Ferritic Steel / CSEF - An Engineering Approach For Weld Creep Lifetime Assessment based on Local Property Measurement

The localized creep failure in the heat-affected zone (HAZ) of Grade 91 steel weldments has been identified as one of the most important factors causing significantly shortened service lifetime and structural integrity issues of welded components in advanced fossil and nuclear power plants. To conduct a reliable creep lifetime assessment, a new engineering assessment approach has been developed by incorporating the experimentally determined local properties of the heterogeneous HAZ. By creep testing a purposely simulated HAZ specimen with in situ digital image correlation (DIC) technique, the nonuniform creep deformation across the HAZ of Grade 91 steel was quantitatively measured. A physical creep cavitation constitutive model was proposed to investigate the local creep deformation and damage accumulation within the heterogeneous HAZ, which takes into account the nucleation of creep cavities and their growth by both grain boundary diffusion and creep deformation. The relationship among the local material property, creep strain accumulation, and evolution characteristic of creep cavities was established. The approach was then utilized to investigate the creep deformation behavior for cross welds of Grade 91 steel. The predicted results exhibited quantitative agreement with the DIC measurement of both nominal/local creep deformation as well as creep rupture lifetime over a wide range of applied temperature and stress conditions. Furthermore, the proposed approach provided quantitative remaining life prediction for an after-service 9% Cr steel weldment (>100,000 service hours) by including the specific aspects regarding the effects of pre-existing damages which developed and accumulated during long-term services. The present work established an effective engineering approach for remaining life assessment of creep-resistant steel weldments.