Influence of shot peening on the fatigue life of Cu–Ni austempered ductile iron

Shot peening (SP) was carried out in order to improve the fatigue life of an austempered ductile iron alloyed with copper and nickel austenitised at 900 °C for 2 h and austempered at 360 °C for 1 h. The optimal peening intensity using S330 steel shot, was found to be 0.38 mmA producing residual compressive stresses with a maximum of approximately 1000 MPa at the surface. The microhardness at the surface was increased by SP from 370 to 535 HV. The fully reversed fatigue tests of SP-specimens showed a 61% increase in fatigue life over that of polished samples. The improvement in fatigue life is explained by the compressive residual stresses in the near surface shifting crack initiation to sub-surface. Fractography analysis showed multiple crack nucleation sites at the surface for the polished specimens, while SP-specimens showed sub-surface crack nucleation. In addition, a mixed fracture mode was also identified, with the graphite nodules and copper islands affecting crack propagation.