Microstructure and Tensile Properties of a 1.25Cr-0.5Mo Main Steam Pipe After Long-Term Service

Metallographic tests, micro-hardness tests and tensile tests were conducted for a 1.25Cr-0.5Mo main steam pipe weldment served for more than 26 years. The results were compared with those for virgin material. Microstructural evolution of 1.25Cr-0.5Mo base metal was investigated. Degradation in micro-hardness and tensile properties were also studied. In addition, the tensile properties of subzones in the ex-service weldment were characterized by using miniature specimens. The results show that obvious microstructural changes including carbide coarsening, increasing inter lamella spacing and grain boundary precipitates take place after long-term service. Degradation in micro-hardness is not obvious. However, the effects of long term service on tensile deformation behavior, ultimate tensile strength and yield stress are remarkable. Based on the yield stress of micro-specimens, the order of different subzones is: WM > HAZ > BM, which is consistent with the order of different subzones based on micro-hardness. However, the ultimate tensile strength and fracture strain of HAZ are lower than BM. Brittle failures can happen more easily for HAZ due to its high yield ratio.