Uniaxial Creep Rupture Property of Mod.9Cr-1Mo Steel Weld Joint and Proposal of Creep Damage Evaluation Method
Takashi OGATA, Takayuki SAKAI and Masatsugu YAGUCHI
Abstract:Modified 9Cr-1Mo steels (P91) have been used for high temperature steam pipes in ultra super critical power plants not only in Japan but also in other countries. Since weld joints of P91 indicate lower creep damage resistance than base metal, it is necessary to establish damage assessment method for weld joints of P91 to maintain reliable operation. In this study, creep tests have been conducted on a P91 to identify creep rupture life property of weld joints, and creep damage process in heat affect zone (HAZ) has been clarified by interrupting the creep tests. As a result, it was found that creep rupture time of the weld joints reduced to about 1/5 of the base metal and creep voids have already initiated at 20% of creep life in the HAZ. Size and number of voids in the HAZ increased with increasing creep damage. Stress distribution within the weld joint specimen was calculated by a finite element analysis of a weld joint model consisting of base metal, weld metal and HAZ. It was indicated that creep strain accumulated in the HAZ preferentially resulting in Type IV failure of the weld joint specimen. Creep rupture time of the weld joint can be predicted by creep strain rate in the HAZ and "limited creep strain" defined in this study. Authors proposed the creep void growth simulation method in the previous study based on void kinetics. This method applied to creep void growth simulation in the HAZ of the P91 weld joint. It was confirmed that quantitative void growth behavior could be predicted by the void growth simulation method. Key Words:Mod.9Cr-1Mo steel, Weld joint Creep rupture, Type IV damage, Creep void, Void growth simulation