Particularly in the transition temperature range, reliable use of toughness data, from small specimens of structural steel can be helped by improved understanding of the initiation of cleavage. For this purpose, fracture details in local regions were studied. Methods of examination were scanning electron microscopic (SEM) fractography, microstructural analysis, hardness tests, and topographic examination of fracture surfaces.

The results indicate that examination of fracture surface morphology through SEM fractography is essential for improved understanding of cleavage fracturing in the transition range. Fracture surface details reflect influences of both ferrite and prior-austenite grain size. In addition, for some specimens, fracture morphology is also influenced by a large size inhomogeneity due to dendritic solidification. In heavy section A533B steel, explanations for large scatter of small specimen K_Ic estimates may be closely related to dendritic inhomogeneity.