Effects of small charge explosions upon AISI 304Cu steel | Effetti di esplosioni di piccola carica sull'acciaio AISI 304Cu

Microstructural variations induced on AISI 304Cu steel disks by explosions of small charges which caused limited or null macro-deformation are illustrated. Explosions were performed in the Cottrau ballistic facility of the Italian Military Navy at La Spezia, Italy, using either 50 g or 100 g NSP explosive spherical charges (54.5 and 109 g TNT equivalent) and explosive-to-target distances in the range from 0.065 to 0.815 m, in order to achieve peak pressures at the specimens’ surfaces in the calculated 2250 to 5 bar range. Calculated peak pressures were verified through some instrumented explosions in the 113 to 450 bar range. The grain size influence was evaluated by using specimens with two different grain sizes (60 and 32 µm). Microstructural features of the material were studied after the explosions by analyzing the specimens’ previously polished and etched surfaces by X-ray diffraction (XRD), optical microscopy (OM), scanning electronic microscopy (SEM) and scanning tunneling microscopy (STM). After being exposed to the explosion, the surfaces showed mainly mechanical twins, in addition to regions undergoing phenomena of ricrystallization and of localized melting at the grain borders at the shortest charge-to-target distances. The features, observed trough OM, SEM and XRD, decrease in intensity with increasing distances. X-ray diffraction allowed to verify a local re-orientation of the grains upon the superficial layers; the crystal rotation is connected to a micro-twinning process, that causes asymmetric peak profiles. The twinning on a microscopic scale has been confirmed by the STM profiles, that show regularly spaced surface relieves, which represent the traces left by this process upon the surface. The XRD analysis also showed that the explosions of small charges do not induce the formation of the ε and α’ martensitic phases. The specimens’ sections, observed by OM, showed only the presence of mechanical twins. These can be observed within a limited distance from the surface exposed to the blast wave and also in a limited layer localized near the opposed surface. Again, the twins are gradually more evident while decreasing the distance from the charge. For each charge and each grain size the charge-to-target distance at which they disappear was identified, both inside the specimen and, at larger distanes, upon the surface. In order to relate such a distance to the critical component if the twinning stress for the steel used, a stress analysis was developed, starting from the results of the tests performed by using instrumented targets.