Metallographic and fractographic examination of fatigue loaded PM-steel with and without MnS additive

This study examined the fatigue fracture morphology and microstructure of two powder metallurgy steels containing 0.0 or 0.5% manganese sulphide (MnS) powder at three density levels: nominal 7.15, 7.45, and full density. Fractography showed that MnS had not participated in fatigue crack initiation or in changing the crack propagation mechanisms. Fatigue crack origins were initiated at pores located at specimen surfaces (7.15 and 7.45 materials), or at subsurface hard inclusions (full density materials). Fatigue crack growth occurred by transparticle serrated and striated mechanisms, by interparticle neck separation from microvoid coalescence, and by transparticle cleavage fracture.