The influences of impurity content, tensile strength, and grain size on in-service temper embrittlement of CrMoV steels
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I.
INTRODUCTION
T H E CrMoV-type steels with upper bainitic microstructures have been used worldwide for fossil highpressure (HP) and intermediate-pressure (IP) steam turbine rotors. Typical inlet and exhaust temperatures for these HP and IP turbine elements are 538 ~ (1000 ~ and 260 ~ (500 ~ respectively. Therefore, an HP or IP rotor operates in a temperature range where temper embrittldment can occur during service. The temper embrittlement is manifested by an increase in the ductileto-brittle transition temperature (FATI') of the steel and is reversible. It is now well established that segregation of residual elements, such as P, Sn, Sb, and As, at the prior austenite grain boundaries is responsible for temper embrittlement of low alloy steels. I~ 4j Thus, the embrittlement susceptibility of a steel generally increases with increasing impurity element content in the steel. Although CrMo- and CrMoV-type steels were generally believed to be immune to temper embrittlement, the results of step-cooling experiments I5-81carded out over the last decade reveal that these steels are also susceptible to temper embrittlement. The degree of embrittlement, however, depends upon the transformation product and the tensile strength of the steel. Viswanathan and Joshi tSl showed that at a given hardness level, CrMoV steel with tempered martensitic structure was more sus-
N.S. C H E R U V U , Principal Fngineer, and B.B. SETH, Advisory Engineer, are with the Power Generation Operations Division, Westinghouse Electric Corporation, Orlando, FL 32826-2399. Manuscript submitted November 23. 1988. METAI.I.URGICAL TRANSACTIONS A
ceptible to temper embrittlement than tempered bainite, and the embrittlement susceptibility increased with increasing hardness of the martensitic steel. In the tempered bainitic condition, the embrittlement susceptibility was dependent on hardness when the steel was tempered to hardness values greater than Rc 30, and below this value, temper embrittlement susceptibility was almost independent of hardness. 151 In fact, at hardness levels comparable to those of rotor forgings (Re 30), the stepcooling treatment increased the FATT by only 10 ~ which was significantly lower than the shift in FATT reported for service-exposed rotors. I9'~~ Hence, it is not possible from the conventional step-cooling studies to predict the influence of hardness or tensile strength on the in-service temper embrittlement behavior of CrMoV rotor steels. Furthermore, from the regression equations developed for determining the shift in FAT'I" of step-cooled and isothermally embrittled CrMo steels, Shaw I81 estimated that the 30-year in-service embrittlement was equal to three times that of the shift produced by the step-cooling treatment. Investigations by Argo and Seth I~21further showed that the degree of in-service temper embrittlement of CrMoV rotor steels was also a function of the temperature at which the rotors had been austenitized. Their results showed that the rotor steel which was austenitized at 1010 ~ (1850 ~ exhibited si
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