Hot Forming, Cold Forming and Warm FormingMetal forming
processes may be carried out in hot as well as in cold
state. Temperatureof re-crystallization is the boundary
between the two.Hot forming or hot working is dened as forming at
temperatures above the re-
crystallization temperature of the metal. In fact the actual
temperatures are muchhigher than the recrystallization temperature.
High temperatures reduce the owstress of metals which results in
low deforming forces.Cold forming or cold working refers to forming
at temperatures below the re-crystallizationtemperature of the
metal. The strain hardening during the process improves
themechanicalproperties of the product. !lso close dimensional
tolerances can be obtained in cold
forming.
Fig. "#ect of temperature on the yield strength
Advantages of hot forming
$i% !t high temperatures the metals become soft its yield
strength decreases $&ig.%and hence low forces are re'uired for
forming. This reduces the cost of e'uipmentneeded for the
process.$ii% Metals are more ductile at higher temperatures and
their formability in hot stateis higher than in cold state.
Therefore large deformations may be given in hotwor(ing.$iii% The
casting defects in ingots li(e internal shrin(age cavities $not
those incontact with atmosphere% and blow holes get welded during
hot wor(ing. Thestructure becomes more homogeneous resulting in
better mechanical properties.$iv% )ue to low ow stresses at high
temperatures very large components may bemade by plastic
deformation.Disadvantages of hot working$i% The products have low
surface 'uality due to o*idation of surface layer.
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$ii% The components formed have low dimensional accuracy.$iii%
There is little improvement in mechanical properties.$iv% The
forming tools also get heated up due to contact with hot metal and
wear of tools is rapid.Advantages of cold forming
$i% +ood surface nish of the product.$ii% High dimensional
accuracy.$iii% ,uperior mechanical properties e.g. hardness and
strength increase due tostrain hardening.$iv% ,train hardening may
eliminate the surface hardening heat treatment re'uiredin some
components such as cold rolled gears.$v% The material develops
anisotropy which may be used to advantage insubse'uent forming
processes. &or instance the anisotropy developed in cold
rolledsheet metal helps in getting deeper draws.Disadvantages of
cold forming$i% High loads on the e'uipment re'uire it to have high
strength and rigidity. Thisincreases the cost of machines.$ii% ith
strain hardening the material becomes less ductile.
