Expanding the operation window of RAFM steels by optimized chemical compositions and heat treatments

9%-Cr reduced activation steels are the designated structural materials for future fusion reactors. The improvement of this class of alloys, especially the extension of the operation limits, is the present scope of the EUROfusion materials project for advanced steels. Within this programme, new alloys are designed and fabricated to overcome some of the limitations of EUROFER97.
In the present study, three 9%-Cr alloys with some variations in the chemical compositions are compared to standard EUROFER97 batches. The main focus lies in the possible extension of the operation window to both higher and lower temperatures.
This is achieved by a variation of the amount of the minor alloying elements which form precipitates and secondary phases. Combining the modified chemical compositions with special heat treatments tailored for the specific alloys leads to a refined distribution of the carbide and nitride phases. Moderate variations in the alloy compositions open the possibility for extended temperature windows for these unconventional heat treatments (e.g. higher tempering temperature). Thermo-mechanical treatments showed the predic ... mehrted effect on hardening and other properties.
Lowering the carbon content proved to be effective to reduce the amount of M23C6 carbides. The predicted changes in precipitate composition and size were confirmed by TEM studies. They showed that an increase in V increased the amount of VN particles, but lead to a tendency towards larger particles than the ones found in the EUROFER97 reference alloys.
Strong effects of the heat-treatments on the toughness and strength were observed. As expected, hardening the materials beyond the conventional treatment (980°C/30min + 750°C/2h) showed a major increase in tensile strength and decrease in toughness. Toughness from Charpy impact tests, creep to rupture lifetime and tensile strength are compared for the different treatments.
Microstructural changes caused by the element variation and/or different treatments were characterized by SEM combined with EBSD mapping.