SERVICE EMBRITTLEMENT TRENDS IN LARGE CRMOV STEEL STEAM-TURBINE BOLTS

The present paper examined the reverse temper embrittlement, RTE, char acteristics of nearly two hundred large intermediate pressure (IP) and high pressure (HP), CrMoV steel turbine bolts which have been removed from two identical 120 MW units which had been in service for over 12 0000 hours at elevated temperatures of around 500 degrees C. It was im mediately clear that the extent of toughness loss or RTE response was primarily controlled by three parameters, viz, grain size, d, accumula ted service strain, % epsilon, and bulk phosphorus level, wt.% P. In a n effort to establish the various average RTE responses from a signifi cant bolt population they were separated into groups in terms of bolt size and unit from which they were removed. It was shown that, essenti ally, the various IP bolt groups exhibited strong influences of accumu lated strain on the degrees of embrittlement recorded while the differ ent HP bolt groups revealed significant effects of grain size. Finally in order to predict the average RTE response of the present CrMoV ste el bolts the average embrittlement trends were portrayed in terms of a n embrittlement factor, Q, which was a function of grain size, accumul ated strain and bulk phosphorus level. It was established that the deg ree of embrittlement, % EMB, could be adequately predicted from an exp ression containing root Q. Hence, the individual influences of the var ious parameters affecting embrittlement could be easily portrayed by u se of Q and % EMB.