“HOT TOPICS” Issue # 7, 2006

Effect of Bismuth in Ductile Iron

Bismuth can be both harmful and helpful in ductile iron depending on the amount that is in the iron and the remaining amounts of rare earth metals. Basic Elemental Information

Element Atomic Symbol Atomic Number Atomic Weight Melting Point Boiling Point Bismuth Bi 83 208.98 520.7F 2847F

Methods of introduction One method for the introduction of bismuth is through charge materials. Besides its use as an alloy in non-ferrous metals, bismuth can be found in malleable iron castings. With its low melting point alloys, bismuth can also come in through fire suppression items like sprinkler heads. Some permanent magnets might also have bismuth as a base. And as always, bismuth can be present in some solders. Bismuth can also be added on purpose through the use of specialized inoculants. Bi cones are also sometime used. The recovery of Bi is very low and should be calculated for your addition type. Concentrations and addition rates need to be tightly monitored and controlled. Negative Effects of Bi Bismuth reacts in ductile iron similar to antimony and lead. Bismuth is a graphite shape-forming element. Its low melting point most likely forms a film in the cell boundaries where bismuth can then have a very adverse effect on graphite shape. The result will be that of low nodularity iron, areas of flake graphite. Bismuth can also have a negative effect on the recovery of the magnesium addition. It has been reported that up to a 25 % (1) reduction in magnesium recovery can be seen. Also surface defects in green sand molds have been reported due to the interaction with bentonite binders. Eliminating the Negative Effects Minor additions of Ce or other rare earths can offset the negative effects of bismuth. Rates as low as 0.005% Ce can offset the deleterious effects of bismuth and return the structure to fully nodular. Of course this amount can vary depending on other deleterious elements present and other offsetting rare earth additions.

Positive Effects of Bi When bismuth is added in conjunction with cerium the results can be quite positive. With minor addition rates of 0.003% to 0.005% bismuth and the addition of cerium, nodule counts have been reported to increase up to 6 times in thin wall castings. Others report increases of 2 to 3 fold. This is due to the very stable Ce-Bi compounds that form. These compounds act as more nucleation site. In heavy section casting this combination lead to smaller or more nodules. Again with the increase in nodule forming sites and the fact that these compounds are reported to be slightly denser, less carbon floatation is reported. Mechanical Property Effects For the majority of mechanical properties reported, the effects of bismuth with or without cerium additions correlate with the resultant microstructure. Hence if the structure is degraded, the properties are degraded. If the microstructure is improved so are the properties. There does seem to be some change reported in Charpy V-notched results. Maximum values at ambient temperatures are reported lower when bismuth is used in the presence of cerium. However, the impact transition temperature is lowered with the bismuth addition and therefore the results can be superior when castings are required to operate below –20oC. (1) Reference DIS Project #27

w/ Bi Addtion w/o Bi Addition

Scott Gledhill , Manager of Technology, Thyssen Krupp Waupaca References: R. Barton & C. Moss: :Effect of bismuth and other trace elements on graphite nodule count in nodular iron”, BCIRA Journal, Vol. 15, 1967.

F.Lotaert, P. Hilaire, C. Staroz: “Development of More Poweful Inoculants for Spheroidal Graphite Irons:, The Bulletin of the Bismuth Institute, 1983. M. Fallon: The Effect of Some Trace Elements in Cast Iron”, Indian Foundry Journal, June, 1980. S. Subramanian et al: “Rare Earths in Ductile Iron Production: A Critical Review”, DIS Project #27, 1996. AFS: The Ductile Iron Handbook. AFS Des Plaines, IL. 1992. ASM: Metals Handbook Vol 15. ASM Int. 1992.