ODS Steels: Microstructure and properties of oxide-dispersion strengthened Eurofer steels

If the conventional high-chromium ferritic-martensitic steels, such as modified 9Cr-1Mo or reduced-activation steels, such as 9Cr-2WVTa and EUROFER were used for a fusion power plant first wall and blanket structure, the upper operating temperature would be limited to 550-600°C.

One plausbile appraoch to shift this limit to higher temperatures and maintain the advantages inherent in ferritic-martensitic steels (i.e. high thermal conductivity and low swelling) is to use oxide dispersion-strengthened (ODS) steels.
In the tempered state, this steel contains fine particles dispersed in the ferritic matrix. More specific, the elevated temperature strength in these steels is obtained through microstructures that contain a high density of small Y2O3  and/or TiO2 particles dispersed in a ferrite matrix.

Hence, such oxide dispersion strengthened reduced-activation ferritic–martensitic steels are promising candidates for applications in future fusion power plants.

ODS steels are currently being developed and investigated for both, nuclear fission and fusion applications.

Important microstructural changes occur during annealing of deformed metals including recovery of structural defects, recrystallization, grain growth and other phase transformations. These changes can be followed by several characterization techniques including classical metallography, hardness testing and electron backscatter diffraction. In this work, we investigate the annealing
behavior of a ferromagnetic material presenting coercivity below 30 Oe for the initial condition. The magnetic properties of ferritic steels are strongly dependent on their microstructure [1]. In particular, the microstructure affects the motion of magnetic domain walls and, in consequence, the characteristic parameters of the hysteresis loop. High and low angle boundaries, dislocations, precipitates and solute atoms act as pinning sites against magnetic domain wall motion [1–6]. Among several magnetic parameters the coercive field (Hc), which is the intensity of the magnetic field needed to reduce the magnetization of a ferromagnetic material to zero after it has reached magnetic saturation, can be used to monitor microstructural changes in different materials. 

More specific, we suggest that oxide dispersion strengthened reduced-activation ferritic–martensitic steels are promising candidates for applications in future fusion power plants. Samples of a reduced activation ferritic–martensitic

9 wt.%Cr-oxide dispersion strengthened Eurofer steel were cold rolled to 80% reduction in thickness and annealed in vacuum for 1 h from 200 to 1350◦C to evaluate its thermal stability. Vickers microhardness testing and electron backscatter diffraction (EBSD) were used to characterize the microstructure. The
microstructural changes were also followed by magnetic measurements, in particular the corresponding variation of the coercive field (H c ), as a function of the annealing treatment. Results show that magnetic measurements were sensitive to detect the changes, in particular the martensitic transformation,
in samples annealed above 850◦C (austenitic regime).