Effects of Hardness of Shot Peening Media on Surface Modification Behavior of Gas Carburized Steel by Micro Shot Peening

Effects of the hardness of fine media used in shot peening on the surface modification behavior of workpiece with high hardness were investigated. Gas carburized JIS SCM822 steels with the surface of 720 HV in Vickers hardness were shot peened by fine media with the range from 390 to 1550 HV in Vickers hardness, followed by the examination of their surface modification behavior. Shot peening was carried out by using suction type air peening machine with 0.6 MPa in air pressure, 10 s in peening time and 50 mm in peening distance. Various gas atomized powders sieved from 45 to 125 pm in particle diameter were used as fine peening media. On the peened surface by the fine media with remarkably higher hardness than that of workpiece, deep nanocrystalline layer with around 1 mu m in depth and less than 15 nm in grain size was generated. In these nanocrystalline layers, the thickness increased and grain size decreased with increasing Vickers hardness of used peening media. In addition, Vickers hardness on peened surface also increased with increasing Vickers hardness of used peening media. Especially, the hardening effect on peened surface was outstanding, in the case of using fine peening media with more than 1020 HV in Vickers hardness. It seemed that this hardening effect was caused by grain refinement strengthening in nanocrystalline layer.