The influences of impurity content, tensile strength, and grain size on in-service temper embrittlement of CrMoV steels

Abstract

The influences of impurity levels, grain size, and tensile strength on in-service temper embrittlement of CrMoV steels have been investigated. The samples for this study were taken from several steam turbine CrMoV rotors which had operated for 15 to 26 years. The effects of grain size and tensile strength on embrittlement susceptibility were separated by evaluating the embrittlement behavior of two rotor forgings, which were made from the same ingot, after giving an extended step-cooling treatment. The results reveal that among the residual elements in the steels, only P produces a significant embrittlement. The variation of P and tensile strength of the steels in the ranges investigated has no effect on in-service temper embrittlement susceptibility, as measured by the shift in fracture appearance transition temperature (FATT). However, the prior austenite grain size plays a major role on in-service embrittlement. The fine grain steels with a grain size of ASTM No. 9 or higher are virtually immune to in-service embrittlement. In steels having duplex grain sizes, the embrittlement susceptibility is controlled by the size of coarser grains. For a given steel chemistry, the coarse grain steel is more susceptible to in-service embrittlement, and a decrease in ASTM grain size number from 4 to 0/1 increases the shift in FATT by 61°C (110°F). It is demonstrated that long-term service embrittlement can be simulated, except in very coarse grain steels, by using the extended step-cooling, treatment. The results of step-cooling studies also show that the coarse grain rotor steels take longer time during service to reach a fully embrittled state than the fine grain rotor steels. This difference in the kinetics of embrittlement is believed to be related to the variations in Mo content in the matrix and the grain size of the steels.