INFLUENCE OF MNS ON FATIGUE FRACTURE SURFACE-MORPHOLOGY OF NICKEL ALLOYED PM STEEL

The influence of pore fractions of 0.05 and 0.09 and 0.5%MnS on the fa tigue fracture surface morphology of two nickel alloyed powder metallu rgy steels, was investigated. The alloys were produced by diffusion bo nding and a combination of prealloying together with diffusion bonding respectively. Fatigue fracture of smooth rectangular specimens was ob tained by testing at constant amplitude and zero mean stress, at a fre quency of 30 Hz. Fractographic examination showed that the same fractu re mechanisms are found in alloys with and without MnS addition. All f atigue cracks initiate from pores located at the surface or just below it. Fatigue crack growth occurred by four propagation modes associate d with: transparticle striated and serrated mechanisms, interparticle neck separation from microvoid coalescence, and transparticle cleavage fracture. The occurrence of various combinations of these fracture me chanisms depends on the porosity and the applied stress amplitude.