R O L L I N G

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R O L L I N G

  1. 1. ROLLING<br />BULK DEFORMATION PROCESSES<br />
  2. 2. INTRODUCTION<br />
  3. 3. CLASSIFICATION<br />PRIMARY WORKING operations are those that take a solid piece of metal (generally in form of ingot) & break it down successively into shapes such as slabs, billets & plates.<br />Traditionally this includes processes like forging, rolling & extrusion.<br />SECONDARY WORKING operations involve further processing of the products from primary working in to final products like bolts, sheet metal parts & wires.<br />
  4. 4. CLASSIFICATION<br />BULK DEFORMATION is the processing of work pieces whose surface area-to-volume ratio (or surface area-to-thickness ratio) is relatively small. In bulk forming processes there is always a change in thickness or cross-section of work piece.<br />Includes: rolling, forging, extrusion & drawing of rod & wire.<br />SHEET-FORMING operations the surface area-to-thickness ratio is relatively high. In general the material is subjected to shape changes. No thickness changes.<br />
  5. 5. ROLLING<br />This is the process of reducing the thickness or changing the cross-section of a work-piece by compressive forces exerted by a pair of rotating rolls.<br />The products are flat products, like: plates & sheets.<br />Plates are used for structural applications like bridges, ships & nuclear vessels.<br />Sheets (generally 6mm or less in thickness) are used for automotive, beverage cans, office & kitchen equipment.<br />
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  8. 8. ROLLING<br />Hot rolling is a hot working process where large pieces of metal, such as slabs or billets, are heated above their recrystallization temperature and then deformed between rollers to form thinner cross sections. <br />While cold rolling increases the hardness and strength of a metal, it also results in a large decrease in ductility. Thus metals strengthened by cold rolling are more sensitive to the presence of cracks and are prone to brittle fracture.<br />Recrystallizationtemperature The minimum temperature at which complete recrystallization occurs in.<br />
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  10. 10. HOT ROLLING<br />Hot rolling produces thinner cross sections than cold rolling processes with the same number of stages. Hot rolling, due to recrystallization, will reduce the average grain size of a metal while maintaining a certain soft microstructure, where as cold rolling will produce a hardened microstructure.<br />Hot rolling is primarily concerned with manipulating material shape and geometry rather than mechanical properties.<br />Primary working is always hot rolling.<br />Recrystallization & Annealing temperatures.<br />
  11. 11. ROLLING<br />Ingots Billets/blooms/slabs bars/rods wires, nails, pipes, sheets, plates<br />Bloom: square cross-section<br />Slab: rectangular<br />Billets:A billet is a bar of steel with a square cross-section whose dimensions are usually less than about 6 inches (15 cm) by 6 inches (15 cm). <br />
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  13. 13. Making barbed wire <br />A steel ingot is heated until it is about 2192°F (1200°C), then rolled between grooved rolls until it has reached the proper size. Giant shears cut the billet to the desired length; then it is allowed to cool.<br />The steel billet is again heated and rolled until it has been shaped into a round bar 0.2 inch (5.6 mm) in diameter, known as a wire rod. The wire rod is rolled into a coil weighing as much as 3,969 pounds (1,800 kg), which is shipped to the wire manufacturer. <br />
  14. 14. Hot rolling Advantages :<br />1. Larger deformation can be accomplished and more rapidly by hot working since the metal is in plastic state.<br />2. Porosity of the metal is considerably minimised.<br />3. Concentrated impurities, if any in the metal are disintegrated and distributed throughout the metal.<br />4. Grain structure of the metal is refined and physical properties improved.<br />
  15. 15. Hot rolling Disadvantages :<br />1. Due to high temperature a rapid oxidation or scale formation takes place on the metal surface, leading to poor surface finish and loss of metal.<br />2. On account of the lost of carbon from the surface of the steel piece being worked the surface layer loses its strength, which is a disadvantage when the part is put to service.<br />3. This weakening of the surface layer may give rise to crack which may ultimately result in fatigue failure of the part.<br />4. Close tolerances cannot be maintained.<br />5. It involves excessive expenditure on account of high cost of tooling. This, however, is compensated by the high production rate and better quality of products<br />
  16. 16. Cold Rolling<br />Quarter Hard, Half Hard, Full Hard stock have higher amounts of reduction. This increases the <br />yield point; <br />grain orientation and <br />material properties assume<br />ductility decreases. <br />Quarter Hard material can be bent (perpendicular to the direction of rolling) on itself without fracturing. <br />Half hard material can be bent 90º; full hard can be bent 45º.<br />
  17. 17. Cold rolling advantages and limitations<br />1.Better dimensional control than hot working is possible because the reduction in size is not much.<br />2.Surface finish of the component is better because no oxidation takes place during the process.<br />3.Strength and hardness of the metal are increased.<br />4.It is an ideal method for increasing hardness of those metals which do not respond to the heat treatment.<br />5.Only ductile metals can be shaped through cold working.<br />6.Over-working of metal results in brittleness and it has to be annealed to remove the same.<br />7. Subsequent heat treatment is mostly needed to remove the residual stresses set up during cold working.<br />

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