Crack-tip gradient microstructure: Formation and influence on impact toughness behavior of low-carbon ferritic steel

The present study proposes a novel approach to extend the service life of structural components with pre-existing cracks by utilizing targeted heat treatment to selectively tailor the crack-tip microstructure, thereby enhancing mechanical properties. Charpy V-notched (CVN) specimens were subjected to tensile pre-straining and annealing to form distinct notch-root microstructures. Pre-straining induced heterogeneous plastic deformation, forming a kidney-shaped plastic zone (rp*). Subsequent annealing led to the formation of gradient ferrite grains at the notch-tip primarily through recrystallization and strain-induced boundary migration (SIBM). Lower pre-strains exhibited a steep gradient in ferrite grain size (GS) upon annealing, whereas higher pre-strain levels revealed a gradual transition from fine to coarse ferrite grains in all directions surrounding the notch. Strain above a critical threshold induced complete recrystallization, forming fine, strain-free ferrite grains near the notch-root, while strain below this level caused structural restoration via abnormal grain coarsening by SIBM. Additionally, statistical analysis indicated a power relation between GS and strain in the modified zone upon annealing at 650 degrees C for 3 h. Superior impact toughness resulted from a gradual ferrite GS gradient, owing to fine grains resisting crack propagation, while poor impact toughness arose from a steep gradient with coarse ferrite grains within the active zone facilitating cleavage crack initiation. In summary, this investigation explores the role of distinct localized gradient microstructures at the crack-tip on the impact toughness behavior of low-carbon ferritic steel.