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01 engineering materials 01 engineering materials Document Transcript

  • -Engineering Materials- © Rohan Desai- Automobile Department - New Polytechnic, Kolhapur. Page 1 INTRODUCTION Materials are probably more deep-seated in our culture than most of us realize. Transportation, housing, clothing, communication, recreation, and food production -virtually every segment of our everyday lives is influenced to one degree or another by materials. In fact, early civilizations have been designated by the level of their materials development (Stone Age, Bronze Age, and Iron Age). The earliest humans had access to only a very limited number of materials, those that occur naturally: stone, wood, clay, skins, and so on. With time they discovered techniques for producing materials that had properties superior to those of the natural ones; these new materials included pottery and various metals. Furthermore, it was discovered that the properties of a material could be altered by heat treatments and by the addition of other substances. At this point, materials utilization was totally a selection process that involved deciding from a given, rather limited set of materials the one best suited for an application by virtue of its characteristics. It was not until relatively recent times that scientists came to understand the relationships between the structural elements of materials and their properties. This knowledge, acquired over approximately the past 100 years, has empowered them to fashion, to a large degree, the characteristics of materials. Thus, tens of thousands of different materials have evolved with rather specialized characteristics that meet the needs of our modern and complex society; these include metals, plastics, glasses, and fibers. The development of many technologies has been associated with the accessibility of suitable materials. For example, automobiles would not have been possible without the availability of inexpensive steel and electronic devices rely on components that are made from semiconducting materials.
  • -Engineering Materials- © Rohan Desai- Automobile Department - New Polytechnic, Kolhapur. Page 2 Q. Give classification of engineering materials. Q. Classify various properties of materials with examples. All important properties of solid materials may be grouped into six different categories: Mechanical, Electrical, Thermal, Magnetic, Optical, and Deteriorative. For each there is a characteristic type of stimulus capable of provoking different responses. Category Stimulus Example Mechanical Force Strength, ductility Electrical Electric field Electrical conductivity Thermal Heat Thermal conductivity Magnetic Magnetic field Magnetic flux Optical Radiation Index of refraction Deteriorative Chemical reaction Corrosion resistance
  • -Engineering Materials- © Rohan Desai- Automobile Department - New Polytechnic, Kolhapur. Page 3 Q. Define the mechanical properties. STRENGTH It is the ability to withstand the force to which it is subjected. It is termed as shear strength, tensile strength, and compressive strength. Unit of strength is N/mm2 Typical tensile strength values of some important materials are given below: Structural Steel 400 N/mm2 Grey Cast Iron 170 N/mm2 ELASTICITY Elasticity is that property of a material which enables it to regain its original shape and size after load is removed. PLASTICITY The plasticity of a material is its ability to be permanently deformed without rupture or failure. Plastic deformation will take place only after the elastic range has been exceeded. DUCTILITY Ductility is that property of a material which enables it to draw out into thin wire. Mild steel is a ductile material. The percent elongation and the reduction in area in tension are often used as measure of ductility. MALLEABILITY Malleability of a material is its ability to be flattened into thin sheets without cracking by hot or cold working. Aluminium, copper, tin, lead, steel, etc. are malleable metals. TOUGHNESS Toughness is a property of metal by virtue of which it can absorb maximum energy before actual fracture or failure takes place. For example, if a load is suddenly applied to a piece of mild steel and then to a piece of glass, the mild steel will absorb much more energy before failure occurs. Thus mild steel is much tougher than a glass.
  • -Engineering Materials- © Rohan Desai- Automobile Department - New Polytechnic, Kolhapur. Page 4 HARDNESS Hardness is defined as the ability of a material to resist to scratching, abrasion, cutting, indentation, or penetration. Many methods are now in use for determining the hardness of a material. They are Brinell, Rockwell and Vickers. BRITTLENESS The brittleness of a material is the property of breaking without much permanent distortion. There are many materials which break or fail before much deformation takes place. Such materials are brittle, e.g. glass, cast iron. Therefore a non-ductile material is said to be brittle material. RESILIENCE Resilience is the capacity of a material to absorb energy elastically. On removal of the load, the energy stored is given off exactly as in spring when the load is removed. CREEP Creep can be defined as the slow and progressive deformation of a material with time under a constant stress at temperatures approximately above 0.4 Tm (where Tm is the melting point of the metal or alloy in degrees Kelvin). FATIGUE When subjected to fluctuating (repeated) loads, the material tends to develop a characteristic behavior which is different than that under steady load. This behavior is called as fatigue. Q. List the ferrous metals and their alloys. Metals:- 1. Pig iron 2. Wrought iron 3. Pure iron Alloys:- 1. Cast iron 2. Alloy Cast iron
  • -Engineering Materials- © Rohan Desai- Automobile Department - New Polytechnic, Kolhapur. Page 5 3. Carbon Steel 4. Alloy Steel Q. Write short note on the Pig iron. All iron and steel products are derived originally from pig iron. This is the raw material obtained from the chemical reduction of iron ore in a blast furnace. The main raw materials required for pig iron are: (1) iron ore, (2) cooking coal, and (3) flux. Iron ores are generally carbonates, hydrates or oxides of the metal, the latter being the best. The coke used in the blast furnace should be a very high class hard coke obtained from good quality coking coals containing as low phosphorus and sulphur as possible. Flux combines with the ashes of the fuel and the ore to form fusible products which separate from the metal as slag. The most commonly used blast furnace flux is limestone. Q. Write short note on wrought iron. It is a highly refined iron with a small amount of slag forged out into fibres. It is produced by remelting pig iron in a puddling furnace. It is the purest form of pig iron. The chemical analysis of the metal shows as much as 99 % of iron. Properties: It is ductile when cold. It is good corrosion resistant than mild steel. It is tough, malleable and has high tensile strength. It can not be melted but can be forged. It can be easily welded. Applications: Bolts, nuts, chains, Pipe & its fittings, sheets, plates, crane hook and boiler tubes. Q. Give short note on Cast irons. Cast irons are basically the alloys of iron and carbon in which the carbon content varies between 2 to 6.67 %. Commercial cast irons are complex in composition and contain carbon in the range of 2.3 to 3.75 % with
  • -Engineering Materials- © Rohan Desai- Automobile Department - New Polytechnic, Kolhapur. Page 6 other elements such as silicon, phosphorous, sulphur and manganese in substantial amount. Properties: Poor ductility and malleability, they can not be forged, rolled, drawn, or pressed into desired shape, but are formed by melting and casting to the required final shape and size. Characteristics: 1. They are the cheapest amongst the commercial alloys. 2. They are easier to melt due to their lower melting temperature (1150-1250 0 C) as compared to steels (1350-1500 0 C). 3. They can be easily cast due to high fluidity of melt and low shrinkage during solidification. 4. Their corrosion resistance is fairly good. 5. In general, they are brittle and their mechanical properties are inferior to steels. Q. Give classifications of Cast iron on various bases. Cast irons are classified according to various criteria as below: (a) On the basis of furnace used in their manufacture: (1) Cupola cast irons (2) Air furnace cast irons (3) Electric furnace cast irons (4) Duplex cast irons (b) On the basis of composition and purity: (1) Low carbon, low silicon cast irons (2) High carbon, low sulphur cast irons (3) Nickel alloy cast irons (c) On the basis of microstructure and appearance of fracture: (1) Grey cast irons (2) White cast irons (3) Malleable cast irons (4) Nodular cast irons
  • -Engineering Materials- © Rohan Desai- Automobile Department - New Polytechnic, Kolhapur. Page 7 (5) Mottled cast irons (6) Chilled cast irons Table showing typical composition of irons & Cast Irons Material Carbon Silicon Manganese Sulphur Phosphorous Pig Iron 3.0 to 4.0 0.5 to 3.0 0.1 to 1.0 0.02 to 0.1 0.03 to 2.0 Wrought iron 0.02 to 0.08 0.1 to 0.2 0.02 to 0.1 0.02 to 0.04 0.05 to 0.2 Grey cast iron 2.50-3.75 1.00-2.50 0.40-1.00 0.06-0.12 0.10-1.00 White cast iron 1.75-2.30 0.85-1.20 0.10-0.40 0.12-0.35 0.05-0.20 Malleable cast iron 2.20-3.60 0.40-1.10 0.10-0.40 0.03-0.30 0.10-0.20 Q. Write process, characteristic and applications of Grey C. I. Process: Grey cast iron is obtained by melting pig iron, coke and steel scrap in a cupola furnace and allowing it to cool and solidify slowly. While solidifying, the iron contains carbon in the form of graphite flakes. It has a dull grey crystalline or granular structure and a strong light will give a glistering effect due to reflection of the free graphite flakes. In tension, the ultimate tensile strength is 120-300 N/mm2 while in compression it is 600-750 N/mm2 Characteristics: (a) They have excellent damping capacity (b) Cheaply available (c) Low melting temperature (between 1150 to 1200 0 C) (d) Good machinability and Graphite on the surface acts as lubricant Applications: Grey cast irons are widely used for machine bases, engine frames, drainage pipes, and elevator counter weights, pump housings, cylinders and pistons of I.C. engines, fly wheels, etc.
  • -Engineering Materials- © Rohan Desai- Automobile Department - New Polytechnic, Kolhapur. Page 8 Q. Write process, characteristics and applications of White C. I. Process: White cast iron is obtained by melting pig iron, coke and steel scrap in a cupola furnace and allowing it to cool and solidify rapidly. While solidifying, the iron contains carbon in the form of iron carbide. (Cementite- Fe3C compound) Characteristics: (a) White cast iron is very hard, brittle and wear resistant. (b) Its fractured surface appears white because of absence of graphite and hence the name white cast iron. (c) It has poor machinability and mechanical properties. Application: wearing plates, road roller surface, grinding balls, dies and extrusion nozzles. White cast irons are widely used for making malleable cast iron. Q. Write the process, characteristics and applications of malleable C.I. Process: These are produced from white cast irons by malleabilizing heat treatment. The heat treatment consists heating the white cast iron slowly to a temp. at around 9000 c and holding at this temp. for long time followed by cooling to room temperature.
  • -Engineering Materials- © Rohan Desai- Automobile Department - New Polytechnic, Kolhapur. Page 9 Fig: Malleablizing heat treatment cycle Upto 1= heating 1-2 = holding period= cementite converted into graphite in rosset form 2-3= moderate cooling= gets pearlitic malleable cast iron 2-3’= slow cooling= gets ferritic malleable cast iron Properties: Good mechanical properties like ductility and malleability Applications: connecting rods, transmission gears, differential cases, flanges, pipe fittings, valve parts, marine services. Q. Write process, properties and applications of Nodular C. I. Process: These cast irons contain graphite in the form of nodules or spheroids. These are produced from grey cast iron by addition of small quantity of magnesium or cerium just before pouring. Due to this addition, instead of graphite flakes, spheroids are formed. Properties: Good mechanical properties like ductility and malleability Applications: Valves pump bodies, crankshafts, gears, and other automotive and machine components. Q. Write short note on mottled and chilled C.I. MOTTLED CAST IRON These cast irons show free cementite as well as graphite flakes in their microstructure. For certain compositions, particularly in terms of carbon and silicon content, such structures are observed under the existing conditions of cooling. For a given composition, faster cooling gives white structure and slow cooling results in gray structure. For intermediate cooling rates, mottled structure is observed. Hence, mottled structure is also observed in certain region between the surface and centre of a chilled casting. Mottled structures do not have good properties and should be avoided. CHILLED CAST IRON Process: This type of cast iron shows white structure at surface and gray structure in the centre. The composition of melt is adjusted in such a manner that rapid cooling gives white structure and usual cooling gives gray structure.
  • -Engineering Materials- © Rohan Desai- Automobile Department - New Polytechnic, Kolhapur. Page 10 Properties: hardness, wear resistance, machinability, damping capacity and low notch sensitivity Applications: railway-freight-car wheels, crushing roll, grinding balls, road rollers, hammers and dies.
  • -Engineering Materials- © Rohan Desai- Automobile Department - New Polytechnic, Kolhapur. Page 11 MICROSTRUCTURES OF VARIOUS CAST IRONS Fig (a): Grey cast iron (the dark graphite flakes are embedded in α ferrite matrix) Fig (b): Nodular cast iron (the dark graphite nodules are surrounded by α ferrite matrix) Fig (c): White cast iron (the light cementite regions are surrounded by pearlite) Fig (d): Malleable cast iron (dark graphite rosettes in α ferrite matrix)
  • -Engineering Materials- © Rohan Desai- Automobile Department - New Polytechnic, Kolhapur. Page 12 Q. Write short note on plain carbon steel. (A) Low Carbon Steels: Composition: 0.008% to 0.30% Carbon and remaining iron with impurities. Properties: They are soft, ductile, malleable, tough, machinable, weldable and non-hardenable by heat treatment. Applications: Steel with 0.008% to 0.15% carbon are used for fabrication work. For example wires, nails, rivets and screws. Steels with 0.15% to 0.30% carbon are widely used as structural steels (mild steel) and finds applications as building bars, grills, beams, angles, channels, etc. (B) Medium Carbon Steels: Composition: 0.30% to 0.60% Carbon and remaining iron with impurities. Properties: They are medium hard, not so ductile and malleable, medium tough, slightly difficult to machine, weld and harden. They are also called as Machinery Steels. Applications: They are used for bolts, axles, lock washers, large forging dies, springs, wires, wheel spokes, hammers, rods, turbine rotors, crank pins, cylinder liners, railway rails and railway tyres. (C) High Carbon Steels: Composition: 0.60% to 2.0% Carbon and remaining iron with impurities. Properties: They are hard, wear resistant, brittle, difficult to machine, difficult to weld and can be hardened by heat treatment. The hardness produced after hardening is high. They are also called as Tool steels. Applications: They are used for forging dies, punches, hammers, chisels, vice jaws, shear blades, drills, knives, razor blades, balls and races for ball bearings, mandrels, cutters, files, wire drawing dies, reamers, and metal cutting saws.
  • -Engineering Materials- © Rohan Desai- Automobile Department - New Polytechnic, Kolhapur. Page 13 Q. What are effects of alloying elements on properties of steel? Molybdenum promotes hardenability, increases tensile and creep strength at high temperature. Chromium improves corrosion resistance, toughness and hardenability. Nickel provides toughness, corrosion resistance, and deep hardening. Silicon increases strength without decreasing ductility and resists high temperature oxidation. Tungsten increases hardenability, wear and abrasion resistance. It reduces the tendency of decarburization. Q. What do you mean by alloy steels and explain any one of them? Alloy steel may be defined as steel to which elements other than carbon are added in sufficient amount to produce an improvement in properties. The chief alloying elements used in steel are nickel, chromium, molybdenum, cobalt, vanadium, manganese, silicon, tungsten. Alloying elements are added in steel for the following purpose: 1. To improve elasticity. 2. To improve corrosion and fatigue resistance. 3. To improve hardness, toughness and tensile strength. STAINLESS STEEL Composition Range of Stainless Steel Class C% Cr% Ni% Uses Ferritic 0.1 to 0.25 16 to 30 — Dairy components, kitchen- ware, automobile fittings Martensitic 0.1 to 0.7 10 to 25 — Turbine blades, ball bearings table cutlery. Austenitic 0.08 to 0.25 15 to 25 5 to 25 Tableware, cutlery, chemical plants, ornamental goods.
  • -Engineering Materials- © Rohan Desai- Automobile Department - New Polytechnic, Kolhapur. Page 14 Properties: High ductility, weldability, machinability and formability Good mechanical properties at low and high temperatures It has good creep resistance, surface finish and appearance TOOL STEELS The selection of proper tool depends upon many factors like the operation to be performed, characteristics of material to be cut, machine tool to be used and rate of cutting. The society of automotive engineers has classified tool steels into the following six major groups. 1. Water hardening tool steels 2. Shock resistant tool steels 3. Cold working tool steels 4. Hot working tool steels 5. High speed steels 6. Special purpose tool steels. Water hardening tool steels: These are used for files, twist drills, chisels, hammers, etc. Shock resistant tool steel: These steels are used for coal cutter picks, cold chisels, pneumatic chisels and punches. Cold working tool steels: These are used in master tools, gauges, dies. They are also for twist drills, taps milling cutters, drawing dies, boring tools. Hot working steels: It is used for hot drawing, hot forging and extrusion dies for casting aluminium, brass, zinc, and their alloys. Special purpose tool steels: These steels are used for special purposes like stainless and heat resisting components. HEAT RESISTING STEELS Composition: 23 to 30% chromium, carbon less than 0.35% and remaining steel. Properties: Heat resisting steels are those which are particularly suitable for working at high temperatures. This steel provides a useful combination of nonscaling and strength-retaining properties together
  • -Engineering Materials- © Rohan Desai- Automobile Department - New Polytechnic, Kolhapur. Page 15 with resistance to acid corrosion comparable with that of stainless steels. Applications: Furnace parts and annealing boxes SHOCK RESISTING STEELS Composition: 0.50% carbon, 2.25% tungsten, 1.50% chromium and 0.25% vanadium. Properties: They resist shock and severe fatigue stresses. Applications: Leaf and coil springs. Q. How C. I. and steels are designated as per IS? Cast iron IS code designation contains following: 1. Symbols indicating the type of casting. 2. Symbols for mechanical properties, CS-Steel castings: CS1250: Unalloyed steel castings with minimum tensile strength 1250 N/mm2 . FG -Grey iron casting: FG 150: Grey iron casting with minimum tensile strength 150 N/mm2 Steels According to IS, steels are designated on two criteria: 1. Steels designated on the basis of mechanical properties. 2. Steels designated on the basis of chemical compositions. Steels designated on the basis of mechanical properties. The code designation shall consist of the following in the order given: 1. Symbol Fe' or FeE' depending on whether the steel has been specified on the basis of minimum tensile strength or yield strength. 2. Figure indicating the minimum tensile strength or yield stress in N/mm2 . 3. Chemical symbols for elements the presence of which characterize the steels. 4. Symbol indicating special characteristics covering method of deoxidation, steel quality, surface condition, weld-ability, formability, etc. Fe 410: steel with a minimum tensile strength of 410 N/mm2 . FeE 270: steel with a minimum yield strength of 270 N/mm2
  • -Engineering Materials- © Rohan Desai- Automobile Department - New Polytechnic, Kolhapur. Page 16 Steels designated on the basis of chemical composition (a) Unalloyed steels 1. Figure indicating 100 times the average percentage of carbon 2. Letter C 3. Figures indicating 10 times the average percentage of manganese content. The result shall be rounded off to the nearest integer. 4. Symbol indicating special characteristics. 45 C 10 G: Steel with average 0.45 per cent carbon, 1 per cent manganese and guaranteed hardenability. (b) Unalloyed tool steels The designation shall contain of: 1. Figure indicating 100 times the average percentage of carbon. 2. Symbol T' for tool steel. 3. Figure indicating 10 times the average per cent manganese content. 75 T 5: Unalloyed tool steel with average 0.75 per cent carbon and 0.5 percent manganese. (c)Low and medium alloy steels (total alloying elements not exceeding 10%) The designation of steels consists of 1. Figure indicating 100 times the average percentage of carbon. 2. Chemical symbols for alloying elements each followed by the figure for its average percentage content multiplied by a factor as given below. Element Multiplying factor Cr, Ni, Mn, Si and W 4 Mo: 10 P,S,N: 100 3. Symbol indicating special characteristics. 25Cr4Mo2G: Steel with guaranteed hardenability and having average 0.25 per cent carbon, 1 per cent chromium and 0.25 per cent molybdenum.
  • -Engineering Materials- © Rohan Desai- Automobile Department - New Polytechnic, Kolhapur. Page 17 (d)High alloy steels (total alloying elements more than 10 per cent). 1. Letter 'X' 2. Figure indicating 100 times the percentage carbon content. 3. Chemical symbol for alloying elements each followed by the figure for its average percentage content. 4. Chemical symbol to indicate specially added element to attain the desired properties. 5. Symbol indicating specific characteristics. X 10 Cr 18 Ni 9 S 3: Steel in pickled condition with average carbon 0.10 per cent, chromium 18 per cent and nickel 9 percent and Sulphar 3% . (e)Alloy tool steels The steel designation shall be as for low, medium and high alloy steels as given above except that the symbol T' will be included in the beginning of the designation of low alloy and medium alloy tool steels and 'XT' instead of X' in the case of high alloy tool steels. XT 75 W 18 Cr 4 V1: high alloy steel with average carbon 0.75 %, Tungsten 18%, chromium 4 % and vanadium 1%. Q. Why nonferrous metals are used? Nonferrous metals are used for the following reasons: 1. Resistance to corrosion. 2. Special electrical and magnetic properties. 3. Softness and facility of cold working. 4. Low density. 5. Attractive colour. Q. write short note on Aluminium. Aluminium Aluminium is a white metal produced by electrical processes from its oxide (Alumina) which is prepared from a mineral called Bauxite. In India, it is chiefly available in Bihar, Madhya Pradesh, Karnataka, Maharashtra and Tamilnadu.
  • -Engineering Materials- © Rohan Desai- Automobile Department - New Polytechnic, Kolhapur. Page 18 Properties: 1. It is light in weight (Specific gravity 2.7) 2. It has very good thermal and electrical conductivity. On weight to weight basis, it carries more electricity than copper. 3. It has excellent corrosion and oxidation resistance. Applications: Cooking utensils, electrical conductors, food containers, ashtrays, bicycles, motorcycles, trucks and buses, aeroplanes and marine vessels. Q. Explain any one of the alloy of aluminium. Duralumin Composition: 3.5-4.5%Cu, 0.4-0.7%Mn, 0.4-0.7%Mg and aluminium the remainder. Properties: High tensile strength, high electric conductivity, very hard and can be easily forged. Application: It is widely used in wrought condition for forging, stampings, bars, sheets, tubes and rivets. Y-alloy Composition: 4%Cu, 2%Ni and 1.5%Mg. Properties: This alloy has the characteristic of retaining good strength at high temperatures. Application: Piston and other components of aero engines. It is also largely used in the form of sheets and strips Q. Write the properties and applications of copper. Properties: 1. It has good ductility and malleability. 2. It has high electrical and thermal conductivity. 3. It is non magnetic and has a pleasing reddish colour. 4. It has fairly good corrosion resistance to general atmospheric conditions. Applications: Electrical conductors, bus bars, automobile radiators, roofing, pressure vessels, kettles and utensils.
  • -Engineering Materials- © Rohan Desai- Automobile Department - New Polytechnic, Kolhapur. Page 19 Q. How copper alloys are classified? Explain them briefly. 1. BRASSES: Brasses are the alloys of copper and zinc. Brasses are classified either on the basis of structure i.e. α-brasses and α-β brasses or colour i.e. red brasses and yellow brasses. α- brasses contain zinc less than 30% and α - β brasses contain zinc between 30 to 44%. Below 20% zinc, the colour of brasses is red and above 20% zinc, the colour is yellow. (1) α-Brasses They are soft, ductile, and malleable and have fairly good corrosion resistance in annealed condition. All the a-brasses are suitable for cold rolling, wire drawing, press work, and such other operations. Some of the important brasses from this group are as below: (i) Cap copper: Composition: 2 to 5% Zinc and remaining Copper. Properties: Zinc is used as a deoxidizer for the deoxidation of copper. If zinc is not added, copper oxide present in the structure reduces ductility and malleability. Applications: caps of detonators in ammunition factories. (ii) Gilding metals: Composition: 5 to 15% Zinc and Copper remainder. Properties: They have different shades of colour from reddish to yellowish according to the zinc content. Applications: Bullet envelopes, drawn containers, condenser tubes, coins, needles and dress jewellery. (iii) Cartridge brass: (70-30 Brass) Composition: 30% zinc and Copper remainder. Properties: It has maximum ductility and malleability amongst all the brasses. Application: Cartridge cases, radiator fins, lamp fixtures, rivets and springs.
  • -Engineering Materials- © Rohan Desai- Automobile Department - New Polytechnic, Kolhapur. Page 20 (2) α - β Brasses Commercial α - β brasses contain zinc between 32 to 40%. Some of the important brasses from this group are given below: (i) Muntz metal: Composition: 40% zinc with balance copper. Properties: Hot worked 60-40 brass (i.e. Muntz metal) shows good a tensile strength and hardness. Application: Utensils, shafts, nuts and bolts, pump parts, condenser tubes and similar applications where corrosion is not too severe. (ii) Naval brass: Composition: 1% tin to Muntz metal. Properties: Corrosion resistance. The brass is called as naval brass or Tobin bronze. Applications: Marine hardware, propeller shafts, piston rods, nuts and bolts, and welding rods. (3) Brazing brass Brass with 50-50 composition is used for brazing purpose. The 50% zinc brass melts at lower temperature (~ 870°C) and can be used for joining commercial brasses. Since the alloy is brittle, it has no other engineering application than for brazing purpose. 2. BRONZES: Bronzes are the alloys of copper containing elements other than zinc. In these alloys zinc may be present in small amount. Commercially important bronzes are discussed below: (i) Aluminium Bronze: Composition: 4 to 11% aluminium and remaining copper. Properties: (a) Good strength, ductility and toughness (b) Good bearing properties, corrosion and fatigue resistance Applications: Jewellery, heat exchangers, heavy duty parts, marine equipments, gear bearings and bushes.
  • -Engineering Materials- © Rohan Desai- Automobile Department - New Polytechnic, Kolhapur. Page 21 (ii) Tin Bronze: Composition: 88% Cu, 10% Sn and 2% Zn. Properties: They have good ductility and malleability. They also have good corrosion resistance. Applications: Coins, pumps, gears, heavy load bearings and marine fittings. (iii) Gun metal: Composition: It consists of 2 to 5% of zinc, 5 to 10% of tin and remainder is copper. Properties: (a) Corrosion resistant (b) High tensile strength (c) Zinc acts as deoxidizer and also improves fluidity of melt. Applications: Used for gun barrels, ordnance parts, Marine castings, gears, bearings and steam pipe fittings. (iv)Phosphor Bronze: Phosphor bronzes can be divided into two main groups (a) Cast phosphor bronze (b) Wrought phosphor bronze (a) Cast phosphor bronze: Composition: 5 to 13% phosphorus and remainder as copper. Properties: It possesses good tensile strength with 5% elongation. Applications: Bearings, gear wheels, slide valves and gudgeon pins. (b) Wrought phosphor bronze: Composition: 2.5 to 8.5% tin, 0.1 to 0.35% phosphorus and remainder as copper. Properties: It possesses high strength, good corrosion resistance. Applications: It is mainly used as a spring. Q. Write short note on bearing materials or sliding bearings. These are used in construction of machines, engines or parts of equipment which requires rotary or reciprocating motions. These materials are used to transmit loads to a shaft rotating relative to the bearing. Requirement of bearings:
  • -Engineering Materials- © Rohan Desai- Automobile Department - New Polytechnic, Kolhapur. Page 22 The friction between bearing and the rotating part should be small. The affinity between the shaft and the bearing material should be minimum. It should be hard and wear resistant. However, it should not be harder than shaft so as to avoid the damage to the shaft. It should have high fatigue strength. Q. Lists the bearing materials and explain White metal alloys (Babbitts): White metal alloys Copper-Lead alloys Tin bronzes Aluminium alloys Grey cast irons White metal alloys Composition: 88% tin, 8% antimony and 4% copper. Properties: It is a soft material with a low coefficient of friction and has a little strength. Applications: Babbitt metal makes a fine bearing and does not affect the shaft very easily when the lubricant fails. Q. Write short note Polymeric materials. Polymeric materials include the familiar plastic and rubber materials. Many of them are organic compounds that are chemically based on carbon, hydrogen, and other nonmetallic elements; furthermore, they have very large molecular structures. Plastics are superior to metals in the following respects. 1. They have good insulating properties. 2. Many plastics are transparent. 3. They possess good colouring properties. 4. They possess good surface finish. 5. Easy formation in different shapes is possible. 6. They possess good corrosion resistance. Classification of Polymers:
  • -Engineering Materials- © Rohan Desai- Automobile Department - New Polytechnic, Kolhapur. Page 23 Polymers are broadly classified in to two major groups as below: (i) Thermoplastic polymers (ii) Thermosetting polymers (i) Thermoplastic Polymers: The materials which can be remelted to manufacture fresh new products are called as thermoplastics. Properties: 1. They are highly plastic 2. They are easily moulded or shaped. 3. They have low melting point 4. As they can be repeatedly used so they have good resale value. Applications: Polystyrene, PVC, copper wire insulation, water tubes, Polyethylene, nursing bottles and ice cube trays. (ii) Thermosetting Polymers: Polymers which can be melted once and can not be remelted again are known as thermosetting plastics. Examples: Epoxies, Phenolics and formaldehydes. Epoxy resin: It is very tough, chemical resistant and transparent with creamy colour. Use: used in foundry and in transformer as an insulating material. Phenolics: It is cheap, strong, rigid and nonconductor of electricity. Use: Plugs, radio cabinets, knife handles and vacuum cleaner parts. Urea formaldehyde: It is hard and brittle. Use: Cosmetic container, pen bodies and mixer bodies Q. Write short on Rubbers 1. Natural rubber 2. Synthetic rubber, Natural rubber: It is generally found in countries which are lying up to 12 degrees on either side of the equator, e.g. South Africa, Malaysia, Singapore, Mexico, Peru and Sir Lanka. It is found in the juice of many plants, like shrub quayule, Russian dandelion, milkweed and many other shrubs, vines and
  • -Engineering Materials- © Rohan Desai- Automobile Department - New Polytechnic, Kolhapur. Page 24 trees. The chief source of rubber is Heveabrassiliencis tree that produces the best rubber latex. The latex is coagulated by acids or by a smoking operation, and the resulting spongy mixture is passed through rollers to form a sheet. This rubber is known as smoked rubber or crude rubber. The crude rubber is further treated by filters, plasticizers or softeners to produce commercial rubber. Synthetic rubber: Synthetic rubber is obtained by suitable combinations of selected monomers. These rubbers are based on models of natural rubber. Actually these are synthetic elastomers. Different types of synthetic rubbers are: 1. Styrene-butadiene rubber (SBR) 2. Butyl rubber 3. Nitrile rubber. Q. Write short note on Ceramics. Ceramics are inorganic, nonmetallic materials. Most of the ceramic materials are silicates, aluminates, oxides, carbides, borides and nitrides. Ceramics are generally classified as Clay products Refractories Glasses Applications: Tiles, sanitary ware, insulators, semiconductors, fuel elements in nuclear power plant, cutting tools, concrete and variety of glasses. Q. Write short note on Composites. The materials produced by combining two or more materials are known as composites. The various types of composites used in industry are 1. Whiskers: Whiskers are produced by introducing Al2O3, SiC and Si3N4 materials into resin and metallic materials. Another form of whisker contains silica fibre coated with aluminium. Whiskers were developed by Rolls Royce and are widely used in furniture and utensils. 2. Glass fibres or resins: Glass fibers possess good strength while the polymers have good toughness. The fibres are woven together and pressed into mats to form the composite.
  • -Engineering Materials- © Rohan Desai- Automobile Department - New Polytechnic, Kolhapur. Page 25 3. Carbon fiber reinforced plastics: These composites possess properties similar to glass fibre reinforced resins. They possess lesser density, good strength and fatigue resistance.