The purpose of this study is to make clear effect of material degradation on creep and creep-fatigue properties of a weldment. Materials used in this study were a weldment of a boiler super header operated for about 200,000 hours (U weld joint) and a simulated as-received weldment (S weld joint). In the U weld joint, although creep cavities were not observed at all, significant coarsening of carbide precipitation and low dense dislocation structure were observed at all locations in the weldment compared with microstructure of the S weld joint. Creep rupture time of the U weld joint is shorter than that of the S weld joint at higher stress region, while the discrepancy of the rupture strength tends to disappear as stress level becomes lower. Strain controlled creep-fatigue tests with 1 hour and 10 hours strain hold time at the maximum tensile strain were performed at strain range of 0.3% and 0.5% on S and U weld joints. No difference of failure life between materials was observed. Reduction in failure life by introducing strain hold period becomes larger as decreasing strain range level indicating that creep-fatigue interaction is enhanced under lower strain range. Based on the nonlinear damage accumulation model, a simplified creep-fatigue life prediction method of weldments under creep-fatigue loading conditions was proposed. It was demonstrated that failure lives under the long-term creep-fatigue conditions were well predicted by the simplified prediction method.

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