THE MECHANISMS AND DETECTION OF EMBRITTLEMENT IN CR-MO PRESSURE-VESSEL STEELS

2 1/4Cr1Mo steel and 1 1/4Cr1/2Mo steel have been widely been used for hydro-processing units such as hydro-desulphurising and hydro-crackin g reactors. These reactor pressure vessel steels have a potential for temper embrittlement that leads to toughness degradation and a reducti on of the critical flaw size for brittle fracture. These steels are al so susceptible to hydrogen embrittlement, especially in aged steels wh ere cracks may propagate in the base metal up to the critical flaw siz e. A vessel with adequate toughness when originally constructed may th erefore embrittle during service and such changes may require pressure restrictions during start-up and shut-down. A survey of the literatur e shows composition to be the controlling parameter for both temper em brittlement (TE) and hydrogen embrittlement (HE), in-particular the pr esence of residual impurity elements such as P and the presence of ele ments such as Mo which nullify the effect of impurity segregation. Muc h information is available to describe embrittlement phenomena for Cr- Mo steels. This paper reviews the mechanisms of TE and HE and describe s a microstructural characterisation route which subsequently allows t he structural integrity of potentially embrittled vessels to be examin ed for the purposes of remaining life assessment and plant life extens ion.