Effect of Normalizing Temperature on Creep Strength of 8Cr-W-V-Ta Oxide Dispersion Strengthened Steels
Kei SHINOZUKA, Manabu TAMURA, Hisao ESAKA, Hiroyasu TANIGAWA and Kiyoyuki SHIBA
Abstract:In international thermonuclear experimental reactor (ITER), reduced activation ferritic/martensitic steels will be used for plasma-facing materials. However, raising the operating temperature is planned in order to elevate efficiency of electric power generation in a prototype fusion reactor. Oxide dispersion strengthened (ODS) steels are promising candidate for high temperature and plasma-facing materials. In the 8Cr-W-V-Ta ODS steels, -ferrite grains which exist in martensite and elongate to the hot-rolling direction affects creep deformation behavior. The ratio of -ferrite is dependent on normalizing temperature. In this work, the relation between normalizing temperature and creep strength of the 8Cr-W-V-Ta ODS steel was investigated. Two kinds of as-received ODS steels, 8Cr-2W-V-Ta and 8Cr-1W, were normalized at 1050C for 1 h. Specimens of different normalizing temperatures were prepared by re-normalizing the as-received specimens at 950C and 1250C for 1 h. Creep tests were performed at 700C and 205 MPa in parallel to the hot-rolling direction. Microstructures of specimens were observed using OM, and Vickers hardness tests were performed. The area fraction of -ferrite decreased with the increase of normalizing temperature in 8Cr-2W-V-Ta, but the fraction of the specimen normalized at 1050C was highest in 8Cr-1W. Minimum creep rates decreased as the area fraction and Vickers hardness of -ferrite increased. The time-to-rupture increased with increasing the area fraction of -ferrite. Although creep-cavities formed along -ferrite in martensite, the progress toward main-crack was obstructed by -ferrite. Therefore, the increase in the area fraction of -ferrite contributed not only to the decrease in the minimum creep rate but also to the delay of the main-crack propagation. Key Words:Reduced activation ferritic/martensitic (RAF/M) steel, Oxide dispersion strengthened (ODS) steel, -ferrite, Minimum creep rate, Creep cavity