Fragmentation of Boron‐Alloyed steel rings under dynamic loading

Expanding-ring fragmentation under dynamic loading provides an effective means to investigate high-strain-rate fracture mechanisms in metals. However, the fragmentation response of boron-alloyed steel rings under varying loading conditions remains insufficiently clarified. In this study, 40SiMnB and 40SiMnB3 steel rings subjected to three heat-treatment routes were investigated by gas-gun-driven expanding-ring tests at loading velocities of 160, 210, and 260 m/s. Electron backscatter diffraction characterization, quasi-static and dynamic tensile tests, and fracture-toughness measurements were performed to determine the microstructural characteristics and mechanical properties of the heat-treated boron-alloyed steels. Weibull analysis was used to evaluate the fragment-mass distribution of the recovered fragments. Finite-element simulations were further used to reproduce the overall fragment-count trend, reconstruct the load-transfer process, and describe the fragmentation development and circumferential strain-rate evolution of the expanding ring. Among the heat-treated states, 40SiMnB3 in the T1 state, quenched at 880 °C for 10 min and tempered at 350 °C for 15 min, exhibited the smallest characteristic fragment-mass scale, with a Weibull scale parameter λ of 0.207 g at 260 m/s. Correlation analysis between λ and material descriptors shows that the high-angle grain-boundary fraction is the most stable microstructural correlate, whereas the association with fracture toughness weakens at higher loading velocities. These findings provide guidance for the design of steels with tailored fragmentation performance.