Effects of Antimony and Wall Thickness on Graphite Morphology in Ductile Iron Castings
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DUCTILE iron is a very specific engineering material in the family of cast irons. It is used in many applications which require strength and toughness, such as various automotive parts, windmill parts, pipes, etc. Low production costs and good machinability and castability are also important positive features of this material. Ductile iron has a wide range of mechanical properties which are primarily determined by its microstructure. In the as-cast condition, the microstructure of a typical ductile iron consists of graphite nodules which are surrounded by a ferritic and/or pearlitic metal matrix. The graphite morphology is a very important factor for obtaining high mechanical properties of ductile iron.[1–4] Graphite particles must have a nodular (spheroidal) shape. The size and uniformity of distribution of graphite nodules also affect the properties of ductile iron, but to a lesser extent than graphite shape.[5] Nodularity of ductile iron is defined as the percentage of graphite particles that have nodular (spheroidal) shape. In general, nodularity must be minimum 80 pct. Higher nodularity may be required for castings subjected to severe loading. Low nodularity, i.e., the presence of irregularly shaped graphite spheroids or nonspheroidal graphite forms, such as chunky, exploded, spiky, vermicular and intercellular flake graphite affects the mechanical properties. These graphite forms are usually present in heavy sections. Increasing the proportion of non-nodular graphite reduces all properties relating to ZORAN GLAVAS and ANITA STRKALJ, Associate Professors, are with the Faculty of Metallurgy, University of Zagreb, Aleja Narodnih Heroja 3, 44103 Sisak, Croatia. KRESIMIR MALDINI, Analyst, is with Croatian Waters Company, Ulica Grada Vukovara 220, 10000 Zagreb, Croatia. Contact e-mail: [email protected] Manuscript submitted October 31, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS B
strength and ductility. The form of non-nodular graphite has an important influence. Sharp edges of graphite have a more adverse effect on tensile properties than rounded ends. Graphite particles in ductile iron act as voids in tensile loading condition.[6] Crack propagates by connecting to the neighboring graphite particles (voids). Irregularly shaped nodules and non-nodular graphite act as flaws or stress raisers that facilitate crack initiation and propagation. Graphite spheroids act like ‘‘crack-arresters’’ which allows better utilization of the mechanical properties of the metal matrix. The chemical composition is one of the most significant factors that affect the graphite morphology. The addition of spheroidizing element, such as Mg, Ce, and La, is necessary. These elements are effective modifiers of the graphite shape during solidification of the treated melt. When added to molten iron in proper amount, graphite structure changes from flake to spheroidal. Furthermore, the content of S and O must be very low. The subversive elements (Ti, Al, As, Sn, Sb, Pb, and Bi) significantly hinder the formation of spheroidal graphite. Therefore,
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