SOME OBSERVATIONS CONCERNING REVERSE TEMPER EMBRITTLEMENT AND CREEP DAMAGE IN A SERIES OF CRMOV STEEL TURBINE BOLTS AFTER 120000 H OF SERVICE

This paper describes a detailed investigation which was specifically a imed at providing a simple practical assessment procedure which identi fied reverse temper embrittlement (RTE) in a series of high-pressure ( HP) and intermediate-pressure (IP) CrMoV steel turbine bolts from two separate 120 MW sister units which had been subjected to 120 000 h of service. A small section of material was removed from a noncritical lo cation from all 182 bolts. From this section the chemical composition, average hardness and prior austenite grain size were assessed. The to ughness of selected bolts was measured by either (i) Charpy impact tes ting or (ii) Auger electron spectroscopy. It was established that grai n size and bulk phosphorus content were the primary factors which cons istently correlated with the embrittled condition. An embrittlement es timative diagram was established by plotting grain size d versus perce ntage phosphorus. This portrayed two distinct regions, namely on embri ttled region and a non-embrittled region, which were separated by a cr itical interface which could be described by the expression d(% P) = C where C is a constant. The value of C is, however, strongly dependent upon the level of accumulated bolt strain during service. Indeed, the higher the value of average bolt strain E(AV), the smaller the numeri cal value of C, i.e. with increasing E(AV) embrittlement was promoted in bolts with finer grain sizes and lower phosphorus levels. Indeed, i t was projected that at E(AV) = 0.5% nearly all the bolts in the prese nt investigation would have undergone embrittlement. From a creep dama ge assessment conducted on a selection of bolts in the thread root loc ation was observed that, at average strain levels, aproaching 0.5%, bo lts with grain sizes of about 20 mum will have suffered RTE and a meas urable amount of secondary creep damage. The limited amount of data su ggested that, for coarser grain sizes, creep damage or grain boundary cavitation occurred at lower accumulated strains. Finally it was shown that, in the few instances where secondary creep damage was recorded, the accumulated strain at the thread root location was 2.7 times high er than the average bolt strain E(AV) obtained by bolt length measurem ents.