Spheroidal graphite cast iron is a group of iron alloys specially formulated to create molded products with heightened elasticity, tensile strength and durability. Despite its wide use, grey iron tends to be brittle and is therefore discounted in many metallurgical applications. The creation of a spheroidal or nodular graphite microstructure, however, produces ductile cast iron that can be used more broadly.
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Industries such as agriculture, irrigation, ventilation, automotive, water and sewage treatment, hydraulics, manufacturing, transportation, construction and building utilize spheroidal graphite cast iron products which also offer improved corrosion and wear resistance, machinability, and thermal tolerances as compared to comparable grey iron castings. Whereas most iron castings develop graphite flakes, or three dimensional rosettes, spheroidal graphite cast iron is processed at temperatures and cooling rates conducive to the formation of nodular or spheroid graphite. The lower aspect ratio of spheroids as compared to flakes reduces length and increases particulate separation. The rounded or blunt edges of the nodules also help to inhibit cracking and alleviate the internal stresses inherent to graphitic flakes. The elemental composition of spheroidal graphite iron alloys is also slightly different than the average gray iron as fractional amounts of magnesium or cerium must be included in order to effectively control the morphological structure of the graphite.
Grades of these alloys are standardized and can be used to determine the tensile strength and elongation of a particular product or material. A basic chemical constitution of these alloys is as follows. 95% of the total mass is iron with carbon amounts between 3.3 and 3.4% and silicon levels ranging from 2.2 to 2.8% of the weight. Manganese levels are between 0.1 and 0.5% with sulfur and phosphorous each comprising around 0.005% of the total mass. Spheroidal graphite cast iron necessitates the presence of between 0.03 and 0.05% magnesium, cerium or a combination. To achieve the production of nodules low sulphur, low silicon iron is liquefied and magnesium or cerium added. This is followed by the addition of silicon to prevent the formation of cementite and ensure that the carbon precipitates as graphite spheroids. The molten metal is poured into a pre-formed mold designed with the impression of the desired part. Cooling is performed at specific rates to solidify the material and facilitate the formation of a pearlite or ferrite matrix structure. In this way iron foundries can produce ductile castings of such varied products as gears, crankshafts, cylinder heads, support beams and pipes.