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Metal Casting Technologies : March 2011
44 www.metals.rala.com.au BacktoBASICS form shortly before casting to give residual magnesium content in excess of about 0.03% in the solidified casting. There are however, a number of elements, which may interfere with the development of acceptable spheroidal graphite structures. These elements are commonly referred to in various terms including deleterious and subversive. Ductile irons are typically produced from mixed charges, which may contain pig iron, steel scrap from various sources, foundry returns and purchased cast iron scrap. As a result, impurity trace elements such as aluminium, antimony, arsenic, bismuth, lead, tellurium, titanium etc., may be introduced in small amounts. These elements can have a significant adverse effect on properties of the alloy by forming undesirable graphite and matrix forms in cast structures. Additionally, the effect of even trace amounts of these subversive elements can be cumulative when they are present in combination. For example, the presence of lead limits the tolerance for titanium, antimony and bismuth, and that the adverse effects of titanium are observed more strongly in copper alloyed irons. For this reason recommended "acceptable" levels of these elements must be treated with some caution. The levels of such elements must therefore be minimised by avoiding contaminated scrap such as lead painted or enamelled material and automotive scrap containing aluminium components. In many cases, analysis of these elements is difficult without specialised equipment, however, based on the severity of the problems they can cause, concentrations should be periodically analysed by an outside laboratory. Fortunately, the undesirable effect of many of these elements can be neutralised with the addition of some cerium either as mischmetal or as a cerium bearing magnesium ferrosilicon. The amount of cerium required to neutralise harmful effects will depend upon the effectiveness of the cerium and the sum of all the tramp elements present. Typically, a residual cerium content of about 0.01% is usually sufficient for most situations. Some of the more commonly experienced deleterious elements include: Lead (Pb) Lead is perhaps one of the most common of the subversive elements found in charge materials. Lead may be introduced to the base iron through the melting of leaded (free-cutting) steel scrap, lead containing pig irons, steel scrap coated with lead based paints, leaded copper scrap which may be deliberately added to the charge to stabilise pearlite or contaminated melt stock such as shown in Figure 3. As little as 0.002% lead in the base iron can neutralise the spheroidising effect of magnesium resulting in a flake form of graphite and dramatically reduced mechanical properties. Since such a small amount can have a dramatic effect on graphite shape, it can be difficult to monitor effects by chemical analysis. The adverse effect of lead becomes more pronounced with heavier casting sections. The use of cerium or cerium Figure 1. Well formed graphite nodules in ductile iron Figure 2. Undesirable graphite morphology Figure 3. Lead insert in cast iron scrap