Remaining Creep Life Prediction Method for In-Service Boiler Pipe Weldment Using Small Scale Specimen

The author proposes a remaining creep life prediction method for weldment of Grade 91 steel using a small specimen taken not from the weldment but from the base metal. Analysis of creep data obtained from long-term used steels indicates that the shape of the master rupture curve given by the Larson-Miller parameter remains almost the same regardless of the heat-to-heat variation of the creep life of the steels, and the life of each steel can be expressed by the parallel displacement of the master rupture curves along the abscissa direction. Therefore, we can estimate the life of used steels with only one datum even if it is obtained with a short-time creep test. This approach of extrapolation is effective for the proposed method because the number of test specimens taken from the pipe is at most two or three in power plants in Japan. The results of investigation of the relationships among creep strength, deformation, and ductility suggest that the creep deformation property primarily determines the creep life of the materials used in this study. Finally, the proposed method is applied to the in-service boiler pipe weldment of Grade 91 steel, and its features are described in comparison with the conventional life assessment method.