The influence of accumulated strain on the temper embrittlement response of a low alloy bolting steel during service

Abstract

The present paper describes part of a larger study which investigated the effects of accumulated service strain (ε) on the reverse temper embrittlement (RTE) response of a series of large HP and IP turbine CrMoV steel bolts which had been in service for 122 000 h at elevated temperatures or around 490°C. It is shown that accumulated service strains exert significant effects on toughness losses in the CrMoV bolting steel, especially at low levels of strain, and that such effects can be adequately described by the expressions

Cn = 7.6 × 10⁻² ε^−0.8

and

FATT(K) = 418–13.28 ε^−0.8

where Cn is the normalized Charpy energy and FATT is the Charpy fracture appearance transition temperature. It is suggested that the extent of the RTE response 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 localized accumulated strain at the bolt thread root, it is shown that brittle failure of the bolts is unlikely, even at strain levels of 1.5%, inasmuch as although toughness levels are low, no macrodefects exist since the creep damage is in the form of isolated cavities of around 1 μm in size.

Keywords:

• accumulated strain
• reverse temper embrittlement
• low alloy steel