Influence of MnS on Fatigue Fracture Surface Morphology of Nickel Alloyed PM Steel

AbstractThe influence of pore fractions of 0·05 and 0·09 and 0·5%MnS on the fatigue fracture surface morphology of two nickel alloyed powder metallurgy steels, was investigated. The alloys were produced by diffusion bonding and a combination of prealloying together with diffusion bonding respectively. Fatigue fracture of smooth rectangular specimens was obtained by testing at constant amplitude and zero mean stress, at a frequency of 30 Hz. Fractographic examination showed that the same fracture mechanisms are found in alloys with and without MnS addition. All fatigue cracks initiate from pores located at the surface or just below it. Fatigue crack growth occurred by four propagation modes associated with: trans particle striated and serrated mechanisms, interparticle neck separation from microvoid coalescence, and transparticle cleavage fracture. The occurrence of various combinations of these fracture mechanisms depends on the porosity and the applied stress amplitude. PM/0656