Effect of pre-strain aging on microstructure and mechanical properties of a structural steel for high-altitude cold regions

Structural steels employed in bridges, tunnels, and roadbeds are inevitably subjected to deformation, which may induce the degradation of performance with the aid of aging effect caused by long-term service. This study investigates this effect by exerting pre-strain aging processing on a freeze–thaw resistant structural steel (FTR steel). The experimental results indicate that increasing the pre-strain from 0% to 15% raises the yield strength by approximately 135 MPa and the tensile strength by about 60 MPa, while reducing the elongation by 13%, while the impact toughness can still maintain around 350 J, even at −40 °C. Microstructural analyses show that pre-strain aging promotes the interaction and proliferation of dislocations and breaks the cementite lamellae. This promotes the formation of dislocation structures within ferrite. As the pre-strain level increases, the consistently high fraction of high-angle grain boundaries maintains the impact toughness at a stable level. During the impact process, the pre-formation of dislocation structures in the pre-strained samples increases the energy required for crack initiation, thereby influencing the energy dissipation mechanisms during fracture.