In this work, characteristics of premature creep failure in the over-tempered base metal (OT-BM) of a Grade 91 steel weldment are investigated with specially designed creep experiments and advanced microstructure characterization. In situ digital image correlation (DIC) strain measurements reveal that local creep strain as high as 100% accumulated in the OT-BM, compared with only 10% nominal strain over the gauge length. The creep strain rate in the OT-BM is identical to that in the intercritical heat-affected zone and fine-grained heat-affected zone at the secondary creep stage and early tertiary stage, but faster at the late tertiary stage. Microstructural analysis shows that highly recovered microstructure in the OT-BM leads to the lowest hardness, the largest grain size, the lowest fraction of coincidence site lattice boundaries, and the lowest localized strain energy. Dislocation creep and transgranular creep fracture are the dominant deformation and fracture mechanisms in the OT-BM under the current creep testing condition of a low temperature (550 °C) and a high stress (215 MPa).

在这项工作中，通过专门设计的蠕变实验和先进的微观结构表征，研究了91级钢焊件的回火母材（OT-BM）的过早蠕变破坏特性。现场数字图像相关性（DIC）应变测量显示，OT-BM中累积的局部蠕变应变高达100％，而在整个标称长度上只有10％的名义应变。OT-BM的蠕变应变率与次级蠕变阶段和第三纪早期的临界热影响区和细粒热影响区的蠕变应变率相同，但在第三纪后期更快。显微组织分析表明，OT-BM中高度恢复的显微组织导致最低的硬度，最大的晶粒尺寸，最低的重合部位晶格边界分数，和最低的局部应变能。在目前的低温（550°C）和高应力（215 MPa）蠕变试验条件下，位错蠕变和跨晶蠕变断裂是OT-BM的主要变形和断裂机理。