Development of innovative steels and thermo-mechanical treatments for DEMO high operating temperature blanket options

Highlights: • Effectiveness of "Ausforming" TMT to improve tensile and creep properties. • No direct correlation between tensile and creep properties for the same material. • Susceptibility of the DBTT to tempering temperature and W content. • Conservation of room temperature ductility hinders the enhancement of creep resistance. - Abstract: Among the options currently taken into account for the realization of the first DEMO reactor there are the "helium-cooled" and the "dual coolant" breeding blanket. Therefore the high temperature (650 °C) behavior of the proposed innovative martensitic alloys should be improved, namely the frame of the hereby reported activities is the development of martensitic alloys more resistant to creep, suitable to tolerate such a high operating temperature. In order to improve the high temperature mechanical properties, concerning the alloy design strategies, two alternative routes are proposed; the effect of Nitrogen and Tungsten increase are taken into account as well as the addition of carbo-nitride forming elements, like Vanadium, combined with the "ausforming" thermo-mechanical treatments. Two alloys have been designed and a special thermo-mechanical treatment on Eurofer 97-2 is proposed. The "ausforming" treatment, consisting in a sort of hot-working at a lower temperature with respect to the austenitization one after the austenitization stage, is aimed at the achievement of a beneficial dislocation "pinning" at high temperature due to carbide precipitation. Generally the improvement of tensile properties is associated to the hardening of the steel due to dislocation network and precipitation effects. This hardening is accompanied by a DBTT increase to markedly higher values with respect to Standard Eurofer. The proposed materials should be, in any case, at least room temperature ductile in order to undergo safe manufacturing and assembling processes. Therefore the issue of the DBTT increase has been taken into account by tuning the tempering temperature adequately. The two variations from chemical composition of Eurofer 97 have been casted and the thermo-mechanical treatments have been selected by means of SEM and hardness measurements to tune grain size and precipitation of carbides. The outcomes of the preliminary mechanical characterization (tensile, creep and impact tests) will be discussed in this paper.