Microstructural insights into EUROFER97 batch 3 steels

Extensive analytical electron microscopical analyses were carried out from the micrometer scale down to the nanometer scale to characterize three variants of the 9% reduced activation ferritic martensitic (RAFM) steel EUROFER97/3. No huge microstructural differences were observed between the three grades. Electron backscatter diffraction (EBSD) in a scanning electron microscope (SEM) was used to determine prior austenite grain (PAG) and lath sizes of the martensite matrix. The PAG size varied between 4.5 µm and 6.5 µm depending on the reconstruction algorithm. Furthermore, the martensitic lath sizes determined by SEM-EBSD are only half or 1/3 of that determined manually from transmission electron microscopy (TEM) images, which might be related to the limited statistics in this type of TEM data evaluations. The SEM-EDX shows that M$_{23}$C$_6$-type phases are preferentially located on lath and grain boundaries due to preferential diffusion of elements like Cr, W, and C to and along grain boundaries, which agrees with TEM-EDX measurements. TEM techniques like STEM-EDX and high-resolution TEM were used to describe the occurring precipitates i.e., M$_{23}$C$_6$, VN, TaC morphologically, structurally, and chemically. In addition, the thermodynamic calculations were carried out to explain phase formation, phase fraction and phase composition. The results are in good agreement with the experimentally determined values. These results will provide a profound basis to explain the mechanical performance of these materials. Furthermore, it will lay a good reference basis of comparison for the material after neutron