Degradation of mechanical properties of Cr-Mo-V and 2.25Cr-1Mo steel components after long-term service at elevated temp
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I. I N T R O D U C T I O N
Low
alloy ferritic steels of the Cr-Mo-V and C r - M o type are extensively used for high temperature applications in the power, chemical, and oil industries, for their excellent elevated temperature strength and good resistance to oxidation and hydrogen embrittlement. These steels are used in the power industry for components such as high pressure (HP) and intermediate pressure (IP) fossil steam turbine rotors, cylinders, blade rings, nozzle chambers, steam chests, steam pipes, boilers, and headers. The inlet and exhaust temperatures of a typical fossil HP or IP turbine element are 538 ~ (1000 ~ and 288 ~ (550 ~ respectively. Thus, the steam turbine components operate in the temperature range where creep and temper embrittlement is a concern. Though Cr-Mo-V and Cr-Mo steels were believed to be immune to temper embrittlement, the results generated over the last decade indicate that they are highly susceptible to temper embrittlement, tl,2,31 Thus, prolonged service exposure of these turbine components leads to degradation of their mechanical properties. Hence, the remaining service life of these components is limited by the extent of in-service material degradation that has occurred. Evaluation of inservice material degradation after long-term service is therefore necessary for meaningful remaining service life assessment. When low alloy steel components are exposed to elevated temperature during service, their properties deteriorate due to changes in microstructure and diffusion
N.S. CHERUVU is Senior Engineer, Power Systems Division, Westinghouse Electric Corporation, The Quadrangle, University Blvd., Orlando, FL 32826. Manuscript submitted April 22, 1988. METALLURGICAL TRANSACTIONS A
of impurities such as P, Sn, As, and Sb to the grain boundaries. It is now well established that segregation of impurities at the grain boundaries temper embrittles these steels. The segregation-induced temper embrittlement is manifested as an increase in the transition temperature and is reversible. The changes in microstructure, such as carbide coarsening and precipitation of more stable carbides during service, can cause softening and irreversible embrittlement. The carbideinduced embrittlement is manifested as a decrease in the impact toughness without affecting the ductile-to-brittle transition temperature t4'5'6] and is irreversible, unlike segregation-induced embrittlement. Recent investigations by several investigators revealed that Cr-Mo, tS~ 2 . 2 5 C r - l M o , t7'81 and Cr-Mo-V rotor steels t9'~1~ had softened significantly as a result of elevated temperature exposure. Zhe e t a l . t1~ studied the influence of operating temperature on the hardness of a rotor, which was referred to as Buck rotor, after seventeen years of service and reported that the steel had softened when the service temperature was greater than 308 ~ (586 ~ However, this temperature is too low to cause any significant changes in microstructure during service since the rotor forgings are normally tempered, prior to placi
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