| Abstract: | This paper describes a detailed examination which could be conducted during a planned outage. It is concerned with assessing reverse temper embrittlement of CrMoV steel turbine bolts after 120 000 h of service. A small section of material was removed from a non-critical location of all the 92 IP and HP bolts. From this section, the chemical composition, average hardness and average prior austenite grain size were measured. The toughness of the bolts was measured by Charpy impact testing and/or Auger electron spectroscopy. From the various parameters investigated, it was established that grain size and phosphorus level were the only factors which consistently identified whether a bolt was embrittled or non-embrittled. Indeed, at a phosphorus level of 0.01%, bolts with grain sizes less than 20 μm were not embrittled while those with larger grain sizes suffered reverse temper embrittlement during service. An embrittlement estimative diagram was established by plotting grain size versus phosphorus level (%P). This portrayed two distinct regimes, an embrittled and non-embrittled regime which were separated by a critical embrittled-non-embrittled interface which could be described by d × (%P) = 0.18 where d is the grain size in μm. Such an embrittlement estimate diagram represent a cogent and practical route in the identification of in-service embrittled bolts. |
