Abstract
Microstructural investigations of the steel 9Cr0.5Mo2W (NF616), a Japanese steel developed for advanced power station components, have been carried out by optical microscopy and by transmission electron microscopy, using thin foils and extraction replicas prepared after normalization at 970–1145°C and tempering at 715–835°C. After normalization, the steel exhibited a martensitic structure with only small amounts of retained austenite. During tempering recovery and precipitation processes occurred which reduced the hardness of the martensite. Elongated martensite laths were observed within the prior austenite grains, the boundaries of which were decorated with M 23C 6 precipitates. In addition to the M 23C 6 precipitates, three types of MX (where X is C and/or N) precipitate were detected; spheroidal Nb-rich carbonitrides, plate-like V-rich nitrides and complex particles consisting of the plate-like particles nucleated on the spheroidal ones. Statistical analyses of the microstructure were carried out using both thin foils and extraction replicas to determine the subgrain size and the particle size distributions and relate them to the heat treatments.
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