CLASSIFICATION 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. Traditionally this includes processes like forging, rolling & extrusion. SECONDARY WORKING operations involve further processing of the products from primary working in to final products like bolts, sheet metal parts & wires.
CLASSIFICATION 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. Includes: rolling, forging, extrusion & drawing of rod & wire. SHEET-FORMING operations the surface area-to-thickness ratio is relatively high. In general the material is subjected to shape changes. No thickness changes.
ROLLING 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. The products are flat products, like: plates & sheets. Plates are used for structural applications like bridges, ships & nuclear vessels. Sheets (generally 6mm or less in thickness) are used for automotive, beverage cans, office & kitchen equipment.
ROLLING 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. 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. Recrystallization temperature The minimum temperature at which complete recrystallization occurs in.
HOT ROLLING 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. Hot rolling is primarily concerned with manipulating material shape and geometry rather than mechanical properties. Primary working is always hot rolling. Recrystallization & Annealing temperatures.
ROLLING Ingots Billets/blooms/slabs bars/rods wires, nails, pipes, sheets, plates Bloom: square cross-section Slab: rectangular 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).
Making barbed wire 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. 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.
Hot rolling Advantages :1. Larger deformation can be accomplished and more rapidly by hot working since the metal is in plastic state.2. Porosity of the metal is considerably minimised.3. Concentrated impurities, if any in the metal are disintegrated and distributed throughout the metal.4. Grain structure of the metal is refined and physical properties improved.
Hot rolling Disadvantages :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.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.3. This weakening of the surface layer may give rise to crack which may ultimately result in fatigue failure of the part.4. Close tolerances cannot be maintained.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
Cold Rolling Quarter Hard, Half Hard, Full Hard stock have higher amounts of reduction. This increases thei) yield point;ii) grain orientation andiii) material properties assumeiv) ductility decreases. Quarter Hard material can be bent (perpendicular to the direction of rolling) on itself without fracturing. Half hard material can be bent 90º; full hard can be bent 45º.
Cold rolling advantages and limitations1.Better dimensional control than hot working is possible because the reduction in size is not much.2.Surface finish of the component is better because no oxidation takes place during the process.3.Strength and hardness of the metal are increased.4.It is an ideal method for increasing hardness of those metals which do not respond to the heat treatment.5.Only ductile metals can be shaped through cold working.6.Over-working of metal results in brittleness and it has to be annealed to remove the same.7. Subsequent heat treatment is mostly needed to remove the residual stresses set up during cold working.
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