The material used must
have a good "forgeability". By forgeability we mean the
capability of the material to undergo deformation by forging without
cracking. A material that can be shaped with low forces and without
cracking is said to have a good forgeability. Usually forgeability
competes with other desirable properties of the material and of
the geometry of the workpiece, such as strength, corrosion resistance,
toughness, fatigue resistance, heat resistance, size and section
thickness. Therefore, the material is often selected on a compromise
It is important to point
out that the combined effects of temperature and deformation change
the properties of the starting material. While it is often desirable
to pick the starting properties based on compatibilities with the
process, it is necessary to know or to be able to predict how the
process will alter them.
Grain Alteration and
Upon solidification of
metals, especially in large sections,coarse dentritic grains form
and a certain amount of segregation of impurities occurs. Hence,
as-solidified metals typically have a nonuniform grain structure
with a rather large grain size.
Reheating the metal without
prior deformation will simply promote grain growth and a concurrent
decrease in properties. However, when deforming the metal sufficiently
at temperatures above the recrystallization temperature, the distorted
structure is rapidly replaced by new strain-free grains. Grain growth
can then occur. The metal can be cooled to "freeze in"
the current structure or be further deformed and recrystillized.
In general, the final structure will be a fine, randomly oriented,
spherical-shaped grain structure. Such structure results in a net
increase in strength, ductility, and toughness.
Forging, hot and cold,
also affects the orientation of inclusions or impurity particles
in the metal. With normal melting and cooling, the impurities locate
along the grain boundary interfaces. It can assist a crack in its
propagation along the metal. When a piece of metal is plastically
deformed, the impurity material often distorts and flows along with
the metal. Those impurities, often nonmetallic, don't recrystallize
with the base metal and often produces an aligned fiber structure.
Such a structure clearly have directional properties, being stronger
in one direction than in another. An impurity originally oriented
so as to aid crack movement through the metal can be reoriented
into a "crack-arrestor" configuration, perpendicular to
the direction of crack propagation.
Most commonly forged
materials are steels, copper, forging brass, naval brass, bronze,
and copper alloys. As for steels, their forgeability decreases as
their carbon and alloy content increases.
Following is a table
of materials and their 'forgability' ranking. The ranking indicates
the material suitability for the forging process.
A value of zero means
that the corresponding material is never used with this process,
a ranking of 100 means that it is excellent for use with this process.
Black, Kohser, Materials and Processes in Manufacturing.