Effect of heat treatment under magnetic field on creep characteristics of advanced heat resistant ferritic steels for USC power plants has been investigated to explore fundamental guiding principles for improving creep rupture strength of the steel at elevated temperatures over 873 K. A model steel with a composition of Fe-0.08 C-9 Cr-3.3 W -3 Co-0.2 V-0.05 Nb-0.05 N-0.005 B-0.3 Si-0.5 Mn (in mass%) has been prepared by vacuum induction melting and processed by hot-forging and rolling into a 16

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... mm square bar. Specimens taken from the bar were heated at 1373 K for austenitization and subsequently cooled under the magnetic field up to 3 Tesla for controlling the martensitic transformation. Creep testing were performed at 923 K and the microstructural evolution during creep was examined using FE-SEM observation and chemical analysis of the extracted residue. Creep rupture strength of the steel is found to increase by the heat treatment under magnetic field, during which martensitic transformation occurs. This is attributed to the finely distributed precipitates such as MX and M23C6 in the specimens heat treated under the magnetic field. It is thus suggested that the heat treatment under magnetic field increases the nucleation sites for precipitation during tempering, resulting in increase in the amount of the fine precipitates and retardation of growth of the precipitates.