Influence of hardness of substrates on properties of surface layer formed by fine particle bombarding

This study investigated the influence that the hardness of substrates has on the properties of surface layers formed by fine particle bombarding (FPB). For this purpose, the hardness of mild steel JIS S45C was changed by quenching and tempering, and then FPB was performed under the same conditions. The properties of the formed surface layers, such as the microstructure, surface hardness, residual stress and hardness distribution, were systematically examined. The surface layers were nano-crystallized through severe plastic deformation induced by FPB, regardless of the hardness of the substrates. The grain size of nano-crystals in the surface layers decreased with increasing hardness of the substrates, so that the surface hardness markedly increased. FPB introduced compressive residual stress and its absolute value became high with increasing hardness of the substrates. Since plastic deformation induced by FPB reached deeper positions when the hardness of the substrates was reduced, the thickness of hardened regions increased.