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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.

In 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 it malleable

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

Hand 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 parts.

The manufacture 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.

A distinction 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.

More 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.

A 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.

For 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 shaping dies.

Forging 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 industry.

In 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 :

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 ratio.

Many 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.