“HOT TOPICS” Issue #6, 2007 Lanthanum in Ductile Iron Lanthanum can be damaging or valuable in ductile Method of introduction iron depending on proper addition and balance with Lanthanum can be found in many types of equipment others elements. which are NOT used as charge material in the foundry process; consequently its method and amount of addition Basic Elemental Information can be kept under tight control. Element Lanthanum Lanthanum metal (99% La) is commercially available Atomic Symbol La under the form of ingots or lumps. It oxidizes easily, becomes flammable when exposed to air and the molten Atomic Number 57 iron homogeneity can be questioned when lanthanum −1 Atomic Mass 138.90547 g.mol cones are added. Density 6.162 g.cm −3 Melting Point 920°C, 1688°F Lanthanum is commonly added in the iron through the use of misch metal (35% La content in average + Boiling Point 3464°C, 6267°F Cerium, Praseodymium, Neodymium and Yttrium). History Lanthanum is also added on purpose through MgFeSi or (Greek lanthanein : to lie hidden). Carl Gustav Mosander, specialized inoculants. The first US patent (12) proposed a Swedish chemist, extracted lanthana (La 2O3) from in 1956 claims that the use of lanthanum alone during impure cerium nitrate and recognized the new element in nodularization and inoculation practices improves 1839. Lanthanum was isolated in relatively pure form in machinability. Later, two European patents (13-14) in 1923. 1978 and 1979 claim the use of lanthanum as an efficient element to minimize micro shrinkage. The use of Notable characteristic lanthanum bearing MgFeSi is universally used for in- Lanthanum metal is a silvery white mold nodularization process since 1979. metallic element soft enough to be cut with a knife. Found in some rare- Industrial context and Ductile Iron requirements earth minerals, usually in Certain ductile iron processes require extended holding combination with cerium, it is one of times of the treated iron. The chemical reactivity of the most reactive of the rare-earth nodularizing elements, particularly magnesium, can lead metals. Lanthanum reacts directly to a deterioration of the microstructure and mechanical with elemental carbon, nitrogen, properties over time. boron, silicon, phosphorus, sulfur, and with halogens. It oxidizes Positive and Negative effects: A question of balance rapidly when exposed to air. Lanthanum, added with cerium, produces higher nodule count than cerium alone. Lalich (8) and Wallace (9) Sources and application reported that nodule counts increase as the La:Ce ratio Lanthanum is one of the 17 elements that define the rare increases toward 2. In inoculant alloys, Stefanescu et al earths group. It is found in rare-earth minerals such as (10) reported that nodule count is at a maximum when ‘Monazite’ and ‘Bastnasite’ ores. the La:Ce ratio is about 1 and the total rare earth is 4.1%. Lanthanum is employed in many applications including carbon arc lights used for studio lighting and projector Early studies, assuming that lanthanum as a true main lights, hydrogen storage material, NiMH battery, special effect on nodule count, show that the average nodule glasses, metallurgy and specialty alloy. counts has a tendency to decrease as lanthanum is added, World production of rare earths was estimated at perhaps due to an effect of the formation of sulfides that 98,200mt of equivalent rare-earth oxides in 2002. China are less effective graphite nuclei than La S . was by far the largest producer with 90 percent of the 2 3 In a later study Horie et al (7) added La as a pure metal world’s total. The use of lanthanum is still growing to base melts containing 0.021% and 0.08% S. At both today. sulfur level, an optimum range for the La:S ratio was 2.5 See Appendix A, B for source and market. Jerome Fourmann, FerroPem, Cleveland Ohio – USA to 6.0. For the low sulfur level, nodule counts increased with increasing lanthanum, but excessive lanthanum resulted in lower nodule counts and increased carbide formation. Effect of Lanthanum on Graphite Crystallization in Ductile Iron Pure Magnesium and magnesium ferrosilicon are commonly used in the nodularisation process. Other elements such as cerium and lanthanum can also have a strong effect on crystallization and structure formation of ductile iron. It was found (11) that cerium and lanthanum additives increased duration and temperature range of eutectic transformation and prevented the primary graphite precipitation. It was established that the change in the crystallization pattern of iron when introducing cerium and lanthanum is caused by their effect on the process of nuclei formation and the growth of graphite in the melt. Non-metallic inclusions formed by cerium and lanthanum with sulphur are characterized by high density and high melting temperatures. This makes lanthanum (& Ce) sulphides to serve as effective substrates for heterogeneous graphite nucleation. It is known that lanthanum has positive effect on nucleation and graphitization. Lanthanum has the advantage of generating stable sulphides and oxy- sulphides thus increasing the nucleation power of the liquid bath. Effect of Lanthanum on the Structure of Eutectic Ductile Iron. Stefanescu and Loper (17) demonstrated that the effects of cerium and lanthanum added to cast iron are similar; their influence is observed at different levels of addition and can be affected by postinoculation process. Lanthanum, up to 0.04%, has a graphitizing effect in cast Ferrosilicon post inoculation of lanthanum-treated cast irons resulting in a lower chilling tendency. At higher iron with 0.3% of 75% FeSi significantly changed the levels, compacted graphite is formed. The matrix is free pattern of solidification and subsequent structure. The of carbides until the La content reaches 0.075%. At range of lanthanum over which compacted graphite higher values, the carbides content increases with the could be obtained is extended, as well as further presence of irregular graphite. increasing the graphitizing tendency of lanthanum. It was also shown that the cooling curve correlated well Cerium did not exhibit a graphitization effect in the irons to the solidification structure obtained. In the absence of studied. At cerium contents in excess of 0.014%, postinoculation, ∆T = Temperature of the Eutectic compacted graphite is formed and carbides may be Recalescence (max) – Temperature Eutectic present. However, at cerium contents in excess of the Undercooling, values of more than 25 °C (45 °F) were second critical level the chilling tendency of cerium- associated with compacted graphite cast irons free of containing irons is lower than that of lanthanum- carbides and a maximum of 9% nodularity. Lower values containing irons. of ∆T corresponded to either flake graphite, or irregular graphite and carbides in the cast irons. 2 Effect of Lanthanum on Ductile Iron solidification Siclari et al (16) presented different stages of grain growth process through the molten iron solidification: First solids appear on mold walls, near locations of severe heat loss. Grains grow toward the opposite direction of the heat flow, usually perpendicular to mold walls. This growing behavior is named “columnar growth”, referring to the growing grain shape. Growth and definition of these grains are determined by the thermal properties of the mold. Graph 2: Effect of lanthanum plus FeSi postinoculation, Ce and La on the carbides content and Brinell hardness of the eutectometer samples Ductile Iron containing 0% La Graph 1: Effect of lanthanum plus FeSi postinoculation, Ce and La on the TEU and TER of the cast irons studied. When heat flow through mold walls has reduced, columnar growth is less favored. Particles of the columnar grains detach to grow freely in the liquid. This second solidification behavior is named “equiaxed growth”. Liquid iron convection motions and viscosity property dictate, in the mold, directions taken by Ductile Iron inoculated 0.8% La bearing inoculant growing grains. Within growing grain particles, heat flow and solidification directions are identical. Harding At a given solidification stage, when lanthanum is added, et al (15) research works show that lanthanum element the equiaxed solidification behavior is favored. promotes the equiaxed solidification behavior by: Consequences for shrinkage and micro-shrinkage are - developing a higher nucleation power, therefore more that: solidification sites within the molten iron. - the thickness of the columnar zone is reduced, leaving - modifying the molten iron viscosity favoring stirring larger free flowing passages for the remaining molten motions within molten iron. iron to travel within channels feeding casting areas that - restricting the growth of columnar grains. are in need of liquid iron and is essential to minimize the This phenomenon is dictated by the physico-chemical formation of shrinkage defects. properties of the Fe-La alloy binary diagram. 3 - The semi-solid iron contains more solid particles. After References having fed a given casting volume, less liquid is needed 1) Handbook of Chemistry and Physics – 87 th Edition. to compensate for the last solidifying areas.. 2) American Chemical Society 3) "The Industrial Chemistry of the Lanthanons, Yttrium, Thorium and Uranium", by R.J. Callow, Pergamon Press 1967 4) "Nouveau Traite de Chimie Minerale, Vol. VII. Scandium, Yttrium, Elements des Terres Rares, Actinium", P. Pascal, Editor, Masson & Cie 1959 5) "Extractive Metallurgy of Rare Earths", by C.K. Gupta and N. Krishnamurthy, CRC Press 2005 6) M.J Gross, Commonwealth Aluminium Corporation – G. Wistehuff, H. Colthurst GM Powertrain Div. GMC Saginaw, Michigan – D.R. Askeland University of Missouri Rolla, Missouri 7) H. Horie, S. Hiratsuka, T. Kowata, Y. Iwamochi, Y. Shobuzawa and M. Nakumara, “Effect of Eight Individual Rare-Earth Elements on the Graphite Dendrites formation from the wall Nodule Count of Thin-Section of Spheroidal Appendix Graphite Cast Iron”, Cast Metals, vol 3, No 2 (1990), pp 73-81. A- Principal sources of US imports of rare earths, by 8) M.J. Lalich, “Effective Use of Rare Earths in weight, in 2005.