ASSESSMENT OF EMBRITTLEMENT IN 12 CrMoV STEEL DURING ELEVATED TEMPERATURE SERVICE

摘要

Within the last few years all 1%CrMoV high temperature studs have been replaced on four of ESB's large steam turbines because of a number of failures of the main casing studs due to creep embrittlement/reverse temper embrittlement. These 1%CrMoV studs were replaced with the higher creep strength material AFNOR: Z20 CD NbV 11 (DIN: X19 CrMoVNb 11.1) i.e. 12%CrMoV. This material enters service with inherently lower toughness, in terms of fracture appearance transition temperature (FATT), than the 1%CrMoV steel. Recently, a limited amount of Charpy testing carried out on a 12%CrMoV main casing stud removed from service after 30,000 hours revealed that the Charpy properties had dropped below the minimum values quoted for virgin material. This reduction in toughness may then explain the short term failures reported in the literature for similar components. Clearly the serviceability of the 12%CrMoV studs must be assessed to prevent in-service failures. The present paper initially reviews the fitness for service methodology established by ESB for the assessment of 1%CrMoV steels. This approach has been developed based on research to characterise the factors affecting service performance and utilisation of special material tests to monitor time dependent changes in critical properties. Then the details of this technology are discussed in relation to condition assessment of 12%CrMoV steel. Particular emphasis is given to investigation of the mechanism of embrittlement in the higher alloy material and consideration of the capability of miniature sample tests to accurately monitor material properties.