Cryogenic process effects on the dynamic mechanical properties of 100Cr6 bullet core analysed via Split Hopkinson Pressure Bar

This study investigates the high strain rate behavior of 100Cr6 bearing steel bullet core material following austempering (300 °C for 1 or 5 h) and subsequent sub-zero cryogenic processing ( -80 °C for 1 h). Microstructural features, phase transformations, and crystallographic parameters were evaluated via SEM and XRD, while mechanical properties were quantified through HV1 microhardness and Split Hopkinson Pressure Bar testing. Cryogenic treatment after prolonged austempering systematically enhanced microhardness and dynamic yield strength while reducing strain capability. The maximum microhardness reached 780 HV1 in the 5-h austempered and cryogenically treated specimen, representing a ∼250% increase over the initial state. XRD analysis revealed maximum pre- and post-impact dislocation densities of 77.86 x 10 ¹⁴ lines/m ² (1-h austempered + cryogenic) and 154.06 x 10 ¹⁴ lines/m ² (1-h austempered), respectively. Overall, post-austempering cryogenic processing effectively optimizes the dynamic mechanical response of 100Cr6 steel.