The effect of small sulfur additions to postinoculated high-carbon equivalent (CE) ductile iron melts has been investigated. Three postinoculated iron hypereutectic nodular iron melts with 4.74 percent CE were analyzed--the initial iron containing only 0.0010 wt percent S, and irons with subsequent sulfur additions to reach 0.0049 and 0.0059 wt percent S. Both thermal analysis and micro-structural characterization were used to reveal the response of the iron to the late sulfur treatment. In the detection of characteristic temperatures--within the solidification and the eutectoid transformation ranges--raw cooling curves, their numerical derivatives and inverse rate cooling methods were employed. Higher primary graphite arrests were observed in irons treated with sulfur, while the eutectic and recalescence parameters remained unchanged. As a result of the additions, higher nodule counts were measured, while the graphite morphology was not noticeably modified. In combining thermal and image analysis results, it has been demonstrated that the late sulfur additions promote the formation of graphite nodules in postinoculated high-carbon ductile irons. Furthermore, the higher nodule count in S-treated irons aids in the formation of additional amounts of ferrite in the microstructure, as expected.
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