RESTORATION OF CHARPY TOUGHNESS FOR IN-SERVICE TEMPER EMBRITTLED STEELS

This investigation is concerned with the restoration or rejuvenation o f the Charpy toughness of a Cr-Mo-V steam turbine bolting steel that h as suffered reverse temper embrittlement (RTE) during around 120 000 h ours of service at a live steam temperature of up to 540-degrees-C. It was demonstrated that the toughness properties can be partially resto red by certain heat treatments and that the degree of rejuvenations wa s directly related to the bulk phosphorus level of the steel. The exte nt of toughness restoration i.e., the change of Fracture Appearance Tr ansition Temperature (DELTAFATT), was also depend upon the steel compo sition with Ni-containing steels giving higher DELTAFATT levels than C r-Mo or Cr-Mo-V steels. By correcting or normalising both the extent o f toughness benefaction during a de-embrittlement heat treatment and t he original embrittled FATT values using reported vectors for grain si ze, composition factor and phosphorus level effects, it was demonstrat ed that the amount of toughness restoration was insensitive to the deg ree of initial embrittlement and that the Ni-containing steels exhibit ed DELTAFATT values that ranged between + 160 and + 240-degrees-C whil e the non-Ni-containing steel DELTAFATT values were only a third to ha lf these values. Finally it was established that the re-embrittlement kinetics of a de-embrittled steels in general exhibited values that we re two orders of magnitude faster than the original embrittlement proc ess.