Forging is the
term for shaping metal by using localized compressive forces.One
of the most important properties of metals is their deformability.
The term malleability denotes the ability to be mechanically
deformed by forging, rolling extrusion etc., without rupture
and without significant increase in resistance to deformation.
Metals such as lead and tin are malleable only when heated.
The term ductility denotes the ability of metals to be mechanically
deformed when cold.
the course of such deformation most metals become progressively
more resistant to deformation, this latter effect is called
work hardening. It can also be termed as strain hardening.
A distinction is to be made between cold-forming and hot-forming
processes. The former are usually associated with work hardening
and are performed at room temperature. Hot forming processes
involve heating the metal above a certain temperature to make
Forging is an
important hot forming process. It is used in producing components
of all shapes and sizes, from quite small items to large units
weighing several tons e.g., heavy crankshafts. The metal,
which is preheated to the appropriate forging temperature
in a forge fire, in a forging furnace or by induction, is
deformed mainly by upsetting i.e compressive deformation between
impact surfaces or pressure surfaces. In the process the metal
flows in the direction of least resistance, so that generally
lateral elongation will occur unless restrained. The most
important forgeable materials are steel and steel alloys.
Certain nonferrous metals and alloys are also shaped by forging
tools (Fig.1) comprise variously shaped hammers, such as light
and heavy sledgehammers respectively wielded with one hand
or both hands, the square flatter, the cross-peen hammer,
various auxiliary hammers etc. The base on which the work
is supported during the forging is the anvil, which is provided
with a hardened steel face and terminates at one end or both
ends in a horn, or beak, used for bending work. Various accessories
can be inserted into the holes in the anvil. For holding the
work, the smith has at his disposal a range of tongs and pincers
with a variety of jaw shapes, together with other devices
for gripping and handling larger pieces.
For the semimechanized
forging of small to medium sized components, forging hammers
powered by various means are employed. The feature common
to all of them is that, like the hand forging hammer, they
utilize the energy of a falling weight to develop the pressure
needed for shaping the metal. Larger components are forged
by means of forging presses operated by steam or compressed
air or by hydraulic or electric power. Largely automatic forging
machines are used for the quantity production of engineering
of intricately shaped forgings from bar material in very large
quantities may be carried out by forging rollers. These are
matched rotating rollers or segments of rollers which have
impressions sunk in their surfaces. The metal blank is rolled
into these impressions as the rollers turn. Whereas the rollers
of rolling mills rotate continuously, forging rollers perform
only one revolution per shaping operation.
can be made between open die forging, usually in the form
of hammer forging, and closed die forging. In hammer forging
(Fig.2) which is essentially derived from the traditional
craft of the blacksmith, the component is shaped by hammer
blows aided by relatively simple tools. These may include
open dies i.e., dies that do not completely enclose the metal
to be shaped. One of the basic operations of hammer forging
is the elongation of a piece of metal by stretching with hammer
blows (Fig.3), causing it to become thinner and longer.
In hand forging
the work piece is usually tuned 90o after each blow, in order
to forge it thoroughly and prevents its lateral expansion.
A tube can be forged by flattening the metal longitudinally
or tangentially around a mandrel. The opposite of elongation
is upsetting, which produces compressive shortening. For example,
the diameter of a bar can be increased uniformly or locally
by heating and hammering axially.
important is closed-die forging (Fig.4), very widely used
for mass production in industry, in which the metal blank
is shaped by pressing between a pair of forging dies. The
upper die is usually attached to the ram of a forging press
or a forging hammer, while the lower die stationary. Together
they form a closed die. Closed die forging can produce components
of greater complexity and accuracy, with a better surface
finish, than the more traditional methods not using closed
dies. The dies are made of special heat-resistant and wear
resistant tool steels.
piece of hot metal sufficient to slightly overfill the die
shape is placed in the bottom die, and the top die is forced
against it, so that the metal takes the internal shape of
the die. In hammer forging, several blows are struck in quick
succession, forcing the metal evenly into the die impressions.
The surplus metal forms a flash at the meeting
surface of the upper and lower dies. This is subsequently
trimmed off by special tools fixed in a press, the forging
being forced through a hollow tool which cuts off the flash.
Closed-die forging is used for the rapid production of large
numbers of fairly small parts and also for very large components.
the latter E.g., modern jet-aircraft components including
complete wing and airframe units, giant hydraulically operated
presses are highly complex pieces of machinery, equipped with
elaborate electronic and other controlling and monitoring
instruments. Forging produced in closed dies are known as
drop forgings or stampings. For some purposes the forging
operation is performed in two stages, the blanks first being
treated in preliminary shaping dies and then formed in final
is one of the oldest known metalworking processes. Forging
was done historically by a smith using hammer and anvil, and
though the use of water power in the production and working
of iron dates to the 12th century, the hammer and anvil are
not obsolete. The smithy has evolved over centuries to the
forge shop with engineered processes, production equipment,
tooling, raw materials and products to meet the demands of
modern times, industrial forging is done either with presses
or with hammers powered by compressed air, electricity, hydraulics
or steam. These hammers are large, having reciprocating weights
in the thousands of pounds. Smaller power hammers, 500 pounds
or less reciprocating weight, and hydraulic presses are common
in art smithies as well. Steam hammers are becoming obsolete.
Advantages and Disadvantages of Forging :
results in metal that is stronger than cast or machined metal
parts. This stems from the grain flow caused through forging.
As the metal is pounded the grains deform to follow the shape
of the part, thus the grains are unbroken throughout the part.
Some modern parts take advantage of this for a high strength-to-weight
metals are forged cold, but iron and its alloys are almost
always forged hot. This is for two reasons: first, if work
hardening were allowed to progress, hard materials such as
iron and steel would become extremely difficult to work with;
secondly, steel can be strengthened by other means than cold-working,
thus it is more economical to hot forge and then heat treat.
Alloys that are amenable to precipitation hardening, such
as most alloys of aluminium and titanium, can also be hot
forged then hardened. Other materials must be strengthened
by the forging process itself.
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