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Metal Casting Technologies : March 2011
42 www.metals.rala.com.au BacktoBASICS Ductile iron grades uctile iron is not a single material but is part of a group of materials which can be produced to have a wide range of properties through control of the microstructure. The common defining characteristic of this group of materials is the graphite morphology. In ductile irons the graphite is in the form of spherical nodules rather than flakes (as in grey iron), thus inhibiting the creation of cracks and providing the enhanced ductility that gives the alloy its name. The formation of nodules is achieved by addition of nodularising elements, most commonly magnesium into the melt. Besides the requirement that the graphite be in a spheroidal form, the ferrite and pearlite ratios can be controlled through alloying, cooling rate control or post-casting heat treatment to vary the relative amounts pearlite and ferrite from 0% pearlite and 100% ferrite, to 100% pearlite and 0% ferrite. The control of the pearlite and ferrite ratio manipulates the tensile, yield and elongation characteristics of the ductile iron to produce numerous standard grades of the material. Primary elements in ductile iron The primary elements in ductile irons are generally considered to be carbon, silicon, manganese, phosphorous, sulphur and magnesium. Carbon While the carbon content in commercial ductile iron ranges from less than 3% to over 4%, it is more common for carbon to be within a range of 3.5% and 3.9%. As a general rule, the carbon equivalent has little effect on the mechanical properties of ductile iron. For this reason there is a tendency to use high carbon equivalent irons since this improves fluidity and reduces shrinkage tendency. A combination of high carbon content and low solidification cooling rate can result in graphite floatation and the presence of degenerate graphite. For these reasons, it is necessary to adjust carbon equivalent according to section size. For thin sections (less than 12mm) a carbon equivalent of 4.6% is recommended, for intermediate sections (12 to 40mm) a carbon equivalent of 4.45% may be more appropriate whilst for heavy sections (greater than 50mm) it may be necessary to limit carbon equivalent to 4.3%. Silicon Typical silicon contents in ductile irons range from 1.80% to 2.80% although there are applications for lower and substantially higher levels. Some oxidation resistant alloys contain up to 6% silicon. Silicon is a potent graphitiser, decreasing the stability of eutectic and pearlitic carbide. Silicon hardens and strengthens ferrite particularly in the annealed condition. A potential adverse effect of increasing silicon in ferritic ductile irons is the raising of the ductile brittle impact transition temperature. For the low-temperature impact grades silicon contents are generally kept within a range of 1.7% to 2.2%. Manganese Manganese is a pearlite stabilising element; however, it is also a moderately strong carbide promoter tending to form chill carbides in thin sections and intercellular carbides in heavy sections. In as-cast ferritic grades and thin sections in any grade, it is preferable to maintain manganese below about 0.2%. Higher levels may be tolerated in heavier section pearlitic grades. Phosphorus Whilst phosphorus must be considered as an undesirable element, it is present in most charge materials used in ductile iron production. The element forms a low melting point iron phosphide which segregates to cell boundaries resulting in deterioration in elongation and impact properties. It may also cause an increase in porosity. Sulphur and magnesium When magnesium is added to molten iron it acts to deoxidize and desulphurise the melt thereafter cause the graphite to precipitate and grow in a spheroidal form. If the base iron and treatment are not properly controlled, excessive oxygen and/or sulphur can consume the magnesium leaving an insufficient residual to spheroidise the graphite. Excessively high magnesium contents tend to promote carbides. Since sulphur readily combines with magnesium it has no effect on the matrix structure, however, high levels in the base iron are undesirable since magnesium yield is reduced. The effects of common elements in ductile Iron J. F. Meredith Casting Solutions Pty Ltd D