Factors Affecting the High Temperature Performance of Creep Strength Enhanced Ferritic Steels

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Long term creep is potentially life limiting for many components which operate at high stress and temperature. These components are typically designed using Code specified maximum stresses. As more long duration rupture data became available the original estimates of allowable stresses for creep strength enhanced ferritic steels (CSEF) have been reduced. Thus, even in properly processed steel, the long term performance and creep rupture strength is below that predicted from a simple extrapolation of short term data. The microstructural degradation mechanisms responsible for this behaviour are linked to changes in the steel substructure and the development of creep voids. Nucleation, growth and inter linkage of voids also result in a significant loss of creep ductility. Indeed, elongations to rupture of around 5% in 100,000 hours are now considered normal for long term creep tests on many CSEF steels. This relatively brittle behaviour increases the risk of burst rather than leak type fracture. Moreover, the presence of high number densities of voids complicates assessment of condition and weld repair of in-service components. This paper examines background on the high temperature behaviour, including the nucleation and development of creep voids, in 9 to 12%Cr martensitic steels. Cost effective improvements to in-service performance of components made from CSEF steels can be achieved using ‘well engineered’ design and fabrication approaches.