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.
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
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
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
The products are flat products, like: plates &
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 &
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
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
Hot rolling is primarily concerned with
manipulating material shape and geometry rather
than mechanical properties.
Primary working is always hot rolling.
Recrystallization & Annealing temperatures.
Ingots Billets/blooms/slabs bars/rods
wires, nails, pipes, sheets, plates
Bloom: square cross-section
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
2. Porosity of the metal is considerably minimised.
3. Concentrated impurities, if any in the metal are
disintegrated and distributed throughout the
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
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
Quarter Hard, Half Hard, Full Hard stock have
higher amounts of reduction. This increases the
i) yield point;
ii) grain orientation and
iii) material properties assume
iv) ductility decreases.
Quarter Hard material can be bent (perpendicular
to the direction of rolling) on itself without
Half hard material can be bent 90º; full hard can
be bent 45º.
Cold rolling advantages and
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
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