Fracture and ballistic-induced phase transformation in tempered martensitic low-carbon armour steels

Plates with thicknesses of 4.5 and 5.2 mm (reduced from current designs of 6 to 8 mm) of various experimental armour steels were tested according to the standard ballistic test of 5.56 mm rounds fired at a muzzle velocity of 940 ± 10 m/s from a distance of 30 m. The fracture mechanism and phase transformation in various radial zones in and around the impacted areas were investigated by SEM, TEM and XRD. The resistance to ballistic perforation could be correlated directly with the microstructure and its changes upon ballistic impact. The roles of retained austenite, twinned plate martensite interfaces and grain boundaries in determining ballistic performance are explained.