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 t urbine bolts after over 200,000 h at 450 degrees C. A small section of material was removed from a noncritical location of all the 51 bolts. From this section, the chemical composition, average hardness and ave rage prior austenite grain size were measured. The toughness of the bo lts was measured by Charpy impact testing and/or Auger electron spectr oscopy. From the various parameters investigated, it was established t hat grain size and phosphorus level were the only factors which consis tently identified whether a bolt was embrittled or non-embrittled. Ind eed it was established that bolt embrittlement could be predicted usin g the simple product of microstructure grain size d (mu m's) and bulk phosphorus content. Finally, it was observed that bolt embrittlement c ould be predicted using the simple product of microstructure grain siz e and bulk phosphorus level and that such a trend indicated the import ance of grain boundary area available for service induced phosphorus s egregation. (C) 1998 Elsevier Science Ltd. All rights reserved.