Effect of normalizing temperature on creep strength of 8Cr-W-V-Ta oxide dispersion strengthened steels

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, δ-ferrite grains which exist in martensite and elongate to the hot-rolling direction affects creep deformation behavior. The ratio of δ-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 1050degC for 1 h. Specimens of different normalizing temperatures were prepared by re-normalizing the as-received specimens at 950degC and 1250degC for 1 h. Creep tests were performed at 700degC 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 δ-ferrite decreased with the increase of normalizing temperature in 8Cr-2W-V-Ta, but the fraction of the specimen normalized at 1050degC was highest in 8Cr-1W. Minimum creep rates decreased as the area fraction and Vickers hardness of δ-ferrite increased. The time-to-rupture increased with increasing the area fraction of δ-ferrite. Although creep-cavities formed along δ-ferrite in martensite, the progress toward main-crack was obstructed by δ-ferrite. Therefore, the increase in the area fraction of δ-ferrite contributed not only to the decrease in the minimum creep rate but also to the delay of the main-crack propagation. (author)