TEM studies of microstructural evolution in creep exposed E911

Transmission electron microscopy has been used to investigate precipitate evolution in E911 steel samples creep tested to a range of temperatures (600-650°C) for durations of up to 75,000 hours. E911 is a 9%Cr 1% MoNbVNW creep resistant ferritic/martensitic steel that is used for boiler applications in power generation plant. The initial microstructure consists of tempered martensite containing M23C6 precipitates at the prior austenite and martensite grain boundaries together with fine M2X and MX precipitates in the matrix. A small amount of primary MX is also observed within the matrix. After prolonged exposure at high temperature and stresses, coarsening of the original M2X and M23C6 was found to occur together with the precipitation of Laves phase and Z-phase. The paper discusses the evolution of the microstructure and relates this to the hardness and strength changes observed owing to creep testing of the alloy.