Modeling of weld residual plastic strain and stress in dissimilar metal butt weld in nuclear reactors

Primary water stress corrosion cracking (PWSCC) constitutes a major safety challenge for dissimilar metal welds (DMW) in pressurized water reactors. The reliability of structure integrity assessment of DMW is strongly dependent on the reliable determination of the weld residual stress (WRS) field, which is one of the primary driving forces for PWSCC. Recent studies have shown that today's WRS models have varying degrees of success in determining the WRS in DMW that is highly dependent upon the strain-hardening models used in WRS simulation. The commonly used strain hardening model in WRS modeling (isotropic, kinematic, and mixed ones) appear to be inadequate in that they neglect the high-temperature time-dependent (viscous) deformation process during welding. This work presents a new strain-hardening model derived from specially designed experiment that mimics the thermal-mechanical deformation process of a SS304L stainless steel under rapid heating and cooling conditions relevant to DMW. Compared to the time-independent strain hardening ones, the new strain hardening model, termed as Dynamic Strain Hardening model, takes into account the effect of time and temperature dependent dislocation annihilation and microstructure recrystallization processes at the elevated temperatures during welding. Moreover, a novel experimental approach based on micro-hardness testing is developed to quantify the residual equivalent plastic strain in a mock-up DMW. It is found that the new dynamic strain hardening model produces results that are more consistent with experimentally measured plastic strains and residual stresses. It is concluded that it is necessary to include the dynamic strain hardening recovery phenomenon to improve the accuracy of WRS predictions in DMW. Moreover, the newly developed micro-hardness based plastic strain measurement approach would be an effective means to quantify the plastic strain distributions which is another key factor for PWSCC in DMW in addition to the residual stresses.