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Effects of inclusion particles on the microstructure and mechanical properties of high strength austempered ductile iron (ADI) were investigated in this study. Inclusion particles, especially when their sizes are less than 5 mm, were mostly found in intercellular regions. Whether an inclusion particle can induce the formation of acicular ferrite depends on Mn segregation. In intercellular region, acicular ferrite was hard to form in the vicinity of inclusion particles due to (1) serious Mn segregation, and/or (2) the Mg-enriched inclusions here in halo-like. Consequently the surrounding austenite remained to be blocky type after austempering treatment. The fatigue life cycles of ADIs were affected by the particle counts and the microstructure. Increasing the count of fine inclusions along with the effect of Mn segregation deteriorated the fatigue life and elongation of high strength ADIs.

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