Mechanical Technology Grade 12 Chapter 5 Materials


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This slide show accompanies the learner guide "Mechanical Technology Grade 10" by Charles Goodwin, Andre Lategan & Daniel Meyer, published by Future Managers Pty Ltd. For more information visit our website

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Mechanical Technology Grade 12 Chapter 5 Materials

  1. 3. History of IRON <ul><li>Iron has been used since prehistoric times. No one knows how humans discovered the use of iron or how they first learnt to extract iron from its ores. </li></ul><ul><li>It appears that humans were using iron as early as 4000 BC. </li></ul><ul><li>There is evidence that the Egyptians used iron tools as early as 3000 BC when they built the pyramids. </li></ul>
  2. 4. USES OF IRON <ul><li>It provided spears, arrowheads, axes, swords, daggers, maces, bayonets, guns and canons for attack, and helmets and shields for defence </li></ul><ul><li>It can be bent, stretched, twisted, folded, cast, riveted, welded, drilled and cut with precision </li></ul><ul><li>Giant projects like building bridges, ocean-going tankers and oilrigs at sea all use the tremendous strength of metals </li></ul>
  3. 5. Manufacture of steel <ul><li>Iron ore: </li></ul><ul><ul><li>Iron is the most important element in steel and is mined in open-pit mines as a solid or powder </li></ul></ul><ul><ul><li>Most steel comprises at least 98% iron </li></ul></ul><ul><ul><li>The other 2% is either carbon, silicon, sulphur, manganese, nickel, tungsten or other elements </li></ul></ul><ul><li>Pig iron: </li></ul><ul><ul><li>the iron is separated from oxygen and other materials in the iron ore. </li></ul></ul><ul><ul><li>The molten iron is cast into solid slabs or blocks, called pigs and is stored for future use. </li></ul></ul>
  4. 6. Manufacture of steel <ul><li>Furnace: </li></ul><ul><ul><li>Charged with coke and limestone that is ignited and used to extract iron from iron ore using a smelting process </li></ul></ul><ul><li>Smelting: </li></ul><ul><ul><li>separates the iron from the oxygen known as reduction </li></ul></ul>
  6. 8. IRON ORE
  7. 9. Extracting Iron
  8. 10. Iron ore <ul><li>Smelting is the most important method for extracting iron from the ore. </li></ul><ul><li>The ore is dumped into a blast furnace and heated with coke and limestone. </li></ul><ul><li>Oxygen escapes from the iron and combines with the carbon from the coke. </li></ul><ul><li>Other impurities from the iron ore and coke become trapped in the molten limestone. </li></ul>
  9. 11. Furnace <ul><li>Blast Furnace </li></ul><ul><ul><li>a tall, round structure about 30 m high and 9 m in diameter </li></ul></ul><ul><li>Cupola furnace </li></ul><ul><ul><li>a cylindrical blast furnace used in foundries for remelting iron or other metals </li></ul></ul><ul><li>Open-hearth Furnaces </li></ul><ul><ul><li>large, rectangular basins </li></ul></ul><ul><li>Basic Oxygen Furnace </li></ul><ul><ul><li>large bottle-shaped container that holds about 80 tonnes of metal </li></ul></ul><ul><li>Electric Furnace </li></ul><ul><ul><li>uses an electric current to reach the high temperature required for melting </li></ul></ul>
  10. 12. Blast Furnace <ul><li>The blast furnace is charged before smelting begins. </li></ul><ul><li>In charging, the blast furnace is filled with coke, limestone, and iron ore and then ignited. </li></ul><ul><li>Air is heated to 675 °C by smaller furnaces called stoves and is forced in through the bottom of the blast furnace. </li></ul><ul><li>The blast of hot air intensifies the burning of the charge material. </li></ul><ul><li>The temperature at the bottom of the furnace rises to well above the melting point of iron, which is 1 535 °C. </li></ul>
  11. 13. Blast Furnace <ul><li>This high temperature causes chemical reactions to occur, during which pure iron is released from the iron ore. </li></ul><ul><li>The molten iron drops to the bottom of the blast furnace. </li></ul><ul><li>The molten limestone traps the impurities from the iron ore and coke. </li></ul><ul><li>The mixture, called slag, floats on the top of the molten iron. </li></ul><ul><li>The slag is then drawn off through a hole in the furnace called a slag tap hole. </li></ul><ul><li>The molten iron is drawn off near the bottom of the furnace and is either used immediately for making steel or stored as pig iron. </li></ul><ul><li>Foundries make iron castings from re-melted pig iron. </li></ul>
  12. 14. Cupola Furnace <ul><li>A cupola furnace is similar to but smaller than a blast furnace. </li></ul><ul><li>Charges of pig iron, scrap iron, low-sulphur coke or anthracite, and limestone go into the cupola furnace. </li></ul><ul><li>Scrap steel is added to make certain kinds of cast iron. </li></ul><ul><li>The iron is melted and poured into moulds usually made of sand, in which it is allowed to solidify. </li></ul><ul><li>This is a simple, convenient and relatively cheap process to manufacture components of complicated shapes. </li></ul>
  13. 15. Cupola Furnace
  14. 16. Open-hearth Furnaces <ul><li>It is charged with limestone and steel scrap. </li></ul><ul><li>Iron ore may also be added. </li></ul><ul><li>Gas, oil or coal is burned as fuel, and hot air is directed over the charge in the furnace. </li></ul><ul><li>The temperature above the charge reaches about 1 650 °C and the charge melts. </li></ul>
  15. 17. Open-hearth Furnaces <ul><li>When the charge is nearly melted, molten pig iron from the blast furnace is added to the furnace. </li></ul><ul><li>Heating continues, and the impurities combine with the oxygen. </li></ul><ul><li>Some of the oxidised impurities bubble up through the molten metal as a gas. </li></ul><ul><li>Others float to the top and combine with the molten limestone to form slag </li></ul>
  16. 18. Open-hearth Furnaces <ul><li>After the impurities burn away, alloying elements are added to bring the steel to the required composition. </li></ul><ul><li>The steel is then drawn from the furnace into a ladle and </li></ul><ul><li>poured into tall </li></ul><ul><li>moulds to form </li></ul><ul><li>ingots </li></ul>
  17. 19. Electric furnace <ul><li>High voltage causes electricity to arc between the carbon electrodes within the furnace. </li></ul><ul><li>The electric furnace gives the operator precise control of both the furnace atmosphere and the amount of alloying elements added during the process. </li></ul><ul><li>It is the only furnace that can remove all the sulfur from steel. </li></ul><ul><li>Tool steels, high-speed steels and other speciality steels are produced in electric furnaces. </li></ul>
  18. 20. Electric furnace
  19. 21. Electric furnace <ul><li>The finished steel from the furnace is poured into tall, rectangular moulds. </li></ul><ul><li>The steel solidifies in the moulds to form ingots weighing 10 tonnes. </li></ul><ul><li>When the mould is lifted off, the red-hot ingot is lowered into a heated pit called a soaking pit. </li></ul><ul><li>Ingots in the soaking pit stay hot while they wait to enter the rolling mill. </li></ul>
  20. 22. Basic Oxygen Furnace <ul><li>It is charged with molten pig iron. </li></ul><ul><li>A water-cooled pipe called a lance is inserted into the furnace. </li></ul><ul><li>Pure oxygen is forced through the lance into the metal. </li></ul><ul><li>The oxygen combines with the contaminants and removes them from the iron. </li></ul><ul><li>Using this process takes about only one hour to convert 80 tonnes of iron into steel. </li></ul>
  21. 23. Basic Oxygen Furnace
  22. 24. Assessment <ul><li>Discuss the following topics: </li></ul><ul><ul><li>How can you decrease noise pollution in urban areas? (Hint: Think, for instance, of revving car engines and loud radios.) </li></ul></ul><ul><ul><ul><li>How can you improve sanitation, and thus health, in rural areas? (Hint: Think of VIP latrines [toilets], the function of bacteria in these toilets, and preventing ground water from being contaminated.) </li></ul></ul></ul><ul><ul><li>What are the conditions of mine workers and why are certain diseases prevalent amongst them. </li></ul></ul>
  23. 25. Properties of Metals <ul><li>Strength refers to the material’s ability to withstand forces that are applied to it, without breaking, bending, shattering or deforming in any way. </li></ul><ul><li>Elasticity refers to the material’s ability to absorb forces and flex in different directions and return to its original shape when the load is removed. </li></ul><ul><li>Plasticity refers to the material’s ability to change in shape permanently – it is the reverse of elasticity. </li></ul>
  24. 26. Properties of Metals <ul><li>Ductility refers to the material’s ability to change shape by stretching it along its length, or to be drawn into wire form. </li></ul><ul><li>Malleability refers to the material’s ability to be reshaped in all directions without cracking. Lead is a malleable material but lacks ductility because of low tensile strength. </li></ul><ul><li>Brittleness refers to the material’s behaviour when fractures occur with little or no deformation. Glass is a classic example of a material with this property. </li></ul>
  25. 27. Properties of Metals <ul><li>Toughness refers to the material’s ability to withstand shock loads and remain intact after continual bending in opposite directions. </li></ul><ul><li>Softness is the opposite property to hardness. Soft materials may be easily shaped by filing, drilling or machining in a lathe, milling machine or shaping machine. </li></ul><ul><li>Stiffness is the ability to withstand bending. </li></ul><ul><li>Flexibility refers to metals which remain bent after a bending force has been removed. </li></ul>
  26. 28. Properties of Metals <ul><li>Hardness refers to the material’s ability to resist penetration, scratching, abrasion, indentation and wear. </li></ul><ul><ul><li>Unfortunately the harder carbon steel tools are made, the more brittle they become, so some hardness must be sacrificed for toughness in the tempering process. </li></ul></ul>
  27. 29. Production of Metal <ul><li>Cast iron </li></ul><ul><ul><li>Cast iron is produced in a cupola furnace in much the same way as the iron ore is smelted. </li></ul></ul><ul><li>Steel </li></ul><ul><ul><li>Different processes produce different kinds of steel, each process requiring a special furnace. </li></ul></ul><ul><ul><li>Steel-making furnaces include open-hearth furnaces, basic oxygen furnaces and electric furnaces. </li></ul></ul>
  28. 30. Cast Iron <ul><li>Cast iron is an alloy of iron and carbon with small amounts of manganese, silicon, sulfur and phosphorus. </li></ul><ul><li>Cast iron is brittle and relatively weak in tension with low tensile strength and poor shock resistance, but strong in compression and is easily machined. </li></ul><ul><li>Cast iron is not ductile and cannot be bent without fracturing. </li></ul><ul><li>The main advantage of cast iron is that it is easily cast into various shapes. </li></ul>
  29. 31. Cast Iron <ul><li>It absorbs vibrations well, making it suitable for supporting machine tools. </li></ul><ul><li>Cast iron is used in the manufacturing of machine beds, marking-off tables, machine tables and internal combustion engines as well as in the production of pistons, piston rings and cylinders. </li></ul><ul><li>Covers on road drains are usually made from cast iron. </li></ul><ul><li>Carbon is present in cast iron, in the form of graphite. </li></ul>
  30. 32. Cast Iron <ul><li>The carbon flakes act as a lubricant, enabling the cast iron to be machined dry. </li></ul><ul><li>Drilling or tapping of cast iron components is fairly easy and no lubricant is required. </li></ul><ul><li>There is, however, a hard skin in which some of the moulding sand may still be present. </li></ul><ul><li>There are five types of cast iron, depending on the structure of the carbon in the iron. </li></ul><ul><li>These are grey cast iron, white cast iron, ductile cast iron, malleable cast iron and high-alloy cast iron </li></ul>
  31. 33. Ferrous Metals <ul><li>Ferrous metals contain iron as their parent metal. </li></ul><ul><ul><li>For instance, in iron, carbon is the most important supplement ‒ only present in small amounts of approximately 0,05% to 1,7%, and seldom exceeding 1,5%. </li></ul></ul><ul><ul><li>The presence of carbon in steel causes big changes in the nature of the metal, and also determines the hardness of the metal. </li></ul></ul>
  32. 34. Ferrous Metals <ul><li>If the carbon content is increased: </li></ul><ul><ul><li>Greater hardness is obtained. </li></ul></ul><ul><ul><li>Tensile strength is increased. </li></ul></ul><ul><ul><li>Ductility is decreased. </li></ul></ul><ul><ul><li>Welding ability is decreased. </li></ul></ul><ul><li>There are three classes of plain carbon steels. </li></ul><ul><ul><li>low-carbon steel, </li></ul></ul><ul><ul><li>medium-carbon steel and </li></ul></ul><ul><ul><li>high-carbon steel. </li></ul></ul>
  33. 35. Plain carbon steels <ul><li>Low-carbon steel (mild steel) </li></ul><ul><ul><li>commonly known as soft, mild or machinery steel </li></ul></ul><ul><ul><li>Used where ductility and softness are important and a high tensile strength is required. </li></ul></ul><ul><ul><li>carbon content of between 0,15% and 0,3%. </li></ul></ul><ul><ul><li>used for operations such as cold bending and riveting </li></ul></ul><ul><ul><li>easy to press into a new shape, machine, weld or forge </li></ul></ul>
  34. 36. Low-carbon steel (mild steel) <ul><ul><li>May be worked hot or cold cannot be hardened by heating and quenching, but can be case-hardened </li></ul></ul><ul><li>Products like rivets, nuts, bolts, nails, washers, chains, machine parts, wire fence, forged parts and shafting can be made from this type of steel. </li></ul><ul><li>available in sheets of varying thickness, squares, bar form with hexagon, round, wire, plates or flat sections in a ‘black’ or ‘bright’ form. </li></ul>
  35. 37. Medium-carbon steel <ul><li>Has a carbon content between 0,3% and 0,75% </li></ul><ul><li>Is less ductile, harder and has greater tensile strength than low-carbon steel. </li></ul><ul><li>Its hardness and strength can be increased by quenching the metal while it is red hot in water or oil. </li></ul><ul><li>Also has better machining qualities and is suitable for many general engineering purposes where the stresses applied are greater than could be withstood by mild steel. </li></ul><ul><li>These steels are used for shafts, rails, connecting rods, car axles, spindles, gears, heavy forgings and other machine parts requiring medium strength and wear resisting surfaces. </li></ul>
  36. 38. High-carbon steel <ul><li>High-carbon steel is frequently known as tool steel, with a carbon content ranging between 0,75% and 1,7%. </li></ul><ul><li>High-carbon steel has a higher tensile strength and hardness than steels in the lower carbon range. </li></ul><ul><li>It responds readily to heat treatment and is used for most cutting tools especially with alloys after being hardened and tempered. </li></ul><ul><li>High-carbon steels are also used for chisels, files drills, reamers, taps, hammers and crowbars. </li></ul>
  37. 39. Alloying Elements <ul><li>These are mixed with metals: </li></ul><ul><ul><li>to improve its mechanical properties so as to permit higher tempering temperature while maintaining high strength and improving ductility </li></ul></ul><ul><ul><li>to improve mechanical properties at low or elevated temperature </li></ul></ul><ul><ul><li>to increase strength and toughness </li></ul></ul><ul><ul><li>to increase resistance to high temperatures </li></ul></ul><ul><ul><li>to secure greater hardness for wear resistance </li></ul></ul><ul><ul><li>to provide high impact resistance </li></ul></ul><ul><ul><li>to secure better machinability. </li></ul></ul>
  38. 40. Alloying Elements <ul><li>Alloying can also lower the melting point of the metal, increase the resistance to corrosion and rust and change the colour and structure of the metal. </li></ul><ul><li>Alloying may also reduce the cost of a metal. </li></ul>
  39. 41. Ferrous Alloying Elements <ul><li>The most commonly used elements are: </li></ul><ul><ul><li>Chromium (Cr) </li></ul></ul><ul><ul><li>Vanadium (V) </li></ul></ul><ul><ul><li>Manganese (Mn) </li></ul></ul><ul><ul><li>Nickel (Ni) </li></ul></ul><ul><ul><li>Tungsten (W) </li></ul></ul><ul><ul><li>Molybdenum (Mo) </li></ul></ul>
  40. 42. Chromium (Cr) <ul><li>Is essentially a hardening agent. </li></ul><ul><li>Chrome steel is also known as stainless steel . </li></ul><ul><li>Frequently used with nickel as a toughening element to produce superior mechanical properties. </li></ul><ul><li>Steels containing chromium are noted for wear and abrasion resistance. </li></ul><ul><li>Chrome steels are used in machine parts, races for bearings, ball bearings, gears, journals , shafts, dies , coil springs, gauges, nuts and bolts, and flat springs. </li></ul><ul><li>Does not hold size as accurately as manganese steels </li></ul>
  41. 43. Vanadium (V) <ul><li>Improves the elasticity, strength and fatigue-resistance of the steel </li></ul><ul><li>Has a more powerful effect upon the properties of steel than any other element and gives: </li></ul><ul><ul><li>increased hardness </li></ul></ul><ul><ul><li>secondary hardening upon tempering </li></ul></ul><ul><ul><li>increased hardness at elevated temperatures. </li></ul></ul>
  42. 44. Manganese (Mn) <ul><li>Normally present in all commercial steels </li></ul><ul><li>It is essential to steel production </li></ul><ul><ul><li>It is necessary not only in the melting process but also in rolling and other processing methods </li></ul></ul><ul><ul><li>In hot forging, the action of manganese on sulphur improves the hot-working characteristics </li></ul></ul><ul><li>Manganese steels have a greater impact and yield strength than plain carbon steels </li></ul>
  43. 45. Manganese (Mn) <ul><li>It lowers the temperature to which the steel must be heated for hardening </li></ul><ul><li>Easily cast into any shape, although the cast condition is weak and brittle </li></ul><ul><li>Heat treatment can give this steel great wearing power with much ductility. </li></ul><ul><li>This makes it useful in steel crusher jaws, and rammers for crushing ore. </li></ul>
  44. 46. Nickel (Ni) <ul><li>When added to steel nickel increases the ductility, strength, hardness and toughness of the metal. </li></ul><ul><li>Nickel steels are used for machine parts subject to repeated shock and stress. </li></ul><ul><li>They are easily heat-treated because nickel lowers the critical cooling rate. </li></ul><ul><ul><li>This critical cooling rate is necessary to produce hardening by quenching. </li></ul></ul><ul><li>These steels are used for axles, crankshafts, special gears, scientific and measuring instruments, marine shafting and parts for earthmoving equipment </li></ul>
  45. 47. Tungsten (W) <ul><li>One of the principal alloying elements </li></ul><ul><li> found in many alloy tool steels is tungsten. </li></ul><ul><li>When added to steel it increases the strength and toughness at high temperatures and makes a dense fine grain structure in steel. </li></ul><ul><li>When added to high-carbon steel it is used for high-speed cutting tools, dies, shear blades and exhaust valves. </li></ul>
  46. 48. Molybdenum (Mo) <ul><li>Added to steel to improve the heat-treatment properties </li></ul><ul><li>It increases the hardness in steel, resists softening upon heating and prevents steel from becoming brittle when tempered. </li></ul><ul><li>Machine parts such as propeller shafts and transmission shafts, bolts, differential gears, coil springs, stainless steel, roller bearings and leaf springs are made from this type of steel. </li></ul>
  47. 49. Non-ferrous Metals <ul><li>Non-ferrous metals are those metals which do not contain iron </li></ul><ul><li>These are metals such as such as copper, tin, lead, zinc, aluminium and antinomy . </li></ul><ul><li>These metals may be mixed to give us the various alloys, which are of great importance. </li></ul>
  48. 50. Copper (Cu) <ul><li>Copper is red in colour and is tough, ductile and malleable. </li></ul><ul><li>Copper is a pure metal. </li></ul><ul><li>It bends and stretches without fracture. </li></ul><ul><li>It is an excellent conductor of heat and electricity. </li></ul><ul><li>It is usually drawn into wire. </li></ul><ul><li>Pure copper is difficult to cast but has a very high tensile strength when cold drawn. </li></ul><ul><li>It is used for cables, switchboard parts, electrical bolts and nuts, busbars, telephone wires, soldering irons, electrical wiring, tubing for water supply and sometimes for roofing. </li></ul>
  49. 51. Tin (Sn) <ul><li>Tin is a silvery-white shiny metal with a bluish shade. </li></ul><ul><li>It is corrosion resistant, soft and malleable, and is a poor conductor of electricity. </li></ul><ul><li>Tin is used in soft solder, the canning industry and the cladding of steel sheeting. </li></ul><ul><li>It is also used in brasses and bronzes. </li></ul><ul><li>Tin provides a protective coating in copper wires and is the basis of white metal bearings. </li></ul>
  50. 52. Tin (Sn)
  51. 53. Lead (Pb) <ul><li>Lead is a soft, bluish-grey coloured metal. </li></ul><ul><li>It is malleable, ductile, and tough and has very low tensile strength. </li></ul><ul><li>Lead has a very low melting point. </li></ul><ul><li>It is a pure metal, which bends and stretches easily. </li></ul><ul><li>It is often added to other metals to make them free-cutting. </li></ul><ul><li>It is used for soft solder, bullets, lead cables, plumbing, on roofs, and as plates in car batteries </li></ul>
  52. 54. Lead (Pb)
  53. 55. Zinc (Zn) <ul><li>Zinc is a bluish-white colour and is hard, brittle and malleable. </li></ul><ul><li>Zinc is a pure metal which casts well and resists corrosion. </li></ul><ul><li>Zinc is seldom used alone but is alloyed with other metals to make brass and bronze. </li></ul><ul><li>It is used as a coating (galvanising) on steel sheets, water tanks and wire. </li></ul>
  54. 56. Aluminium (Al) <ul><li>Aluminium-bearing ore (bauxite ) is ground into a powder and processed chemically to produce an oxide, alumina. </li></ul><ul><li>Aluminium has a bluish-white colour, is fairly hard, extremely light and resistant to corrosion. </li></ul><ul><li>It is a pure or base metal. </li></ul><ul><li>It is the lightest of the commonly used metals. </li></ul>
  55. 57. Aluminium (Al) A cast aluminium engine block
  56. 58. Aluminium (Al) <ul><li>It is too soft to use in its pure state but is alloyed with copper, magnesium and manganese. </li></ul><ul><li>It is widely used for many components. </li></ul><ul><li>Aluminium is impossible to solder by the usual methods. </li></ul><ul><li>It is non-magnetic and a far better conductor of electricity than copper. </li></ul><ul><li>It is used for cooking utensils, foil (often called silver paper) and electricity conductors. </li></ul>
  57. 59. Antimony <ul><li>Antimony has a bluish-white colour and has a scaly, crystalline structure. </li></ul><ul><li>It is very brittle and is used only in alloys such as pewter, solder and anti-friction metals. </li></ul>
  58. 60. Non-ferrous Alloys <ul><li>Brass </li></ul><ul><li>Bronze </li></ul><ul><li>White metal </li></ul><ul><li>Duralumin </li></ul>
  59. 61. Brass <ul><li>Brass is an alloy of copper and zinc, available in many varying proportions of the two metals. </li></ul><ul><li>Brass turns well, is easily cast and resists corrosion. </li></ul><ul><li>The higher the zinc content, the lower the melting point, which results in lower malleability and ductility. </li></ul><ul><li>The lower the zinc content, the better the corrosion resistance, and the higher the strength and ductility of the metal. </li></ul>
  60. 62. Brass <ul><li>Brass is easily machined. </li></ul><ul><li>Brass is usually tougher than bronze and produces a stringy chip when machined. </li></ul><ul><li>Brass is widely used for its resistance to corrosion and also for the manufacture of condenser parts, tubes, rods and sheets. </li></ul><ul><li>It is also used in bolts and nuts, gears, bushes, electrical components, taps and other water fittings. </li></ul>
  61. 63. Bronze <ul><li>Bronze is found in many combinations of copper and other metals, but copper and tin are its main elements. </li></ul><ul><li>Bronze is sometimes alloyed with zinc, lead or phosphorous, which is used in the production of phosphor bronze. </li></ul><ul><li>Bronze is usually harder than brass, and is easily machined with sharp tools. </li></ul><ul><li>The chip produced is often granular. </li></ul><ul><li>Bronze is used for valves, valve seats, bearings and gear wheels. </li></ul>
  62. 64. White metal <ul><li>White metal is an alloy with either a lead or tin base. </li></ul><ul><li>White metal is also known as Babbitt metal. </li></ul><ul><li>The tin-based white metal is used in heavy duty bearings to withstand greater pressures and speed whilst the lead-based metals are used under less exacting conditions. </li></ul><ul><li>White metal is used for bearings and to reduce friction. </li></ul>
  63. 65. Duralumin <ul><li>Duralumin is an aluminium alloy, which contains magnesium, manganese, copper, and silicon in small percentages. </li></ul><ul><li>It is a light metal with a high tensile strength and good resistance to corrosion, even in seawater. </li></ul><ul><li>Duralumin is used in the manufacturing of bars, sheets, and rivets and in automobile and aircraft parts. </li></ul>
  64. 66. Identification of Metals <ul><li>All metals are normally marked or colour coded on the ends </li></ul><ul><li>If the marking is cut off and the piece of metal is separated from its proper storage rack, it is very difficult to determine the carbon content and alloy group. </li></ul><ul><li>start cutting from the unmarked end and leave the marked end intact. </li></ul>
  65. 67. Identification of Metals <ul><li>By a process of elimination, they can determine which of the several steel types in the workshop is most comparable to the sample. </li></ul><ul><li>These methods of workshop testing include: </li></ul><ul><ul><li>Visual test </li></ul></ul><ul><ul><li>Scratch test </li></ul></ul><ul><ul><li>Spark test </li></ul></ul><ul><ul><li>File test </li></ul></ul><ul><ul><li>Sound test </li></ul></ul><ul><ul><li>Machinability test </li></ul></ul>
  66. 68. Visual test <ul><li>Heat scale or black mill scale is found on all hot rolled steels, that is low-carbon, medium-carbon, high-carbon and alloyed steels. </li></ul><ul><li>Cold-finished steel usually has a metallic lustre. </li></ul><ul><li>Ground and polished steel have a shiny finish. </li></ul><ul><li>Chromium, nickel and stainless steel which is austenitic and non-magnetic, usually have a white appearance. </li></ul><ul><li>When grey cast iron fractures, it appears dark grey and will smear your finger with a grey graphite smudge when touched. </li></ul><ul><li>When white cast iron fractures it appears silvery or white. </li></ul>
  67. 69. Scratch test <ul><li>Be sure that all scale and other surface impurities have been removed before scratch testing. </li></ul><ul><li>Simply scratch one sample with another and the softer sample will be marked. </li></ul><ul><li>A variation of this method is to strike similar edges of two samples together. </li></ul><ul><ul><li>The one receiving the deeper indentation is the softer of the two. </li></ul></ul>
  68. 70. Spark test <ul><li>Tests for carbon content in many steels. </li></ul><ul><li>Always wear safety goggles or a face shield. </li></ul><ul><li>When held against the grinding wheel, the metal tested will display a particular spark pattern depending on the carbon content. </li></ul><ul><li>Adjust the grinding wheel so that the sparks will fly outward and downward, and away from you. </li></ul><ul><li>Use a coarse grit wheel which has been freshly dressed to remove contaminants. </li></ul>
  69. 71. Spark test <ul><li>High-carbon steel: </li></ul><ul><ul><li>short, very white or light yellow carrier lines with considerable forking with many star-like bursts </li></ul></ul>
  70. 72. Spark test <ul><li>Low-carbon steel: </li></ul><ul><ul><li>straight carrier lines with a yellowish colour with a very small amount of branching and very little carbon burst </li></ul></ul>
  71. 73. Spark test <ul><li>Cast iron: </li></ul><ul><ul><li>short carrier lines with many bursts, which are red near the grinder and orange-yellow farther out; considerable pressure is required on cast iron to make sparks </li></ul></ul>
  72. 74. File test <ul><li>Files can establish the relative hardness between two samples, as in the scratch test. </li></ul><ul><li>This method, however, requires skill. </li></ul><ul><li>Take care not to damage the file, since filing on hard materials may ruin the file. </li></ul><ul><li>Testing should be done on the tip or near the edge </li></ul>
  73. 75. Sound test <ul><li>The metal type can also be determined by the sound it makes when it is tapped with a hammer or when it is dropped on the floor. </li></ul><ul><li>If the sound is loud and clear, the metal is a high-carbon steel (hard) but if it is a dull sound, the metal is a low-carbon steel (soft). </li></ul>
  74. 76. Machinability test <ul><li>Machinability can be ‘sample-tested’. </li></ul><ul><li>For example, two unknown samples identical in appearance and size can be cut in a machine tool, using the same speed and feed for both. </li></ul><ul><li>The ease of cutting should be compared and the chips observed for heating colour and curl. </li></ul>
  75. 77. Composites <ul><li>Nowadays there is an ever-growing number of synthetic materials available on the market. </li></ul><ul><li>These materials become plastic above certain temperatures and while plastic, they can be squeezed into dies and moulds to give them the required shape which is retained on cooling. </li></ul><ul><li>These materials hardly ever show signs of plastic properties in their finished state. </li></ul><ul><li>Two main types of plastic materials are thermoplastics and thermosetting plastics (also referred to as thermosets). </li></ul>
  76. 78. Thermoplastics <ul><li>These plastics can be re-heated and therefore shaped in various ways. </li></ul><ul><li>They cannot be used at temperatures much above </li></ul><ul><li>100 °C although they harden again on cooling. </li></ul><ul><li>They tend to be tougher than thermosetting plastic but not as firm. </li></ul><ul><li>This material can be recycled. </li></ul><ul><li>It is tough, of low density, low cost and can be formed in intricate shapes with ease </li></ul>
  77. 79. Thermoplastics <ul><li>Some thermoplastics are transparent, for example celluloid and Perspex, and can be coloured by adding pigment. </li></ul><ul><li>Nylon is one of the best-known and earliest plastics and is used for a variety of purposes including gear wheels and pulleys. </li></ul><ul><li>Polyvinyl chloride (PVC) is a member of this group and is a flexible, rubber-like substance which makes a dull sound when dropped. </li></ul><ul><li>It is commonly used for insulating electrical cables. </li></ul>
  78. 80. Thermosetting plastics <ul><li>Once these plastics set they cannot be re-heated to soften, shape and mould. </li></ul><ul><li>They are firm, hard and relatively fragile. </li></ul><ul><li>They are very durable and of great strength. </li></ul><ul><li>Bakelite falls within this category. </li></ul><ul><li>Thermosetting plastics are mostly used for electrical equipment and components, melamine dinnerware, connectors and surface coating. </li></ul>
  79. 81. Reinforced plastic <ul><li>Laminated plastics like Tufnol® consist of a fibrous material such as woven cloth or paper saturated with phenolic resin. </li></ul><ul><li>The fabric sheets are laid up in a hydraulic press and squeezed and heated so that they become solid sheets, cylinders or shafts. </li></ul>