Grain Refinement and Phase Transformation in a Middle-Carbon Steel after Jet Penetration

Abstract

The “black†layer near the crater surface of a middle-carbon steel target penetrated by a high speed copper jet was observed by SEM. Micro-hardness test showed that the “black†layer near the crater surface was much harder than the matrix. It almost reached to the untempered martensite phase microhardness. The microstructure features divided the penetrated middle-carbon steel into four layers from the crater surface to the matrix: needle-like martensite layer, austenite and ultra-fine grained(UFG) ferrite layer, plastic deformation layer and normal matrix. The phase transformation was caused by the high pressure introduced by the hypervelocity impact and plastic deformation. And the microstructure distribution has something relationship to the different pressure condition and deformation strain and strain-rate because the shock wave decays as it spreads to the deep. XRD patterns confirmed that there were new phases like austenite in the middle-carbon steel after jet penetration.