PM and Rheocasting in Ductile Iron Castings

Permanent mold (PM) and semi-solid (rheo) casting in ductile iron castings (DIC) were studied, and the microstructure was aimed to be chill free in as-cast condition. Denitrification was conducted understanding solubility properties of nitrogen in base molten iron, and re-nitrification was strictly avoided during molten treatment by Mg alloy. As the results, chill-free knuckle castings were possible with both casting methods in as-cast condition. The knuckle castings had no shrinkage cavity also. The solidification time by rheocasting was shorter than that of PM casting. It was approximately one in two and a half. The castings had ultrafine spheroidal graphite which were mostly under 10 µm. The graphite structure by rheocasing method was finer and more uniform than those of PM casting. The tensile strength in castings with both casting methods was approximately 1.3 times keeping elongation over 10% comparing with conventional sand knuckle castings. It was concluded that free nitrogen (N_F) promoted chill formation, and chill could be avoided by promoting denitrification and minimum re-nitrification.