The role of grain size in brittle particle induced fracture of steels

The role of grain size in the control of low temperature brittle fracture is investigated. Quasistatic four point bending tests from two Ti–V steels, in a bainitic or acicular ferrite condition, have been performed. Fractographic examination shows that overall fracture initiation sites are related both to coarse TiN particles and large-size facets in the case of bainitic steels. In addition, a coarser bainitic microstructure causes a significant decrease of fracture stress and normalised applied fracture load. In contrast to the well-known grain size effect by which cracks might arrest at the matrix–matrix boundaries and thus improve fracture toughness, the experimental results indicate an additional influence of grain size. In the case of fine-grained acicular ferrite, TiN particles are irrelevant to the fracture process thus demonstrating an additional effect of the matrix.