Some observations concerning reverse temper embrittlement and creep damage in a series of CrMoV steel turbine bolts after 120 000 h of service

Abstract

This paper describes a detailed investigation which was specifically aimed at providing a simple practical assessment procedure which identified 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 non-critical location from all 182 bolts. From this section the chemical composition, average hardness and prior austenite grain size were assessed. The toughness of selected bolts was measured by either (i) Charpy impact testing or (ii) Auger electron spectroscopy. It was established that grain size and bulk phosphorus content were the primary factors which consistently correlated with the embrittled condition. An embrittlement estimative diagram was established by plotting grain size d versus percentage phosphorus. This portrayed two distinct regions, namely an embrittled region and a non-embrittled region, which were separated by a critical 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 ε_AV, the smaller the numerical value of C, i.e. with increasing ε_AV embrittlement was promoted in bolts with finer grain sizes and lower phosphorus levels. Indeed, it was projected that at ε_AV = 0.5% nearly all the bolts in the present investigation would have undergone embrittlement. From a creep damage assessment conducted on a selection of bolts in the thread root location it was observed that, at average strain levels approaching 0.5%, bolts with grain sizes of about 20μm will have suffered RTE and a measurable amount of secondary creep damage. The limited amount of data suggested 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 higher than the average bolt strain ε_AV obtained by bolt length measurements.

Keywords:

• reverse temper embrittlement
• creep damage
• grain boundary cavitation