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UNIT TWO: CLASSIFICATION AND PROPERTIES OF METALS AND THEIR ALLOYS 2.1 INTRODUCTION The knowledge of materials and their properties is of great significance for design and manufacturing. The machine elements should be made of such a material which has properties suitable for the conditions of operation. In addition to this, a design engineer must be familiar with the effects which the manufacturing processes and heat treatment have on the properties of the materials. In this chapter, we shall discuss the commonly used engineering materials and their properties in manufacturing and design. 2.2 CLASSIFICATIONS OF ENGINEERING MATERIALS The engineering materials are mainly classified as 1. Metals and their alloys, such as iron, steel, copper, aluminum, etc. 2. Non-metals, such as glass, rubber, plastic etc. The metals may be further classified as: a. Ferrous metals, and b. Non-ferrous metals The ferrous metals are those which have the iron as their main constituent, such as cast iron, wrought iron and steel. The non-ferrous metals are those which have a metal other than iron as their main constituent, such as copper, aluminum, brass, tin, zinc, etc. 2.3 FERROUS METALS We have already discussed that the ferrous metals are those which have iron as their main constituent. The ferrous metals commonly used in engineering practice are cast iron, wrought iron, steels and alloy steels. The principal raw material for all ferrous metals is pig iron which is obtained by smelting iron ore with coke and limestone, in the blast furnace. 2.4 CAST IRON The cast iron is obtained by re-melting pig iron with coke and limestone in a furnace known as cupola. It is primarily an alloy of iron and carbon. The carbon content in cast iron varies from 1.7 per cent to 4.5 per cent. It also contains small amounts of silicon, manganese, phosphorous and sulphur. The carbon in a cast iron is present in either of the following two forms: 1. Free carbon or graphite, and 2. Combined carbon or cementite.
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Since the cast iron is a brittle material, therefore, it cannot be used in those parts of machines which are subjected to shocks. The properties of cast iron which make it a valuable material for engineering purposes are its low cost, good casting characteristics, high compressive strength, wear resistance and excellent machinability. The compressive strength of cast iron is much greater than the tensile strength. Following are the values of ultimate strength of cast iron: Tensile strength = 100 to 200MPa Compressive strength = 400 to 1000MPa Shear strength = 120MPa 2.5 TYPES OF CAST IRON The various types of cast iron in use are discussed as follows: 1. Grey cast iron: - it is an ordinary commercial iron having the following compositions: Carbon = 3 to 3.5%; silicon = 1 to 2.75%; manganese = 0.40 to 1.0 %; phosphorous = 0.15 to 1%; sulphur = 0.02 to 0.15%; and the remaining is iron. The grey color is due to the fact that the carbon is present in the form of
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