Effect of Ta rich inclusions and microstructure change during precracking on bimodal fracture of reduced-activation ferritic/martensitic steels observed in transition range

The master curve method for analyzing fracture toughness data depends on the assumption that the fracture initiation points are homogeneously distributed and fracture initiation is independent of test temperature. The reduced-activation ferritic/martensitic steels, such as F82H (Fe-8Cr-2W-0.2V-0.04Ta), form AI₂O₃-Ta(V,Ti)O composite inclusions and its distribution Is not homogeneous throughout one heat, and this microstructural inhomogeneity appears to be correlated with bimodal facture of F82H in the transition range. To investigate this possibility, 1TCT fracture toughness specimens of F82H-IEA steel were fatigue precracked and sliced through the specimen thickness for microstructure analysis around the crack. It was found that the crack penetration was straight in the beginning, and then tended to follow a prior austenite grain boundary and finally to branch into two or three different directions. In addition, the microstructures around the crack and ahead of the crack formed a cell structure and became softer than nearby regions, which Is typical for fatigue-loaded F82H. Possible mechanisms for how this cell structure ahead of cracks affects fracture toughness are suggested.