Clarification of creep damage mechanisms and establishment of remaining life prediction methods of weldment parts of P91 boiler pipings are important subjects to maintain reliable operation of boilers in thermal power plants. In order to develop a creep damage assessment method of weldment parts of P91 pipings, internal pressure creep tests were conducted on P91 steel longitudinally welded tubes and a previously proposed void growth simulation method is applied to predict void growth behavior. Failure occurred at the heat affected zone without significant deformation. It was found from observation of creep damage interrupted specimens that initiation of creep voids concentrated at the mid-thickness region rather than the surface. It was suggested that triaxial stress states caused acceleration of creep damage evolution in the heat affected zone resulting in internal failure of the tube specimens. Void growth behavior in the heat affected zone was well predicted by the previously proposed void growth simulation method. The void growth prediction method is applied to predict creep damage induced by void initiation and growth in a weldment part of an actual P91 pipe. From comparison of void number density between measurement for a weldment part of a retired elbow pipe and prediction by the simulation, good agreement is obtained indicating the void growth simulation method can be applied to creep damage assessment of weldment parts in actual boiler piping.

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