Effect of prior heat treatment on the dynamic impact behavior of 4340 steel and formation of adiabatic shear bands

Adiabatic shear bands are observed in many metallic materials when subjected to both high strain rates (>103 s−1) and large strains. These bands are a manifestation of damage and may lead to catastrophic failure because they are susceptible to crack nucleation and propagation due to their high internal stresses. Minimizing their occurrence is important in many applications and processes. In this study, the effect of prior heat treatment on evolution of adiabatic shear bands in a typical heat treatable steel was investigated. Cylindrical AISI 4340 steel specimens were heat treated at 315 °C, 425 °C and 620 °C, for 1 h or 2 h and impacted using the direct impact Hopkinson pressure bar (DIHPB). Tempering the specimens at 315 °C and 425 °C for 1 h led to the formation of distinct and well-developed adiabatic shear bands after impact. Diffused shear bands were formed in the specimens tempered at 315 °C and 425 °C for 2 h. Specimens tempered at 315 °C and 425 °C for 1 h exhibited higher impact resistance than those tempered at 315 °C and 425 °C for 2 h. The 1 h tempered specimens absorbed much of the impact energy without failing at the impact momentums at which those tempered at 315 °C and 425 °C for 2 h were failing. These observations demonstrate that tempering the steel specimens at 315 °C and 425 °C for 1 h may lead to catastrophic failure through the formation of adiabatic shear bands, while they exhibit higher impact resistance than when they are tempered for 2 h. Tempering the steel specimens at 620 °C for 2 h before impact resulted in the formation of deformed shear bands, which are less susceptible to crack nucleation and propagation. However, at very high impact momentum, a distinct transformed shear band was observed, indicating that catastrophic failure through the formation of adiabatic shear bands can be reduced by increasing the tempering temperature of the steel specimens even though at very high impact momentums, failure through the formation of adiabatic shear bands is likely to occur. Thus the initial conditions (tempering temperature and time) of a heat treatable steel before impact are shown to have a significant influence on the formation of adiabatic shear bands.