Engineering materials used to manufacture of articles or products, dictates which manufacturing process or processes are to be used to provide it the desired shape. Sometimes, it is possible to use more than one manufacturing processes, then the best possible process must be utilized in manufacture of product. It is therefore important to know what materials are available in the universe with it usual cost. What are the common characteristics of engineering materials such as physical, chemical, mechanical, thermal, optical, electrical, and mechanical? How they can be processed economically to get the desired product. The basic knowledge of engineering materials and their properties is of great significance for a design and manufacturing engineer. The elements of tools, machines and equipments should be made of such a material which has properties suitable for the conditions of operation. In addition to this, a product designer, tool designer and design engineer should always be familiar with various kinds of engineering materials, their properties and applications to meet the functional requirements of the design product. They must understand all the effects which the manufacturing processes and heat treatment have on the properties of the engineering materials. The general classification
CLASSIFICATION OF ENGINEERING MATERIALS
A large numbers of engineering materials exists in the universe such as metals and non metals (leather, rubber, asbestos, plastic, ceramics, organic polymers, composites and semi conductor). Some commonly used engineering materials are broadly classified as shown in Fig. 1. Leather is generally used for shoes, belt drives, packing, washers etc. It is highly flexible and can easily withstand against considerable wear under suitable conditions. Rubber is commonly employed as packing material, belt drive as an electric insulator. Asbestos is basically utilized for lagging round steam pipes and steam pipe and steam boilers because it is poor conductor of heat, so avoids loss of heat to the surroundings. Engineering materials may also be categorized into metals and alloys, ceramic materials, organic polymers, composites and semiconductors. The metal and alloys have tremendous applications for manufacturing the products required by the customers.
Metals and Alloys
Metals are polycrystalline bodies consisting of a great number of fine crystals. Pure metals possess low strength and do not have the required properties. So, alloys are produced by
melting or sintering two or more metals or metals and a non-metal, together. Alloys may consist of two more components. Metals and alloys are further classified into two major kind namely ferrous metals and non-ferrous metals.
(a) Ferrous metals are those which have the iron as their main constituent, such as pig iron, cast iron, wrought iron and steels.
(b) Non-ferrous metals are those which have a metal other than iron as their main constituent, such as copper, aluminium, brass, bronze, tin, silver zinc, invar etc.
Ferrous metals are iron base metals which include all variety of pig iron, cast iron wrought iron and steels. The ferrous metals are those which have iron as their main constituents. The ferrous metals commonly used in engineering practice are cast iron, wrought iron, steel and alloy steels. The basic principal raw material for all ferrous metals is pig iron which is obtained by smelting iron ore, coke and limestone, in the blast furnace. The principal iron ores with their metallic contents are shown in Table 1.
Main Types of Iron
1. Pig iron
2. Cast iron
(A) White cast iron
(B) Gray cast iron
(C) Malleable cast iron
(D) Ductile cast iron
(E) Meehanite cast iron
(F) Alloy cast iron
3. Wrought iron
(A) Plain carbon steels
1. Dead Carbon steels
2. Low Carbon steels
3. Medium Carbon steels
4. High Carbon steels
(B) Alloy steels
1. High speed steel
2. Stainless steel Some important ferrous metals, their extraction, composition, properties and their common applications are discussed in detail as under.
Pig iron was originated in the early days by reduction or iron ores in blast furnace and when the total output of the blast furnace was sand cast into pigs which is a mass of iron roughly resembling a reclining pig. It is roughly of 20″ × 9″ × 4″ in size. It is produced in a blast furnace and is the first product in the process of converting iron ore into useful ferrous metal. The iron ore on initial refining and heating in blast furnace becomes pig iron when the impurities are burnt out in a blast furnace. Pig iron acts as the raw material for production of all kinds of cast iron and steel products. It is obtained by smelting (chemical reduction of iron ore in the blast furnace. It is of great importance in the foundry and in steel making processes. It is partly refined in a cupola furnace that produces various grades of cast iron. By puddling processes, wrought iron is produced from pig iron. Steel is produced from pig iron by various steel making processes such as bessemer, open-hearth, oxygen, electric and spray steel making. The charge in the blast furnace for manufacturing pig iron is
(a) Ore Consisting of iron oxide or carbonate associated with earth impurities.
(b) Coke A fuel
(c) Limestone A flux
In addition to iron, pig iron contains various other constituents in varying form of impurity such carbon, silicon, sulphur, manganese and phosphorus etc. It has the following approximate composition which is as given as under.
Carbon — 4 to 4.5%
Phosphorus — 0.1 to 2.0%
Silicon — 0.4 to 2.0%
Sulphur — 0.4 to 1.0%
Manganese — 0.2 to 1.5 %
Iron — Remainder
Carbon exists in iron in free form (graphite) and/or in combined form (cementite and pearlite). Pig iron is classified on the basis of contents of free and combined carbon as follows. These classifications are also termed as grades.
1. Grey pig iron (Grades 1, 2 and 3) Grey pig iron contains about 3% carbon in free form (i.e., graphite form) and about 1% carbon in combined form. This is a soft type of pig iron.
2. White pig iron (Grades 4) White pig iron is hard and strong. It contains almost all of the carbon in the combined form.
3. Mottled pig iron (Grade 5) This type of pig iron is in between the grey and white variety. It has an average hardness and molted appearance. The free and combined forms of carbon are in almost equal proportion in mottled pig iron.