2. Cold working of metal
means the mechanical
working at temperature
below the re-crystallization
Normally, it is taken to be
working of metals at room
3. When a metal is heated and deformed under mechanical
force , an energy level will be reached when the old grain
structure (which is coarse due to previous cold working)
Simultaneously , an entirely new grain structure (equi- axed,
stress free) with reduced grain size starts forming . This
phenomenon is known as “ recrystallization ” and the
temperature at which the phenomenon starts is called “
Recrystallization temperature” .
According to ASM(AMERICAN SOCIETY OF METALS) “
the approx. temperature at which the complete
recrystallization of cold worked metal occurs within a
specified period of approx. 1 hour” is recrystallization
4. Type of metal
Extent of prior cold work
IT IS A FUNCTION OF TIME
AMOUNT OF DEFORMATION IS INVERSELY PROPROTIONAL TO GRAIN SIZE.
Recrystallization temperature is directly proportional to rate of deformation.
Presence of second phase particles decreases recrystallization
5. It is clear from the above graphs that due to work hardening , the strength of the metal increases but
it’s ability to deform further(formability or ductility ) decreases with cold working . So if the
material is excessively deformed , it may fracture before it is formed . To avoid this , large
deformations in cold working are obtained in several stages , with intermediate annealing. This will
soften the cold worked material & restore it’s ductility & hence the formability.
6. Recovery - A low-temperature annealing heat
treatment designed to eliminate residual stresses
introduced during deformation without reducing the
strength of the cold-worked material.
Recrystallization - A medium-temperature annealing
heat treatment designed to eliminate all of the effects
of the strain hardening produced during cold
Grain growth - Movement of grain boundaries by
diffusion in order to reduce the amount of grain
7. 1. Annealing effect :
Photomicrographs showing the
effect of annealing temperature
on grain size in brass. Twin
boundaries can also be observed
in the structures. (a) Annealed at
C, (b) annealed at 650o
(c) annealed at 800o
The effect of annealing
temperature on the
microstructure of cold-
(b) after recovery,
(c) after recrystallization,
and (d) after grain growth
8. Since cold working is done at room temperature or low temperatures , no
oxidation & scaling of the work material occurs . This results in reduced material
Surface defects are removed.
Excellent surface finish which reduces or completely eliminates subsequent
machining resulting in enormous saving in material.
High dimensional accuracy.
Thin gauge sheets can be produced by cold working.
Highly suitable for mass production and automation , because of low working
The physical properties of metals that do not respond to heat treatment can be
improved by cold working.
9. At low temperatures , the strength of metal is very high. So , large forces
are needed for deformation . For this , high capacity equipment is
required which is costly.
The ductility / formability of metals is low at low temperatures . Hence
for large deformation cold working requires several stages of annealing ,
which increases the cost of production.
Due to very high forces , the tool pressures & power requirements are high
too. So , the tooling must be specially designed , which increases the tool
Due to above factors , normally , only the ductile metals are cold worked.
10. THERE ARE TWO FACTORS BY WHICH THE CHOICE OF MATERIAL IS
1.) The ability of the tool material to withstand the required pressures for cold
working of a material. Obviously , the tool material must have a mechanical
strength greater than that of the material to be cold worked . From economic
point of view it must have reasonable working life and it must be able to
withstand working stress for a particular time.
2.) The economic requirement that the max. possible deformation of the material
should be obtained in a single working operation . This will depend upon cold
ductility & cold flowability of the material.
11. In general the requirements of material to be cold worked are:
1.) Yield stress curve of gentle slope.
2.) Early yield point .
3.) Then great elongation with pronounced necking before fracture.
The materials commonly used are :low & medium carbon steel (.25 to .45 %
C),low alloy steels , copper & light alloys such as Al , Mg , Ti & Be.
Cold forming is most suitable for axisymmetric (symmetry about an axis or
cylindrical symmetry) components like shaft components, flanged(an
external/internal rib or rim used to add strength) components , finished gears
, & bearing races etc.
12. COLD ROLLING:
Hot rolled products have a relatively rough surface finish & lack dimensional accuracy
& certain physical properties.
However , hot rolled plate , over 6 mm thick & 1800 to 5000 mm wide , is an important
starting material for ship building , boiler making .
Thinner gauges , better surface finish & tighter tolerances are obtained by cold rolling .
Bars of all shapes , rods , sheets & strips produced are commonly finished by cold
Cold rolled sheets & strips make up an important part of total steel production . Cold
rolling gives springiness to sheets . These sheets are major raw material for some high-
production consumer goods industries , such as for household appliances .
Machinability of steel is improved by cold rolling & for this reason , cold rolled stock
is widely used for fast automatic machining operations .
Sheet steel less than 1.25 mm thick is cold rolled because it cools too rapidly for
practical hot rolling . Prior to cold rolling , steel is pickled to clean the surface and
Copper is rolled to various gauges for roofing , containers , cooking vessels & down to
0.075 mm for radiator fin stock.