Effect of heat treatment on adiabatic shear bands in a high-strength low alloy steel

Adiabatic shear bands (ASBs) were developed in AISI 4340 steel by direct impact loading using Split Hopkinson Pressure Bar (SHPB). These ASBs were found to be inherently very hard and brittle, and they exhibit a tendency to brittle fracture. The present study deals with the effect of heat treatment on these ASBs. Annealing the shear bands was performed at 315°C for 2h and at 650°C for 20min and for 2h. At 315°C, the heat treatment did not produce any considerable effect on the microstructure and hardness of ASBs. Annealing at 650°C for 20min produced a significant effect of reducing the hardness of the shear band to the same level as that of the bulk material outside the shear band. Extending the annealing time to 2h at 650°C led to replacement of the brittle white ASB with a more ductile deformed band that is observable within the magnification of an optical microscope. Results of this study suggest that microstructural changes that occur during adiabatic shearing can be reversed by heat treatment.