. Although the material is not intentionally used in a deformed state, it can become deformed by various manufacturing processes. Welding is believed to be the process that is most likely to cause susceptibility, yet it does not seem quite that detrimental in experiments. The overall purpose of this work was to investigate why weld-induced deformation does not seem to cause the same degree of susceptibility as cold deformation.
The work started with a microstructural investigation to assess if any of the changes caused by welding can explain the difference in SCC behavior. While a beneficial change in the microstructure was observed, it was not enough to explain the differences.
The focus was then turned towards addressing knowledge gaps of the method used to assess weld-induced deformation. This method is based on measuring misorientations using electron backscatter diffraction (EBSD). It was shown that kernel average misorientation (KAM) is closer related to the degree of hardening than the degree of deformation, and that it can be used to obtain a qualitative map of hardness at the micrometer scale. Improvements to the KAM based method have been proposed along with estimates of strain distributions near welds from component mockups.
In the final paper of the study, the validity of using a misorientation-based method on warm deformation was tested. It was shown that the method gives a rough estimate of the degree of strain hardening, although the data suggests it is a small overestimation. The overestimation would mean that weld deformation may have a lower hardness than the strain estimate implies, which is beneficial for SCC resistance.