operator has control over both the amount of pressure applied and the
finishing temperature, and he is able to produce parts of the highest
grade.
This kind of forging, also called smith forging, is used
extensively where only a small number of parts are needed.
Considerable machining and material are saved when a part is smith-
forged to approximately the finished shape.
Pressing is used when the parts to be forged are large and heavy or
where high-grade steel is required. Because a press is slow acting,
its force is uniformly transmitted to the center of the section.
This gives the interior and exterior grain of the metal the best
possible structure throughout.
EXTRUSION
Forcing metal through an opening in a die causing the metal to take
the shape of the die opening is called extrusion. Some metals, such
as lead, tin, and aluminum, can be extruded cold; however, metals are
generally heated before extrusion.
The principal advantage of this
process is its flexibility. For example, aluminum, with its ductile
and malleable properties, can be economically extruded in more shapes
and sizes than is practicable with many other metals. Extrusions are
produced in simple as well as complex shapes.
In addition, many
structural parts, such as stringers, are made by the extrusion
process.
COLD WORKING
Mechanically working metals at temperatures below the critical range
(cold working) results in strain-hardening the metal.
In fact, the
metal becomes so hard that it is difficult to continue the shaping
process without annealing the metal.
Because the errors due to
shrinkage are eliminated in cold working, a more compact and better
metal is obtained than in hot-worked metal. The strength, hardness,
and elastic limit are increased, but the metal's ductility is
decreased.
Because this makes the metal brittle, it must be heated
from time to time during the working to remove the undesirable
effects.
A number of cold-working processes are used in industry;
however, the ones of interest to airframe repairers are cold rolling,
cold drawing, stamping, and pressing.
Each is discussed in the
paragraphs that follow.
Cold Rolling. Cold rolling is the process of reducing the cross-
sectional area of pieces of metal by passing them between revolving
cylinders or rolls at room temperature.
In a steel mill, a rolling
mill has, in addition to cranes and other handling equipment:
Soaking pits
where
ingots
are
brought
to
the
appropriate
temperature.
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AL0992
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