|
Hammer and Press Forging
Process
Forging is a metal forming process used to produce large quantities of identical parts, as in the manufacture of
automobiles, and to improve the mechanical properties of the metal being forged, as in aerospace parts or military
equipment. The design of forged parts is limited when undercuts or cored sections are required. All cavities must
be comparatively straight and largest at the mouth, so that the forging die may be withdrawn. The products of forging
may be tiny or massive and can be made of steel (automobile axles), brass (water valves), tungsten (rocket nozzles),
aluminum (aircraft structural members), or any other metal. More than two thirds of forging in the United States
is concentrated in four general areas: 30 percent in the aerospace industry, 20 percent in automotive and truck
manufacture, 10 percent in off-highway vehicles, and 10 percent in military equipment. This process is also used
for coining, but with slow continuous pushes.
The forging metal forming process has been practiced since the Bronze Age. Hammering metal by hand can be dated back
over 4000 years ago. The purpose, as it still is today, was to change the shape and/or properties of metal into
useful tools. Steel was hammered into shape and used mostly for carpentry and farming tools. An ax made easy work
of cutting down trees and metal knives were much more efficient than stone cutting tools. Hunters used metal-pointed
spears and arrows to catch prey. Blacksmiths used a forge and anvil to create many useful instruments such as
horseshoes, nails, wagon tires, and chains.
Militaries used forged weapons to equip their armies, resulting in many territories being won and lost with the use
and strength of these weapons. Today, forging is used to create various and sundry things. The operation requires
no cutting or shearing, and is merely a reshaping operation that does not change the volume of the material.
Forging
Forging changes the size and shape, but not the volume, of a part. The change is made by force applied to
the material so that it stretches beyond the yield point. The force must be strong enough to make the material
deform. It must not be so strong, however, that it destroys the material. The yield point is reached when the
material will reform into a new shape. The point at which the material would be destroyed is called the fracture
point.
In forging, a block of metal is deformed under impact or pressure to form the desired shape. Cold forging, in which
the metal is not heated, is generally limited to relatively soft metals. Most metals are hot forged; for example,
steel is forged at temperatures between 2,100oF and 2,300oF (1,150oC to 1,260oC). These temperatures cause
deformation, in which the grains of the metal elongate and assume a fibrous structure of increased strength along
the direction of flow. (See Figure)
Normally this results in metallurgical soundness and improved mechanical properties. Strength, toughness, and
general durability depend upon the way the grain is placed. Forgings are somewhat stronger and more ductile along
the grain structure than across it. The feature of greatest importance is that along the grain structure there is
a greater ability to resist shock, wear, and impact than across the grain. Material properties also depend on the
heat-treating process after forging. Slow cooling in air may normalize workpieces, or they can be quenched in oil
and then tempered or reheated to achieve the desired mechanical properties and to relieve any internal stresses.
Good forging practice makes it possible to control the flow pattern resulting in maximum strength of the material
and the least chances of fatigue failure. These characteristics of forging, as well as fewer flaws and hidden
defects, make it more desirable than some other operations (i.e. casting) for products that will undergo high
stresses.
In forging, the dimensional tolerances that can be held vary based on the size of the workpiece. The process is
capable of producing shapes of 0.5 to >50.0 cm in thickness and 10 to <100 cm in diameter. The tolerances vary
from ± 1/32 in. for small parts to ± ¼ in. for large forgings. Tolerances of 0.010 in. have been held in some
precision forgings, but the cost associated with such precision is only justified in exceptional cases, such as
some aircraft work.
Types of forging
Forging is divided into three main methods: hammer, press, and rolled types.
- Hammer Forging(Flat Die)
Preferred method for individual forgings. The shaping of a metal, or other material,
by an instantaneous application of pressure to a relatively small area. A hammer or ram, delivering intermittent
blows to the section to be forged, applies this pressure. The hammer is dropped from its maximum height, usually
raised by steam or air pressure. Hammer forging can produce a wide variety of shapes and sizes and, if sufficiently
reduced, can create a high degree of grain refinement at the same time. The disadvantage to this process is that
finish machining is often required, as close dimensional tolerances cannot be obtained.
- Press Forging
This process is similar to kneading, where a slow continuous pressure is applied to the area to be
forged. The pressure will extend deep into the material and can be completed either cold or hot. A cold press
forging is used on a thin, annealed material, and a hot press forging is done on large work such as armor plating,
locomotives and heavy machinery. Press Forging is more economical than hammer forging (except when dealing with
low production numbers), and closer tolerances can be obtained. A greater proportion of the work done is transmitted
to the workpiece, differing from that of the hammer forging operation, where much of the work is absorbed by the
machine and foundation. This method can also be used to produce larger forgings, as there is no limitation in the
size of the machine.
- Die Forging
Open and closed die operations can be used in forging. In open-die forging the dies are either
flat or rounded. Large forgings can be formed by successive applications of force on different parts of the material.
Hydraulic presses and forging machines are both employed in closed die forging. In closed-die forging the metal is
trapped in recessed impressions, which are machined into the top and bottom dies. As the dies press together,
the material is forced to fill the impressions. Flash, or excess metal, is squeezed out between the dies.
Closed-die forging can produce parts with more complex shapes than open-die forging. Die forging is the best
method, as far as tolerances that can be met, and also results in a finished part that is completely filled out
and is produced with the least amount of flashing. The final shape and the improvement in metallurgical
properties are dependent on the skill of the operator. Closer dimensional tolerances can be held with closed
die forgings than with open die forgings and the operator requires less skill.
|
