Damage characterization of a P91 steel weldment under uniaxial and multiaxial creep

Clarification of creep damage mechanism and establishment of remaining life assessment methods of boiler pipings of P91 steel with weldment are important subjects to maintain reliable operation of thermal power plants. in order to characterize the creep damage of a P91 steel weldment and to discuss creep rupture life prediction methods, uniaxial creep tests on a P91 steel cross weld and internal pressure creep tests on a longitudinal welded tube specimen were conducted. Three-dimensional finite-element creep analysis of the longitudinal welded tube specimen was conducted to identify stress and creep strain distribution within the specimen. Creep rupture time of the weld joints was reduced significantly from that of the base metal due to Type IV failure. It was found from an examination of creep damage in interrupted cross weld specimens that creep voids have already initiated around 20% creep damage, and that the number of voids inside the specimens was larger than those at the surface of the specimens. Creep analysis results indicated that triaxial tensile stress yielded at mid-thickness in the heat affected zone (HAZ) of the tube specimen due to differences of creep deformation properties of the base metal, the HAZ and the weld metal. It was suggested that triaxial stress states caused acceleration of creep damage evolution in the HAZ resulting in internal failure of the tube specimens. Rupture life of the longitudinal welded tube specimen under internal pressure creep is predicted from the circumferential steady-state creep strain rate at the HAZ obtained from creep analysis using a three-materials finite-element model of the tube specimen. (C) 2008 Elsevier B.V. All rights reserved.