of these rolled shapes are sheets, bars, channels, angles,
condition is known as shock and fatigue resistance. It is
I-beams, and the like. In aircraft work, sheets, bars, and
essential that materials used for critical parts be
rods are the most commonly used items that are rolled
resistant to these stresses.
from steel. Hot-rolled materials are frequently finished
The preceding discussion of the properties and
by cold-rolling or drawing to obtain accurate finish
qualities of metals is intended to show why you must
dimensions and a bright, smooth surface.
know which traits in metals are desirable and which are
undesirable to do certain jobs. The more you know
FORGING.--Complicated sections that cannot be
about a given material, the better you can handle
rolled, or sections of which only a small quantity is
required, are usually forged. Forging of steel is a
mechanical working of the metal above the critical
METAL WORKING PROCESSES
range to shape the metal as desired. Forging is done
either by pressing or hammering the heated steel until
When metal is not cast in a desired manner, it is
the desired shape is obtained.
formed into special shapes by mechanical working
Pressing is used when the parts to be forged are
processes. Several factors must be considered when
large and heavy, and this process also replaces
determining whether a desired shape is to be cast or
hammering where high-grade steel is required. Since a
formed by mechanical working. If the shape is very
press is slow acting, its force is uniformly transmitted to
complicated, casting will be necessary to avoid
the center of the section, thus affecting the interior grain
expensive machining of mechanically formed parts. On
structure as well as the exterior to give the best possible
the other hand, if strength and quality of material are
the prime factors in a given part, a cast will be
unsatisfactory. For this reason, steel castings are
Hammering can be used only on relatively small
seldom used in aircraft work.
pieces. Since hammering transmits its force almost
instantly, its effect is limited to a small depth. Thus, it is
There are three basic methods of metal working.
necessary to use a very heavy hammer or to subject the
They are hot-working, cold-working, and extruding.
part to repeated blows to ensure complete working of
The process chosen for a particular application depends
the section. If the force applied is too weak to reach the
upon the metal involved and the part required, although
center, the finished forging surface will be concave. If
in some instances you might employ both hot- and
the center is properly worked, the surface will be
cold-working methods in making a single part.
convex or bulged. The advantage of hammering is that
the operator has control over the amount of pressure
applied and the finishing temperature, and is able to
produce parts of the highest grade.
Almost all steel is hot-worked from the ingot into
some form from which it is either hot- or cold-worked
This type of forging is usually referred to as smith
to the finished shape. When an ingot is stripped from its
forging, and it is used extensively where only a small
mold, its surface is solid, but the interior is still molten.
number of parts are needed. Considerable machining
The ingot is then placed in a soaking pit, which retards
and material are saved when a part is smith forged to
loss of heat, and the molten interior gradually solidifies.
approximately the finished shape.
After soaking, the temperature is equalized throughout
the ingot, which is then reduced to intermediate size by
rolling, making it more readily handled.
Cold-working applies to mechanical working
The rolled shape is called a bloom when its
performed at temperatures below the critical range, and
sectional dimensions are 6 x 6 inches or larger and
results in a strain hardening of the metal. It becomes so
approximately square. The section is called a billet
hard that it is difficult to continue the forming process
when it is approximately square and less than 6 times 6
inches. Rectangular sections that have width greater
than twice the thickness are called "slabs." The slab is
Since the errors attending shrinkage are eliminated
the intermediate shape from which sheets are rolled.
in cold-working, a much more compact and better
metal is obtained. The strength and hardness as well as
HOT-ROLLING.--Blooms, billets, or slabs are
the elastic limit are increased, but the ductility
heated above the critical range and rolled into a variety
decreases. Since this makes the metal more brittle, it
of shapes of uniform cross section. The more common