Effect of heat treatment on stability of impact-induced adiabatic shear bands in 4340 steel

Adiabatic shear bands (ASBs) have been observed in many engineering applications, including armor plates and metal working operations subjected to both high strain rates (>103S−1) and large strains. These bands indicate excessive deformations due to strain localization. Preventing their formation by proper choice of materials or eliminating them when formed may avert the occurrence of catastrophic failures. In this study, impacted specimens of 4340 steel, where ASBs have been observed, were subjected to post-impact annealing to determine the persistence of ASBs. Microstructural observations were made using optical and scanning electron microscopy to determine whether the physical appearance of the ASBs changes or not during the post-impact annealing. These observations were followed by X-ray diffraction analysis on the shear band regions and on the base materials to determine the influence of the heat treatment on the annealed specimens. It was found that annealing at temperatures higher than 600°C shows significant reduction in the physical appearance of the ASB and its hardness. After 30min annealing at 650°C, the hardness of the shear bands and their matrixes reduced to approximately the same level as they were before deformation. The X-ray diffraction analysis also confirmed that recrystallization processes and grain growth cause the high strains of the ASBs to be reduced. Post-impact annealing at 650°C revealed that the fine crystals in the shear bands were replaced by larger crystals compared to the surrounding material due to the early occurrence of recrystallization in the shear band regions. Post-impact annealing of the steel specimens at 750°C and 850°C for 30min led to a homogenous microstructure with no trace of the shear bands. The pre-impact heat treatment properties were not affected because there was no excessive grain growth to significantly alter the strength and hardness of the impacted and annealed specimens as compared to the properties of the undeformed material. The X-ray diffraction analysis revealed that the average size of the crystals in the annealed specimens is smaller when compared to the unimpacted specimen.