Type IV Cracking and Life Evaluation of Weldments on 2.25Cr-1Mo Steel under Creep-Fatigue Loading Condition
Ogata Takashi; Yaguchi Masatsugu
Abstract:Under the circumstance that aging of boiler components proceeds and some 'Type IV' cracking incidents were recently reported in boiler weldment parts, development of an accurate remaining life evaluation method for the weldment parts is an important subject for the utilities. In this study, in order to clarify a cause of the 'Type IV' cracking under creep-fatigue loading condition and to develop a life evaluation method for boiler weldment parts, creep-fatigue tests on the heat affected zone (HAZ) simulated materials, base metal, weld metal and weld joint of 2.25Cr-1Mo steel, and elastic-plastic and creep analysis for the weld joint was conducted. As a result, initiation of many cavities and 'Type IV' cracking were observed in a fine grain region of the weld joint specimen failured under the creep-fatigue loading. It was found from the comparison between experimental evidences and the analytical results that 'Type IV' cracking was caused by two major reasons. One of them is accumulation of creep strain during strain hold in the fine grain region is larger than that in other region suggesting progress of creep damage in the fine grain region prior to other regions. The other one is existence of multiaxial tensile field within the fine grain region caused reduction of failure ductility. Crack initiation portion and failure life under the creep-fatigue test could be well predicted by the non-linear damage accumulation model based on finite element analysis using conventional elastic-plastic and creep theory. Key Words:aging boiler, weldment part, heat affected zone, 2.25Cr-1Mo steel, creep-fatigue, Type VI, cracking creep cavities, life prediction