EFFECTS OF SHOT PEENING TREATMENT ON VERY HIGH CYCLE FATIGUE PROPERTY IN AUSTEMPERED DUCTILE CAST IRON

摘要

Rotating bending fatigue tests were carried out on austempered ductile cast iron (ADI) with shot peening (SP) treatment, and effects of the SP treatment on fatigue properties and fracture morphologies were investigated in the very long life regime. As results, the improvement of fatigue strength for the SP treated ADI was confirmed before the regime of 10{sup}7 cycles. However, the S-N diagram of the SP treated ADI and that of the non-treated ADI crossed at near 10{sup}8 cycles, and the fatigue strength at 10{sup}9 cycles of the SP treated became lower than that of the non-treated ADI. To investigate the reason for the fatigue strength reduction of the SP treated ADI over the regime of 10{sup}8 cycles, the residual stress distribution and the hardness distribution in the internal direction were measured and the fracture origins were also observed. The sites of fracture origins were larger than the hardened layer depth resulted by the SP treatment, and coincided approximately with the transition points where the compressive residual stress on the surface layer decreased and changed into the tensile residual stress in the internal direction.