The term Cast Iron designates an entire family of metals with a wide variety of properties. Steels and cast irons are both primarily iron with carbon as the main alloying element. Steels contain less than two percent carbon, while all cast irons contain more than two percent carbon. In addition to carbon, cast irons must contain from one to three percent silicon, thus they are actually iron-carbon-silicon alloys. However, the various types of cast iron cannot be designated by chemical composition because of similarities between types. For this reason, most iron castings are purchased on the basis of mechanical properties (esp. tensile strength) rather than composition. The various types of cast iron can be classified by their microstructure and based on the form of carbon present in the iron.
Gray Iron is a type of cast iron that has a flake graphite microstructure. When gray iron is broken, the fracture occurs along the graphite, thereby accounting for the characteristic gray color. Gray iron tends to be brittle in thin sections, has limited tensile strength, and no ductility. The flake graphite structure, however, provides gray iron with unique properties such as excellent machinability, effective corrosion resistance, very good compressive strength, and outstanding vibration dampening. Gray iron is the most common and widely produced of the cast irons. Its uses include: pump housings, valve bodies, automotive engine blocks, machine bases, sheaves, and cookware. The most commonly specified gray iron is ASTM A48 Class 30 and Class 40.
Ductile Iron is a type of cast iron that has a nodular or spheroidal graphite microstructure. The addition of a small amount of magnesium to molten iron causes the graphite to form in small nodules rather than flakes, providing the enhanced ductility that gives the alloy its name. Ductile iron castings, duplicating many grades of steel, possess high strength, ductility, corrosion resistance, and resistance to both mechanical and thermal shock. Annealed ductile cast iron can be bent, twisted, or deformed without fracturing. It is considerably less expensive than cast steel and only moderately more expensive than gray iron. Its uses include: valves, gears, housings, pipe, and crankshafts. The most commonly specified ductile iron is ASTM A536 Grades 60-40-18, 65-45-12, and 80-55-06.