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TRANSCRIPT
- 1.ROLLING
BULK DEFORMATION PROCESSES
2. INTRODUCTION
3. 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.
4. 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.
5. 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.
6. 7. 8. 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.
Recrystallizationtemperature The minimum temperature at which
complete recrystallization occurs in.
9. 10. 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.
11. ROLLING
IngotsBillets/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).
12. 13. Making barbed wire
A steel ingot is heated until it is about 2192F (1200C), 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.
14. 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.
15. 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
16. Cold Rolling
Quarter Hard, Half Hard, Full Hard stock have higher amounts of
reduction. This increases the
yield point;
grain orientation and
material properties assume
ductility decreases.
Quarter Hard material can be bent (perpendicular to the direction
of rolling) on itself without fracturing.
Half hard material can be bent 90; full hard can be bent 45.
17. Cold rolling advantages and limitations
1.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.