Inclusion spacing and prior austenite grain size effects on the fracture toughness of structural steels

When fracture occurs by micro-void coalescence the fracture toughnesses of structural steels are controlled by both the inclusion distributions and the fine-scale microstructure. If the inclusions are equiaxed and nucleate voids at low strains the characteristics of the inclusion distributions which most significantly influence toughness are their volume fraction and spacing. For a fixed fine-scale microstructure and inclusion volume fraction the crack tip opening displacement at fracture increases with inclusion spacing until the inclusion spacing reaches some critical value; once the inclusion spacing exceeds this critical spacing the toughness becomes independent of inclusion spacing. However, the effect of inclusion spacing on toughness depends on the fine-scale microstructure. First, for a fixed and small prior austenite grain size this critical spacing decreases as the strength of the material decreases. it has been found for low strength materials that toughness is not significantly influenced by prior austenite grain size at large inclusion spacings but that at small inclusion spacings increasing the prior austenite grain size can significantly reduce toughness.