Abstract
The micromechanisms of cleavage fracture in ferritic steels are reviewed. Mechanisms of crack nucleation by dislocation interaction are compared with those involving the formation of stable microcracks. It is concluded that cleavage in steels containing discrete carbide particles is nucleated by microcracks formed in the carbide particles when fracture obeys a critical tensile stress criterion. The cleavage fracture stress in these steels is seen to depend largely upon the size of the cracked carbide particle. No micromechanisms have been established for cleavage in martensitic or low-carbon bainitic steels but the packet size seems to control cleavage fracture in these materials.
Two models of the relationship between the micromechanism of cleavage and fracture toughness are discussed in some detail. The statistical competition between different sized crack nuclei at a loaded crack tip can be represented by the requirement that the fracture stress be exceeded over a microstructurally determined characteristic distance. This simply expressed fracture criterion can be combined with fracture mechanics analyses to explain many features of the fracture behaviour of ferritic steels.
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