New Analytical Approach to Predict Creep Void Growth in Heat-Affected Zone of High Cr Steel Weldments

This paper proposes a new analytical approach to predict creep void growth and remaining creep lifetime in a heat-affected zone (HAZ) of high Cr steel weldments. Concept of the new approach is based on a relationship between creep void growth rate and a parameter to represent multi-axial stress state obtained by finite element analysis. In this study, creep tests of ASME grade 91 (9Cr-1Mo-Nb-V) and grade 122 (11Cr-2W-0.4Mo-Cu-Nb-V) tubes with longitudinal weldments subjected to various internal pressures have been conducted to reveal creep void growth behavior in HAZ. Some specimens were intentionally interrupted before leakage at a damage level from 40 to 70% for clarification of void growth behavior at intermediate damage level. In addition, finite element creep analyses of the specimens at different creep strain rates in base metal, weld metal and HAZ have been carried out to investigate distribution of stress and stress triaxiality factor in HAZ. A comparison between stress distributions and void distributions revealed that stress triaxiality factor predominantly affects growth behavior of creep voids. From the result, the relationship between creep void growth rates and the parameter as a function of principal stress and triaxiality factor was established. Based on the relationship, a new prediction method was proposed. To verify proposed approach, the new method was applied to the elbow pipe with longitudinal weldment. As a result, predicted creep void growth in HAZ showed a good agreement with the results from diffusion simulation reported by literature. Therefore, the results demonstrated that the proposed approach is applicable to predict void distribution and remaining creep lifetime in the HAZ of high Cr steel weldments.