168 Chapter 7
Forging.
Forging is a process of shaping metal while it is hot and pliable by suitably applying compressive force.
Usually the compressive force takes the form of hammer blows using a power hammer or a press. To
achieve good forgings, a proper lubricant is necessary. The lubricant helps prevent the work piece stick-
ing to the die and also acts as a thermal insulator to help reduce die wear.
Forging refines the grain structure and improves the physical properties of the metal. With proper
design, the grain flow can be oriented in the direction of principal stresses that are encountered in ac-
tual use. Grain flow is the direction of the pattern that the crystals take during plastic deformation. Phys-
ical properties (such as strength, ductility, and toughness) are much better in a forging than in the base
metal, whose crystals are randomly oriented. Forgings yield parts that have a high strength-to-weight ra-
tio. Common forging processes include:
- Open-die or hand forgings are a traditional and antiquated manufacturing process based on re-
peated blows in an open die, where the operator manipulates the work piece in the die. The fin-
ished product is a rough approximation of the die. - Press forging is the shaping of metal between dies on a mechanical or hydraulic press. The action
is that of kneading the metal by relatively slow application of force as compared with the action of
hammering. This results in uniform material properties and is necessary for large weight forgings.
Parts made with this process can be quite large as much as 125 kg (260 lb) and 3m (10 feet) long. - Impression-die forgings or precision forgings are further refinements of blocker forgings (blocker
forging is a forging that approximates the general shape of the final part with relatively gener-
ous finish allowance and radii. Such forgings are sometimes specified to reduce die costs where
only a small number of forgings are desired and the cost of machining each part to its final shape
is not exorbitant). Finished components more closely resemble the die impression. - Upset forgings increase the cross section by compressing the length and are used in making
heads on bolts and fasteners, valves, and other similar parts. - In roll forging (also known as draw forging) a round or flat bar stock is placed between die rollers,
which reduces the cross section and increases the length to form parts such as axles, leaf
springs etc. Draw forging also involves pulling a hollow tube through a series of hardened steel
dies of gradually decreasing diameter. Before each step of the drawing process, the tube is
pointed at one end to fit through the next smaller die and is then gripped by automatic jaws at-
tached to a rotating drawing machine. - In swaging a tube or rod is forced inside a die and the diameter is reduced as the cylindrical object
is fed. The die hammers the diameter and causes the metal to flow inward, causing the outer di-
ameter of the tube or the rod to take the shape of the die. - Net and near net shape forging represents a number of recent developments of the conventional
impression die forging process. Net and near net shape forgings are distinguished by geomet-
ric features that are thinner and more detailed, varying parting line locations, virtual elimination
of draft, and closer dimensional tolerances. The resulting product benefits are much fewer ma-
chining operations (in many cases, the only machining operations required are drilling of attach-
ment holes), reduced weight and lower costs for raw materials and energy. The processes are
quite costly in terms of tooling and the capital expenditure required. Thus, these processes can
only be justified for current processes that are very wasteful where the material savings will pay
for the significant increase in tooling costs.