THE INFLUENCE OF HEAT-TREATMENTS ON THE MICROSTRUCTURE OF 9-PERCENT CHROMIUM STEELS CONTAINING TUNGSTEN

Microstructural investigations of the steel 9Cr0.5Mo2W (NF616), a Japa nese steel developed for advanced power station components, have been carried out by optical microscopy and by transmission electron microsc opy, using thin foils and extraction replicas prepared after normaliza tion at 970-1145 degrees C and tempering at 715-835 degrees C. After n ormalization, the steel exhibited a martensitic structure with only sm all amounts of retained austenite. During tempering recovery and preci pitation processes occurred which reduced the hardness of the martensi te. Elongated martensite laths were observed within the prior austenit e grains, the boundaries of which were decorated with M23C6 precipitat es. In addition to the M23C6 precipitates, three types of MX (where X is C and/or N) precipitate were detected; spheroidal Nb-rich carbonitr ides, plate-like V-rich nitrides and complex particles consisting of t he 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 particl e size distributions and relate them to the heat treatments.