Influence of Shot Peening Parameters on Residual Stresses in Flake and Vermicular Cast Irons

Test samples of grey and compacted graphite cast irons with pearlitic matrix were shotpeened to different surface conditions using twelve different combinations of shot size, peening intensity and peening coverage percentage. Relatively high surface compressive residual stresses varying between 245 to 565 MPa were observed and the compressive residual stresses reached a depth between 280 μm and 770 μm. Within the range of peening parameters used, the compacted graphite cast iron with its vermicular graphite showed a somewhat better response to the same shotpeening treatment than the grey cast iron containing flake graphite, giving a larger peening affected zone with higher compressive residual stresses. For both the cast irons, an increase in peening coverage percentage, shot size or peening intensity led often to a lower surface compressive stress. However, peening using a higher intensity greatly increased the degree and extent of plastic deformation and therefore increased the magnitude and penetration depth of the subsurface compressive residual stresses, while the effect of increasing shot size also depends on the peening intensity. On the other hand, measurements on the grey cast iron samples showed that the peening coverage has little effect on the depth profile of residual stress.