Structural Features of Plastic Deformation Zones Formed in Quenched and Tempered Structural Steel during Dynamic Testing

Systematic microhardness measurements were performed to assess the shape and size of plastic deformation zones formed under the fracture surface in quenched and tempered steel 25 during dynamic testing. It was found that as the tempering temperature of the steel decreases, the size of the plastic deformation zone in the region of dynamic crack initiation decreases, and the dynamic crack becomes flatter. The stress and strain fields at the crack front in high-tempered steel 25 under dynamic loading were modeled by finite element simulations. The simulation results revealed a multistage evolution of the stress and strain fields in tests for dynamic crack resistance. The experimental results on microhardness distribution in the plastic deformation zone under the fracture surface and strain distributions during dynamic crack propagation showed good qualitative agreement with the simulation results. Electron microscopic examination of the starting region of the plastic deformation zone and the hammer impact region revealed adiabatic shear bands in the subsurface of the fractured specimen. Tracks of composition contrast observed along the band boundaries indicate the mass transfer of carbon. Simultaneous studies of the structure and fracture surface in the impact region showed that the dynamic crack propagates mainly along the boundaries of structural elements that induce adiabatic shear, resulting in the formation of characteristic grooves on the fracture surface.