Loss of creep resistance in post-weld P91 alloy occurs mainly due to the change in microstructure particularly in the heat-affected zone under actual service operating conditions as well as residual stress from the welding process that is not often properly addressed in many damage models. In this paper, a validated deformation mechanisms map (DMM) using low temperature creep strain accommodation processes i.e. GBS, is used for the P91 alloy that predicts the creep rates over a wide range of temperature and stress including those arising under in the actual service conditions. These creep rates are further utilized into a microstructure-based creep damage model for accurate life prediction. A 3D transient computational welding mechanics (CWM) modeling of a pipe in a super-critical water loop, predicts the thermal, microstructure and stress state from welding. It also determines the coarse and fine grain heat affected zone (CGHAZ & FGHAZ). The CWM results are coupled with physics-based creep damage modeling to practically predict the creep life under the actual service conditions considering the welding residual stress and microstructure states.

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