THE INFLUENCE OF ACCUMULATED STRAIN ON THE TEMPER EMBRITTLEMENT RESPONSE OF A LOW-ALLOY BOLTING STEEL DURING SERVICE

The present paper describes part of a larger study which investigated the effects of accumulated service strain (epsilon) on the reverse tem per embrittlement (RTE) response of a series of large HP and IP turbin e CrMoV steel bolts which had been in service for 122 000 h at elevate d temperatures or around 490 degrees C. It is shown that accumulated s ervice strains exert significant effects on toughness losses in the Cr MoV bolting steel, especially at low levels of strain, and that such e ffects can be adequately described by the expressions C-n = 7.6 x 10(- 2) epsilon(-0.8) and FATT(K) = 418 - 13.28 epsilon(-0.8) where C-n is the normalized Charpy energy and FATT is the Charpy fracture appearanc e transition temperature. It is suggested that the extent of the RTE r esponse in low alloy steels can be separated into a pure RTE component and a strain-induced or synergistic RTE component. Also, from limited Auger electron spectroscopy data it is suggested that the latter may not be the result of strain-induced enhanced grain boundary phosphorus segregation. Finally, when the data are considered in terms of locali zed accumulated strain at the bolt thread root, it is shown that britt le failure of the bolts is unlikely, even at strain levels of 1.5%, in asmuch as although toughness levels are low, no macrodefects exist sin ce the creep damage is in the form of isolated cavities of around 1 mu m in size.