REVERSE TEMPER EMBRITTLEMENT OF TURBINE BOLTS DURING AN OUTAGE

This paper describes a detailed examination which could be conducted d uring 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 composi tion, average hardness and average prior austenite grain size were mea sured. The toughness of the bolts was measured by Charpy impact testin g and/or Auger electron spectroscopy. From the various parameters inve stigated, it was established that grain size and phosphorus level were the only factors which consistently identified whether a bolt was emb rittled or non-embrittled. Indeed, at a phosphorus level of 0.01%, bol ts with grain sizes less than 20 mum were not embrittled while those w ith larger grain sizes suffered reverse temper embrittlement during se rvice. 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 descr ibed by d x (%P) = 0.18 where d is the grain size in mum. Such an embr ittlement estimate diagram represent a cogent and practical route in t he identification of in-service embrittled bolts.