Service embrittlement trends in large CrMoV steel steam turbine bolts

Abstract

The present paper examined the reverse temper embrittlement, RTE, characteristics 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 120 000 hours at elevated temperatures of around 500°C. It was immediately clear that the extent of toughness loss or RTE response was primarily controlled by three parameters, viz, grain size, d, accumulated service strain, %ϵ, and bulk phosphorus level, wt.% P. In an effort to establish the various average RTE responses from a significant bolt population they were separated into groups in terms of bolt size and unit from which they were removed. It was shown that, essentially, the various IP bolt groups exhibited strong influences of accumulated strain on the degrees of embrittlement recorded while the different HP bolt groups revealed significant effects of grain size. Finally in order to predict the average RTE response of the present CrMoV steel bolts the average embrittlement trends were portrayed in terms of an embrittlement factor, Q, which was a function of grain size, accumulated strain and bulk phosphorus level. It was established that the degree of embrittlement, % EMB, could be adequately predicted from an expression containing √ Q. Hence, the individual influences of the various parameters affecting embrittlement could be easily portrayed by use of Q and % EMB.