Correlation of deformation with damage progression behavior around a notch tip under creep and fatigue conditions for W-added 9Cr steel including weld joint

Research concerning heat-resistant steels for the application in fossil-fired power plants has progressed remarkably during the past 60 years. This has resulted in improvements in the electrical efficiency of fossil-fired power plants. Currently, there are plans and programs to develop ultra-supercritical plants designed to operate at steam temperature and pressure conditions of 600/650 °C and 32 MPa. The W-added 9%Cr ferritic heat-resistant steel, that is, ASME grade P92, has been developed as a boiler material for this ultra-supercritical plant. Boiler materials, whose performance is critical for ultra-supercritical plant, are required to possess high creep resistant properties. In addition, these materials are exposed to fatigue induced by thermal stresses, that is, they are operated under creep-fatigue interacting conditions. In this study, mechanical tests under the condition of high temperature creep-fatigue interaction were conducted for P92 steel under stress-controlled and various load frequency conditions using the in-situ observational creep-fatigue testing machine to observe the damage formation behavior around a notch tip composed of voids in mesoscale. On the basis of these results, the effects of damage formation behavior on crack growth life were clarified. Furthermore, for the case of creep deformation, the numerical analyses of vacancy diffusion and concentration around a notch tip were conducted using our proposed numerical method of local stress-induced vacancy diffusion behavior, which is a nanoscale phenomenon to relate these behaviors to the damage formation behavior in mesoscale (μm scale).