Materials deformed at high strain rates exhibit sometimes the behaviour that is very different from that at static or quasi-static loading. Material and physical properties that are of no concern in low strain rates become crucial in the material response at high strain rates. Material properties such as thermal diffusivity and thermal softening have to be considered in order to understand the mechanism of deformation at high strain rates. The most important aspect of high strain rate deformation is the occurrence of local thermal and mechanical instability that leads to failure of the material, usually characterised by the occurrence of adiabatic shear bands (ASB). In this study, AISI 4140 steel was tested using a torsional split Hopkinson bar at three different strain rates and the impact of strain rate on the material behaviour was examined. The influence of material hardness on failure mechanisms was also investigated by producing different tempers of 4140 steel with distinct microstructures and different hardness. The fracture surface of specimens was examined under the scanning electron microscope (SEM). (Abstract shortened by UMI.)