Embrittlement observed in Cr–Mo turbine bolts after service

The present paper describes a study aimed at investigating the extent of service related embrittlement suffered by a series of Cr–Mo steel turbine bolts after over 200,000 h at 450°C. A small section of material was removed from a non-critical location of all the 51 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 it was established that bolt embrittlement could be predicted using the simple product of microstructure grain size d (μm's) and bulk phosphorus content. Finally, it was observed that bolt embrittlement could be predicted using the simple product of microstructure grain size and bulk phosphorus level and that such a trend indicated the importance of grain boundary area available for service induced phosphorus segregation.