The embrittlement characteristics of large turbine casing bolts: the influence of service induced strain

The present article attempts to describe an assessment of the effects of ac cumulated permanent service strain on the degree of toughness losses, cause d by reverse temper embrittlement (RTE) experienced by large turbine casing bolts during high temperature service. Effectively, it was established tha t (a) reported studies in this area were scarce, (b) the early work exhibit ed somewhat contradictory trends while (c) the post 1990 studies indicated that the presence of strain exacerbated the extent of RTE. The few modern d ay studies suggested that the effects of strain in low alloy steel bolts we re greatest at low levels of up to around %epsilon similar to 0.5% and the trend could be described by the expression: C-N = A(%epsilon)(-0.8), where C-N was the normalised ambient Charpy energy and A was a scaling coefficien t. Also the Charpy fracture appearance transition temperature (FATT) was al so shown to increase in proportion to (%epsilon)(-0.8). One study has repor ted an embrittlement constant, termed C-RTE, which was a function of grain size and bulk %P in the steel, and whose value determined the likelihood of bolt embrittlement during service, i.e., low values predicted embrittlemen t with high values indicating non-embrittlement during service. It was repo rted that strain decreased the value of C-RTE. Within the strain range 0.2% to 0.6% one study established that the effects could be described as follo ws: C-RTE = 0.35 - 0.44 x (%epsilon), while at lower strain values another study reported that C-RTE = 0.0772 X (%epsilon) - 0.64. Note that the expon ent in the latter expression was similar to that which described the normal ised Charpy energy trends. It has been suggested that the total RTE effects , or toughness losses, were the summation of two separate processes, viz., a temperature-time controlled grain boundary phosphorus segregation compone nt and a strain induced grain boundary phosphorus segregation component. Th e latter was thought to be related to enhanced phosphorus movement along ce rtain dislocation networks, caused by plasticity, to grain boundary locatio ns. Although the data was scarce, it was suggested that the degree of grain boundary segregation by phosphorus during high temperature service increas ed with increasing levels of strain. This trend between the atomic fraction of grain boundary phosphorus C-GB(P) and accumulated strain was simply des cribed numerically as C-GB(P) = 10.54 X (%epsilon)(0.35). Finally, actual b olt strain distributions with service times in real working situations are discussed together with considering both embrittlement and creep damage deg radation acting together with time, or accumulated service strain. (C) 1999 Elsevier Science Ltd. All rights reserved.