Fractographic aspects of fatigue of sintered Ni-steels

This study examines the fatigue fracture morphology of two Ni-sintered steels with different contents of pores and MnS. These steels are produced to contain either MnS or pores or a combination of both. A total of 12 alloys are examined. Fatigue fracture of smooth rectangular and circular specimens were obtained by testing at constant amplitude and zero mean stress at 30-Hz frequency. Fractographic examinations were carried out to characterize the location and character of the fatigue origins as well as the propagation mechanisms of the fatigue crack initiated from these origins. For materials where pores are the most populous, multiple fatigue crack origins were observed at surface locations. In cases where MnS were the most populous, single fatigue crack origin was observed either at surface inhomogeneity or at subsurface inclusion. In all samples, fatigue crack growth occurred by four propagation modes: transparticle striated and serrated mechanisms, interparticle neck separation from microvoid coalescence and transparticle cleavage fracture. The combination of these fracture mechanisms depends mainly on the pore level and the applied stress amplitude.