Cleavage fracture of low carbon ferritic steels with fine grain size

Micromechanisms of transgranular cleavage fracture in ferritic steels have been investigated, particular attention being paid to the dependence of cleavage fracture stress σ_f on grain size over the range from 3·5 to 34·3 μm. It is found that σ_f increases less than linearly with d^−1/2, where d is the grain diameter, whereas the Hall–Petch relation holds for the yield stress. Based on the experimental results, where σ_f was measured in the steels and their grain size and carbide size were changed independently, it is suggested that in the grain size range cited, the predominant mechanism was the propagation of a microcrack, which formed in the discrete carbide, into contiguous ferrite grains. The measured values of σ_f were discussed in a quantitative manner based on a model into which an equilibrium distribution of dislocations produced by the double pile-up mechanism was incorporated.