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Forging
Expertise
 The
forging of steel from ancient times has been considered an arduous
physical labour and at the same time an art. Working with steel heated
to a high temperature of nearly 1000 deg. centigrade, to give it a high
degree of deformation in a very short period before it loses its
plasticity, demands a great deal of effort coupled with a high degree of
skill.
When a steel is forged, its micro structure and mechanical properties
undergo changes. These depend on the
following factors :
1) the temperature of the forging process
2) the degree of reduction of the cross-sectional area
3) the method of forging employed
4) the chemical composition of the base metal
During forging, the structure of the metal changes in two diametrically
opposite directions, and becomes fibrous. The grains of steel are drawn
out in the direction in which the metal flows, and changes from a
coarse-grained to a fine-grained structure. Forging, as compared to
rolling, results in a greater degree of disorientation of the fibre of
the metal resulting in superior mechanical properties.
The formation of new grain structure is called recrystallisation. This
occurs at fairly high temperatures for steel. Also, higher the forging
temperature, greater will be the growth of these grains. Consequently,
steel should be completed at temperatures which foster recrystallisation
and should be completed at temperatures at which grain growth no longer
occurs. This will ensure fine-grained forgings, possessing good
mechanical properties. It can therefore be concluded that the structure
of a steel and the mechanical properties of a forging will depend on the
degree to which its grains have been worked as a result of their
deformation and recrystallisation.
The mechanical properties of forgings also depend on the method of their
production. Forgings should be produced without cutting accross the
fibre of the metal. The strength and toughness of a forging is improved
if the fibre of the metal is continuous along its entire length.
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