Microstructural Characterization and Mechanical Properties of Free-Machining Sintered Steel Parts Containing up to 1 Wt Pct MnS

The effect of prealloying MnS up to 1.0 wt pct on the microstructural features of nonmetallic inclusions and their impacts on tensile and fatigue properties of 7.0 g/cm³ powder metal (P/M) steel (Fe-2.0Cu-0.7C) have been investigated. As the MnS content increases, larger, more irregular, and more closely spaced inclusions are obtained. While no significant impact on both static and dynamic properties was observed when prealloying up to 0.65 wt pct MnS, a decrease of more than 15 pct of the ultimate tensile strength and of the endurance limit was found when the MnS content reached 1.0 wt pct. The decrease in the ultimate tensile stress is attributed to a lower ductility of the sinternecks, as void initiation and void growth were promoted at lower stress levels by larger inclusions. The larger size of the MnS particles and the lower mean free path between nonmetallic inclusions also favor the initiation of microcracks and their coalescence into cracks, reaching fatigue fracture earlier.