Innovative RAFM steels with improved impact properties

• Decreasing Carbon and Manganese helps to improve impact properties of a RAFM steel. • The recrystallized alloy exhibits very narrow ferritic grain (1 µm) associated to further improved impact properties at low temperature. • The tensile properties at low temperature result markedly enhanced after recrystallization. • Coarser distribution of precipitates could be correlated to decay in tensile properties. • Toughness appears more sensitive to grain size rather than to precipitate distribution. The contribution of ENEA together with Rina-CSM to the Eurofusion programme “WPMAT" (Work Package MATerials) deals with the development of innovative RAFM steels able to withstand the critical low temperature (280–300 °C in the “water-cooling” option) foreseen for the blanket of the first DEMO reactor. Therefore the target is the development of tougher alloys, suitable to prevent the low temperature irradiation embrittlement. In addition to the DBTT shift due to the dpa damage under irradiation, also the issue of the increased embrittlement due to Helium production must be taken into account. The alloy design focused on chemical composition optimization in terms of contents of some main key elements as Carbon, Manganese, Vanadium, Nitrogen and Tantalum. An overall indication deriving from the last neutron irradiation experiment was to increase Vanadium and Nitrogen at the expense of Carbon and Manganese. The ingot has then been cast with accurate control of impurities. The final product has proven compliant to the specifications concerning RAFM steels for fusion environment. Alternative thermo-mechanical treatments have then been applied on this RAFM steel. On the one hand the DBTT after a standard double normalization and tempering treatment is definitely successful; -140 °C on KLST specimens. The extremely low DBTT must be considered a consequence of the choices done on the chemical composition. On the other hand a particular TMT, whose aim is to induce recrystallization of the laths substructure and obtain an equiaxed nano-grain structure, has been investigated. Supposed to increase the resistance to crack propagation, the decrease in grain size (less than one μm) leads to a transition curve that doesn't show a sharp drop in the absorbed energy, but instead a more gradual transition from the USE to the LSE. The tensile properties in the recrystallized condition result markedly higher than after the conventional treatment.