Cast Iron

Cast Iron

There are two predominant types of cast iron, those being grey iron and white iron. The former has a graphitic structure, the deflection of which provides the namesake color on fracture surfaces. White iron, however, has small white deposits of cementite rather than being completely pallid. The main differences between the two are silicone content and cooling times, both of which have a significant impact on the physical and mechanical behavior of the alloy. White iron has a low carbon content and is cooled at a fast rate to produce a brittle cast part with good hardness and abrasion resistance. These are used in a number of wear applications such as slurry pumps, liners, grinding mills and pulverizers. Grey iron castings on the other hand are produced through the slow cooling of high carbon iron alloys and are less brittle allowing their use as crankshafts, support beams, engine blocks and more. In addition to these two types, iron foundries and metallurgical engineers continue to develop more malleable and ductile irons that exhibit the beneficial characteristics of cast iron, but with significant reductions to brittleness due to a spheroid rather than flaked internal structure. These specialized alloys are becoming increasingly common in the industrial world.

Although pure iron is found only in meteorites, the element is one of the most abundant on Earth making up 5% of the crust and 35% of the total mass. Mining operations extract the element from iron ore and oxides such as magnetite, hematite, limonite, goethite and siderite which contain high levels of iron. These oxides are smelted to produce what is known as pig iron, the base material for cast iron. The stock forms are heated in a special blast furnace known as a cupola. Scrap iron and steel are added to the molten mixture to produce cast iron. Once in a molten state this metal is poured into a cast where it is cooled at controlled rates before a finished or near finished part is ejected or extracted. Some of the more popular methods used today for iron castings are die casting, centrifugal casting and sand casting. Die casting is used to manufacture complex parts at high production rates, centrifugal casting creates cylindrical parts and sand casting uses expendable synthetic or natural sand molds to create rough parts. These processes result in easily machined cast iron components with high compression strength, low melting points, good thermal conductivity and energy dissipation, wear resistance and fluidity.