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Metallurgical characterisation of modi® ed F ± 82H steel was carried out. Microstructural studies together with mechanical testing were conducted on the steel in the as received state and after thermal aging in the temperature range 300 ± 600°C for periods up to 5000 h. In addition, precipitate development during creep tests at 600 and 650°C was studied. During thermal aging, the general microstructure of the steel (tempered martensite with M 23 C 6 precipitates) remained stable up to 500°C. However, at 550 and 600°C, in addition to the M 23 C 6 carbides, Laves phase and precipitates with intermediate composition between carbides and Laves phase were observed. In terms of mechanical properties, the F ± 82H modi® ed steel exhibited the same values of hardness, ultimate tensile stress and 0 . 2% proof stress, and similar creep rupture strength, regardless of the material condition studied. In contrast, aging treatments at 550 and 600°C caused a signi® cant decrease in impact properties, essentially attributable to the Laves phase precipitation.MST/5107