Effect of Shot Peening Treatment on High Cycle Fatigue Property in Long Life Regime of Austempered Ductile Cast Iron

Abstract

Rotating bending fatigue tests were carried out on austempered ductile cast iron (ADI) with shot peening (SP) treatment, and influences of the SP treatment on fatigue properties and fracture morphologies were investigated in the long life regime. As a result, improvement of fatigue strength for shot peened ADI specimen was identified before the region of the 107 cycles. Moreover the S-N diagram of shot peened ADI and fatigue limit of non-shot peened ADI cross at over 108 cycles. Then fatigue strength at 109 cycles of non-shot peened ADI was about 360 MPa and that of shot peened ADI was about 280 MPa. To invistigate the reason for fatigue strength reduction of shot peened ADI, residual stress distribution and hardness distribution were measured and fracture origin was observed by SEM. As a result, the reason why the fatigue strength of shot peened ADI decreases more than non-shot peened ADI was that the tensile residual stress had acted inside the specimen. The fatigue limits of shot peened ADI were estimated by casting defects size and actual stress distribution in the specimen.