Solidification pattern of 4.5%Si ductile iron in metal mould versus sand mould castings

The present work evaluated the solidification pattern of 4.5%Si un-inoculated ductile iron by standard cooling curve analysis and addresses the effects of solidification cooling rate on as-cast structure as determined by metal mould versus sand mould and section size (3–15 mm). The 4.5%Si-alloyed ductile cast iron led to a large ΔT s eutectic interval (closer to 80 °C) and a very low undercooling (only 11% from ΔT s ), while the temperature of the solidification end was more than 18 °C above the eutectic metastable temperature. The graphite amount was up to 65% lower in the metal mould and more depending on the section size. The metal mould versus green sand mould solidification increased nodule count (more than 30%) with a reduced effect of the increasing wall thickness, at higher rate of less than 15 μm size nodules and with 5–10% higher graphite nodularity. No free carbides were noticed in the green sand mould (inclusively at 3 mm section size), but up to 15% carbides in 6 mm thickness was remarked in the metal mould, with decreasing up to 5% for 12–15 mm thickness. In the present experimental conditions, 70–100% ferrite resulted in the green sand mould and 10–20% ferrite in the metal mould. It appears that solidification in metal mould of High-Si ductile cast iron was more sensitive to chemical composition, including minor (trace) elements than in the sand mould, documented as a high capacity to tolerate higher amounts of pearlite and carbide stabilizing elements. The results of the standard thermal (cooling curve) analysis are in agreement with the obtained structure of 4.5%Si ductile iron in the sand mould, but without the prediction of free carbides formation in the metal mould, being necessary to use ceramic cups at higher solidification cooling rate, especially for thin wall castings control.