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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 …

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