EXPERIMENTAL INVESTIGATION OF EFFECTS OF CHEMICAL VARIABLES ON IRON CASTING

Cast iron is an alloy of iron containing more than 2% carbon as an alloying element. It has almost no ductility and must be formed by casting. Ductile iron structure is developed from the melt of cast iron. The presence of silicon in higher amount promotes the graphitization, inhibiting carbon to form carbides with carbide forming elements present. The carbon forms into spheres when Ce & Mg are added to the melt of iron with very low sulphur content. Due to this special microstructure containing graphite in nodular form ductile iron possesses ductility & toughness superior to that of any cast iron & steel structure resulting in numerous successes in industrial application. Ductile iron castings with 3 and 12 mm thickness with varying chemical composition were cast in furan resin sand moulds to identify the effect of sample thickness on micro structural changes and selected mechanical properties.  The effect of melt chemistry and molten metal processing variables  (i.e.,  pre-conditioning  of  the  base  iron,  inoculation  type  and  practice,  and pouring temperature, etc.) on the tensile and impact properties of thin-wall ductile iron castings has been investigated. Comparison of 3 and 12 mm sections within the same casting showed that section size was the main factor influencing tensile properties of ductile irons.  While many samples from 3 mm sections showed low elongation values likely caused by a high pearlite content or presence of carbides, many others showed higher elongations and superior strengths well above those required in ASTM A536 grades. At moderate to high elongations, the thin-wall samples were significantly stronger than samples from identical irons of 12 mm section.

A direct comparison between impact values could not be made due to different test specimen sizes, but it is clear that toughness in the two section sizes was roughly equivalent when account was made for the total cross sectional area.  The main difference between the Impact properties in the two section sizes lay in the relative in sensitivity of the thin section specimens to either melt chemistry or molten metal processing variables. Of the elements contained in the iron, silicon had the greatest effect on the tensile properties of the thin wall sections. The same increase in silicon content of the thin wall sections had little effect on impact toughness. As expected, any processing variable that led to an increase in nodule count (with a corresponding increase in ferrite content) led to greater ductility, lower strength, and improved toughness. Of the variables studied the greatest effect was found to be from late inoculation, base iron pre-conditioning, and the use of an inoculants containing bismuth and rare earths.