ENGINEERING MATERIALS 725 the more common materials with indications of their main fields of application. with a at its tem- until a estab- non- For omitted dis- difficult of a Cast iron Cast iron is an alloy mainly of iron and carbon. It may contain up to 4% carbon, although other elements, or impurities, may also be present. The various types of cast iron differ in the exact structure of the metal and the way in which the carbon mixes with or combines with the iron In broad terms, however, cast iron has a fairly low ultimate tensile stress, of the order of 150-225 Nimm although this may be in creased by the addition of suitable elements and by heat treatment. The compression strength is between 3 and 4 times the tensile strength, being about 770 Nmm while the shear strength is of about the same order he tensile strength 50-225 NLmma). As cast, there is usually a hard skin on the surface of cast iron. Under this skin the material is more readily machinable, but is usually rather brittle. It may break under uneven heating conditions because of distortion produced by internal stresses which exceed the ultimate at the temperature concerned. Under reasonable operating conditions, cast iron offers excellent resistance to corrosion compared with many other ferrous metals. It is an electrical conductor, having a resistivity of about 15 x 10-8 2m, and normally also has good magnetic properties. Cast iron is used, as the name implies, in 'castings', for which the molten metal is poured into suitable moulds and allowed to solidify. This method makes it possible for intricate shapes to be formed with- out the expense and waste of material that would be involved in machining them from a solid block of material. Limitations in the shape and use of such castings are imposed by the relatively low tensile and shear strength of the material. These could lead to cracks in the cast iron as it cools down in the mould, or to the breaking-off of thin projections when it is in use. (i) Steel NImm Steel is also an alloy mainly of iron and carbon, although special may be steels may have amounts of other elements present in excess of the rties of carbon content. These other elements, which include nickel, chromium, ere the molybdenum and tungsten, considerably modify the properties of the resulting steel. The carbon content of steel is lower than that of cast nternal iron, and does not exceed 2%