Creep-fatigue evaluation method for weld joints of Mod.9Cr-1Mo steel. Part I: Proposal of the evaluation method based on finite element analysis and uniaxial testing

In the present study, a method for creep-fatigue life evaluation of Mod.9Cr-1Mo steel weld joint was proposed based on finite element analysis (FEA). Since the point of the creep-fatigue life evaluation in the weld joint is a consideration of the metallurgical discontinuities, FEA was performed using a model with three material properties, a base metal (BM), weld metal (WM) and a heat-affected zone (HAZ) formed in the base metal due to the welding heat input, to consider the mutual relationships among them. The material properties of these three materials were collected and utilized in FEA for considering such metallurgical discontinuities. The creep-fatigue life estimated using the proposed evaluation method based on the FEA results were compared with available creep-fatigue test data, and the proposed method was found to predict the number of cycles to failure within a factor of 3. Moreover, the elastic follow-up factors due to metallurgical discontinuities were calculated using the FEA results for a uniaxial creep-fatigue test. The values of the elastic follow-up factors for both plastic deformation and creep relaxation were less than those defined in the Japanese elevated temperature design code. These considerations will contribute to the codification of evaluation rules for creep-fatigue damage in Mod.9Cr-1Mo steel weld joints.