Handbook-Welding Cast Iron

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gray irons are equally strong, or equally hard. As in steel, tensile strength and hardness are closely related. In gray irons, tensile strength ranges from about 14 MPa (20,000 psi) to more than 35 MPa (50,000 psi). The hardness of the strongest grades is double that of the weakest grades. All gray irons have high compressive strength – three to four times their tensile strength. While all gray cast irons contain free carbon (graphite) in flake form, they also contain combined carbon (iron carbide) in almost every case. This combined carbon is often present in pearlite grains, such as found in most carbon steels. It may also be found as cementite or martensite. The composition of the cast iron, the rate at which it cooled after casting, and heat treatment after casting all have a bearing on the structure. Small amounts of alloying elements are used in the strongest gray irons; they tend to prevent the formation of pearlite. While the hardness and strength of steel almost always increase as carbon content rises, in the case of gray cast iron the strongest, hardest grades have less carbon than some of the lower-strength, less expensive grades. Gray iron is usually cast in sand molds, and allowed to cool normally in the mold. Heat treatment after casting is not always necessary, but is frequently employed, either to increase or to decrease hardness. Almost all gasoline and diesel engine blocks are gray iron castings. Whenever industry desires an intricate form which can be machined to close tolerances, and must withstand abrasive wear, gray iron gets consideration. Only when it is essential that the finished item have some ductility and good shock resistance is some other material – such as nodular cast iron or cast steel, both more expensive – likely to be substituted. White iron, mentioned above, is about the same as gray iron in composition, but has been cooled rapidly so that graphite does not have time to form, and all the carbon winds up in the combined form, as pearlite, cementite, or martensite. Many white iron castings are subsequently converted to malleable iron, which we shall take up next. However, some gray iron castings are made with white iron wearing surfaces, since white iron is much harder than gray iron, although extremely brittle. This is accomplished by inserting metal or graphite chill blocks at appropriate places in the mold. The molten metal that solidifies against those chill blocks cools so rapidly that white iron surfaces are created. Plowshares, railroad car wheels, and various types of dies are often made with such chilled white iron surfaces.