Effects of interactions between nodule count of spheroidal graphite and retained austenite on tensile properties of austempered ductile cast iron heat treated from (α + γ) Range

Abstract

ADI (austempered ductile cast iron) has outstanding balance in strength and ductility, as the retained austenite improves ductility through the strain-induced transformation. In our previous report, the formation and mechanical properties of fine ferrite, bainitic ferrite, and retained austenite on the preheat-treated fine pearlite structure were investigated. However, the investigation into the effect of spheroidal graphite was insufficient. Therefore, the purpose of this study is to clear on the effect that the interaction between the nodule count of spheroidal graphite and retained austenite has on mechanical properties. Two methods were used for changing the nodule count of spheroidal graphite. The first method is that the nodule count is adjusted by changing the solidification speed in which crystallization of the spheroidal graphite occurs during casting. The second method is that the amount of added carbon is reduced to decrease the amount of spheroidal graphite being crystallized, which reduces the nodule count. Based on these conditions, the test results indicate that the condition in which more spheroidal graphite that it could be considered as a defect demonstrated better mechanical properties due to the strain-induced transformation of the retained austenite. However, the material that more nodule counts of spheroidal graphite decrease ductility due to work-hardening ability become too high.