INTERPRETATION OF CLEAVAGE RESISTANCE OF MODERN HIGH-STRENGTH STRUCTURAL-STEELS

To explain a series of experimental data concerning low carbon, high manganese, microalloyed structural steels, a theory of cleavage fracture is presented, based on the assumption that cleavage initiation can occur in the ferrite lattice at the intersection of two slip bands, in the absence of brittle phases. The crack extension force acting on the propagating microcrack is controlled by the grain diameter d which, accordingly, is the microstructural parameter determining the tendency of the material to fracture. The cleavage fracture stress sigma(cf) is found to be strongly influenced by the amount of blunting produced during the stage of subcritical crack growth. Taking this effect into account, sigma(cf) is shown to depend on grain diameter according to a law of the type d(-p), where p is close to 1/4.