Modeling of Weld Residual Plastic Strain and Stress in Dissimilar Metal Butt Weld in Nuclear Reactors.

摘要

Primary water stress corrosion cracking (PWSCC) is a major materials 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 todays DMW WRS models have significant variations in predictions that often depend on the choice strain hardening model. The commonly used strain hardening models (isotropic, kinematic, and mixed) can do not account for high temperature time-dependent (viscous) deformation during welding. Viscous deformation can be a significant effect and not accounting for it can be limitation of current models. 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 models, the new 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 including the dynamic strain hardening recovery phenomenon can improve the accuracy of WRS predictions.