Degradation of mechanical properties of Cr-Mo-V and 2.25Cr-1Mo steel components after long-term service at elevated temperatures

Abstract

The influence of operating temperature on in-service degradation of mechanical properties of high temperature stream turbine components has been investigated. Material samples for this study were taken from a Cr-Mo-V rotor and several 2.25Cr-1Mo cast steel components which had operated over 200,000 hours. The test results revealed that the degree of in-service degradation of strength, toughness, and the fracture appearance transition temperature of both steels were very sensitive to the service temperature. Both steels softened only when they were exposed at a temperature greater than 454°C (850°F) and the degree of softening increased with further increase in service temperature. In Cr-Mo-V steel, the loss in strength was accompanied by an improvement in ductility and toughness. Despite softening of 2.25Cr-1Mo steel in service, elevated temperature exposure resulted in a marked decrease in ductility and toughness. The loss of toughness in this steel was in part irreversible. In contrast, a severe increase in fracture appearance transition temperature, due to reversible temper embrittlement, occurred in both steels at a service temperature of around 427°C (800°F), but not at the highest service temperature. In fact, the Cr-Mo-V steel did not temper embrittle as a result of service exposure at the highest operating temperature investigated. These results are rationalized in terms of changes in microstructure and grain boundary chemistry that occur in service as a function of operating temperature.