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

Abstract

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.

Creep-fatigue evaluation method for weld joint of Mod.9Cr-1Mo steel Part II: Plate bending test and proposal of a simplified evaluation method

“…Therefore, we have developed a method for evaluating creep-fatigue in Mod.9Cr-1Mo steel weld joints. In Part I [9], finite element analysis (FEA) considering the three types of materials properties in weld joints such as the base metal (BM), weld metal (WM), and heat-affected zone (HAZ) was performed to evaluate the creep-fatigue life in the uniaxial test.…”

“…In the present study, to validate the evaluation procedure proposed in Part I [9], plate bending tests with weld joints are performed. FEA is then performed considering the test conditions and using the model based on the three materials in weld joints.…”

“…FEA is then performed considering the test conditions and using the model based on the three materials in weld joints. The material properties for the BM, HAZ, and WM used in the FEA are the same as those used in Part I [9].…”

“…The material properties of the HAZ were also defined based on those of the BM in the JSME FRs code; namely, the creep properties of the HAZ were assumed to be those in the JSME FRs code after applying a time factor, (α R ), of 10 by referring to the material test result of the 8/38 simulated HAZ in Part I [9]. As a result, the creep rupture time of the HAZ was 10 times less than that of the BM.…”

“…As a result, the creep rupture time of the HAZ was 10 times less than that of the BM. In addition, in the same manner as the Part I [9], the creep-relaxation rate was accelerated because the creep strain equation in the JSME FRs code includes the creep rupture equation with time factor (α R ).…”