Effects of long-time service at high temperature on the material strength and J-R curve of Grade 91 steel

The effects of long-time service at high temperature on the material strength, ductility, and J-Resistance behavior of Mod.9Cr-1Mo (ASME Grade 91) steel were investigated based on tensile and J-R tests with virgin and service-exposed Gr.91 steel specimens. The service-exposed Gr.91 steel specimens were sampled from a tee junction in a reheat steam piping system of an ultra-supercritical (USC) plant in Korea with an accumulated operation time of 73,716 h. Comparisons of the test data were made not only between the test results of the virgin materials and service-exposed materials but also between the test results and the material properties in the elevated temperature design (ETD) codes. These test data of yield strength (YS) and tensile strength (TS) for the virgin and service-exposed Gr.91 steel were compared with those of the ETD code, which showed that the current ETD code overestimates the YS and TS properties for long-time service in a non-conservative manner for Gr.91 steel. J-R test data currently not available in ETD codes were produced over a range of temperatures for virgin and service-exposed Gr.91 steel. The effects of long-time service in J-R curve were investigated.