Effects of inclusions on fracture toughness of reduced-activation ferritic/martensitic F82H-IEA steels

Reduced activation ferritic/martensitic steels (RAFs) are recognized as the primary candidate structural materials for fusion blanket systems. F82H is the RAF which has been developed and studied in Japan, and F82H-IEA heat, one of the world's first 5 ton heats of RAF, was provided and evaluated in various countries as a part of the IEA collaboration on fusion materials development. A problem with the steel is that some fracture toughness values obtained by IT type of compact tension (lTCT) specimens showed very low values in ductile brittle transition region. There might be several reasons for the scatter, and one of them could be related to the microstructural inhomogeneity of F82H-IEA. In the present study, this possibility was investigated focusing on inclusions formed in a plate of F82H-IEA steel by scanning (SEM) and transmission electron microscopy (TEM) equipped with EDS. The plates examined in this study were obtained from F82H-IEA heat no. 9753. nominally Fe-7.5Cr-2W-0.15V-0.02Ta-0.1C, in mass%. Analyses by SEM and TEM for the plates revealed that Ta does not form MX precipitates, but instead, it forms composite Al₂O₃ - Ta(V,Ti)O oxide, or single phase Ta(V)O oxide. The composite inclusions are rather dominant in the plate obtained from the bottom of the ingot, but not in the plate from the middle of the ingot. SEM observations also revealed that composite oxide tended to be observed at the crack-initiation site. These results suggest that the scatter of toughness values may be correlated with this microstructural inhomogeneity.