Micromechanism of cleavage fracture at the lower shelf transition temperatures of a C–Mn steel

Abstract

This paper investigates the mechanism of cleavage fracture in precracked specimen made of Q-T C–Mn steel at the lower shelf of ductile-to-brittle transition curves. The results reveal that the lower shelf region can be divided into three domains: (1) at the lower temperature end (−150 to −130°C), cleavage is controlled by crack nucleation in the microscopic scale and is driven by increasing applied load which blunts the precrack tip in the macroscopic scale; (2) at moderate low temperatures (−130 to −90°C) cleavage fracture is controlled by propagation of a second phase particle-crack into contiguous grains in the microscopic scale and is also driven by increasing applied load which further blunts the precrack tip in the macroscopic scale; (3) at the upper temperature end (−90 to −60°C) the cleavage fracture is controlled by the propagation of the carbide crack into contiguous grains in the microscopic scale, yet in addition to the crack tip blunting for compensating the drop of the yield stress a short fibrous crack is needed to extend from the precrack tip to increase the peak normal stress and to move it closer to the precrack tip.The results also reveal that work hardening plays a key role in triggering cleavage fracture in this temperature region. The normal stress induced by dislocation pile-up impingement, which greatly enhances the applied normal stress, is the essential part of the cracking mechanism and is enhanced by increasing the applied load through strain-hardening effects.