Effects of shot peening treatment on very long life fatigue property in ductile cast irons

Rotating bending fatigue tests that covered very long life region (up to over 10⁸), were carried out on specimens of three kinds of ductile cast iron (austempered one, ADI, pearlitic one, PDI and ferrite pearlitic one, FPDI) with shot peening (SP) treatment. Influences of the SP treatment on fracture morphology and fatigue property were investigated. It is expected that the SP treatment results not only the high level work hardening and the high level compressive residual stress in the surface layer of the specimens, but also the decrease in casting defects in near surface. Then, they lead a considerable improvement of fatigue strength. Therefore, initial distribution of the hardness in the surface layer of the SP treated specimens was measured. Furthermore, in the PDI-SP and the FPDI-SP materials, it was observed that the sites of the fatigue crack initiation were transferred from the surface to subsurface in the longer life regions, but in the ADI-SP material, the fatigue crack initiation site in whole region of fatigue life was the subsurface. And the S-N diagrams did not show fatigue limit over 10⁷ cycles. So differences in fatigue fracture morphology in the shorter and the longer region of fatigue life were discussed from detailed observation of crack initiation sites with afield emission scanning electron microscopy (FE-SEM)