Cleavage Fracture In Ferritic Steels Research Articles

Cleavage fracture of low carbon ferritic steels with fine grain size

AbstractMicromechanisms of transgranular cleavage fracture in ferritic steels have been investigated, particular attention being paid to the dependence of cleavage fracture stress σf on grain size over the range from 3·5 to 34·3 μm. It is found that σf 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 σf 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 σf 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.

Cleavage micromechanisms of crack extension in steels

AbstractThe 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 ch...