Effective grain size dependence of crack propagation resistance in low carbon steel

Abstract

Unlike numerous studies focused on the crack initiation occurred in steel materials, the influence of effective grain size on crack propagation resistance does not receive enough attention and the intrinsic mechanism is not elucidated. In this work, comparable matrix microstructures with varying effective grain sizes were designed using different weld thermal simulations. Instrumented Charpy impact tests were conducted on these specimens to document the variation of fracture behaviors. The results showed that the impact toughness changes notably regardless of their similar matrix microstructure. By sub-dividing the total impact absorbed energy into two parts based on the maximum load, we find a new inverse proportional relation between the effective grain size and energy dissipation at the crack propagation stage. This new function, which is akin in form to the classical RKR model, may have board applications for optimizing the fracture behavior because it explicitly displays the effect of metallurgical factors on crack propagation resistance. This relation further implies that the crack propagation resistance is more strongly dependent on the grain size compared to the crack initiation resistance. With focus on the brittle crack propagation stage, a sharp decrease in brittle cracking speed for fine-grained specimens is attributed to the mechanism that numerous tear-ridges and side ligaments formed by local plastic deformation exert a closure stress at the wake of crack tip.