Development of new reduced activation ferritic/martensitic steels with higher temperature capability and improved properties

Reduced Activation Ferritic/Martensitic (RAFM) steels are considered as candidate materials for the first wall of future fusion reactors. Previous development of RAFM steels has primarily focused on 8-9 wt.% Cr compositions containing abundant M23C6 precipitates (M = Cr-rich) and minor MX precipitates (M = Ta/V, X = C/N), which exhibit significant degradation in strength and fracture toughness at elevated temperatures (550~650°C). Thermomechanical treatment (TMT) can be employed to refine grain size and promote MX precipitation to enhance the high temperature mechanical properties of RAFM steels. In this work, the microstructure, precipitation behaviour, TMT processes, and mechanical properties of four RAFM steels were investigated: the conventional Eurofer 97 from EU and CNA2 from Oak Ridge National Laboratory as baseline RAFM steels, along with two newly designed RAFM steels based on these baseline: Ti-RAFM (with 0.1 wt.% Ti addition) and N-RAFM (with 0.077 wt.% N addition). Through multi-stage hot deformation and multiple passes of final deformation followed by optimized heat treatment processes, the two new RAFM steels achieved significant grain refinement and substantial MX precipitation. The dominant mechanism for grain refinement was primarily associated with recrystallization mechanism, while the increased MX precipitation was related to strain induced precipitation (SIP) mechanism and optimized alloy design. Furthermore, both new RAFM steels demonstrated comprehensive improvements compared to Eurofer 97, with a 17% increase in ultimate tensile strength (UTS) and a 75% enhancement in ductility. Simultaneously, both new RAFM steels addressed the drawback of CNAs where strength improvement came at the expense of reduced ductility.