Cast iron is by no means pure iron. In fact, there is less iron in any grade of cast iron than there is in a low-carbon steel, which may be 98% iron. Almost every cast iron contains well over 2.0% carbon; some contain as much as 4.0% . In addition, cast iron usually contains 1.2 to 2.5% silicon, 0.5 to 0.8% manganese, and (as in steel) small percentages of sulphur and phosphorous. It is the high percentage of carbon that make cast iron different from steel in many of its properties. In a finished steel, all the carbon is combined with iron in the form of iron carbides, whether those carbides are in grains of pearlite, in grains of cementite, or in scattered small particles of carbide. In cast iron, most of the carbon is usually present in uncombined form, as graphite. (Graphite is one of the two crystalline forms of carbon; diamond is the other). The differences between the general types of cast iron most widely used arise chiefly from the form which the graphite assumes in the finished iron. Gray Iron. Of the general types of cast iron, gray iron is by far the most widely used. The term ”gray iron” was adopted originally to distinguish it, by color of the fractured metal, from white iron, a form of cast iron in which all the carbon is combined. We’ll have more to say about white iron later. At this point, we wish to stress the point that gray iron is a very broad term. All gray irons contain graphite in the form of flakes. This makes the gray irons readily machinable. All gray irons have almost no ductility, again because of the flake form of the graphite, which causes the metal to break before any appreciable amount of permanent elongation has occurred. However, not all 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.
Gray cast iron can usually be welded without loss of essential properties. For fusion welding, preheating of the
casting is absolutely essential. Since a higher level of preheat is required for oxy-acetylene welding then for arc welding, arc welding is likely to be chosen where fusion welding is essential (as it is whenever good color match is desired). For many repair jobs, however, oxy-acetylene braze welding is the ideal method. Much less preheating is required; in many cases, preheating can be done with the torch. If the work is properly done, the braze-welded joint will have a strength equal to that of the base metal, and excellent machinability. Welding of gray iron castings which have chilled white iron surfaces is seldom attempted, since the desirable properties of white iron will always be affected by welding temperatures. Welding of white iron generally is limited to malleable iron foundries, where castings may be reclaimed by welding before conversion to malleable iron takes place. Malleable Iron. The chemical composition of malleable cast iron is much the same as that of a typical gray iron, but its properties are much different. It is tough; it can resist shock; it has ductility approaching that of mild steel. How is such a remarkable change achieved? By cooling the original casting so rapidly that white cast iron, with no free carbon, is formed; then heating the casting to about 8000C and holding it at that temperature for several days. Under those conditions, virtually all the carbon is released from the iron carbide to form fine rounded particles of graphite (sometimes called temper carbon) scattered among grains of ferrite. Malleable iron has good wear resistance, and is widely used for parts where the toughness of steel is required, and the economy of casting (instead of forming or machining) will result in lower cost. However, malleable iron is substantially more expensive to make than gray iron, and is usually selected only where its toughness and ductility are essential. Malleable iron cannot be successfully fusion welded and retain its unique properties; to put it another way, you can weld malleable iron as easily as you can weld gray iron, but in the act of welding you will convert some of the malleable iron casting into a gray iron casting. Seldom will that yield a satisfactory result. However, malleable iron castings can usually be braze welded successfully. You may wonder how to tell a malleable iron casting from a gray iron casting. There’s one almost infallible method: use a high-speed grinder to make a spark test. The difference between the spark streams produced by gray iron and malleable iron is quite pronounced. Spark testing is covered in the Appendix.
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