Handbook-Welding Cast Iron

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WELDING OF CAST IRON Cast iron is an extremely versatile material, used in thousands of industrial products. It is hard, wear-resistant, and relatively inexpensive. Like steel, it is available in many different grades and compositions. While we usually think of cast iron as being brittle (having low ductility), this is not true of all cast irons, as we shall see shortly. Cast iron, like steel, is an iron-carbon alloy. In composition and structure, and in some of its properties, it is quite different from steel. While many grades of cast iron can be welded successfully, not all cast iron is weldable, and welding of any cast iron presents problems not usually encountered in the welding of steel. Composition and Grades of Cast Iron 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