Forging

I

Introduction

Forging, process of shaping iron and other malleable metals by hammering or pressing them after making them plastic by application of heat. Forging techniques are useful in the working of metal because the metal can be given the desired form, and the process improves the structure of the metal, particularly by refining the grain size of the metal. Forged metal is stronger and more ductile than cast metal and exhibits greater resistance to fatigue and impact.

II

Hand Forging

Sometimes called smithing, or blacksmithing, hand forging is the simplest form of forging and it is one of the methods by which metal was first worked. The metal to be forged is first heated to red heat in the fire of a forge, and then is beaten into shape on a metal anvil with sledges or hammers. The forge consists of an open hearth, made of some durable, refractory substance such as firebrick, which is provided with a number of air openings, or tuyeres, through which air is forced by a bellows or blower fan. Charcoal, coke, and coal are used as fuels in the forge. Hammers and other tools are employed by the blacksmith in the various forging operations.

In general, six basic types of forging exist: upsetting, or decreasing the length and increasing the diameter of the metal; swaging, decreasing the diameter of the metal; bending; welding, joining two pieces of metal together by semifusion; punching, the forming of small openings in the metal; and cutting out, the forming of large holes in the metal.

A piece of metal, called the work, is upset when it is struck along the longest dimension (for example, the end of a rod or bar), which shortens and thickens it. Swaging is accomplished by hammering the metal stock while it is held on the anvil within any one of various concave tools called swages. Bending is accomplished either by hammering the work around a form or by leveraging it against a supporting fulcrum. In forge welding of iron, a flux such as borax is first applied to the heated metal to remove any oxides from the surfaces of the two pieces, and the pieces are then joined by hammering them together at high temperature; a welded joint of this kind, when properly made, is entirely homogeneous and is as strong, that is, uniform, as the parent metal. To punch small holes, the work is supported on a ring-shaped piece of metal atop the anvil, and a punch of the proper shape is driven through the work by hammer blows. Larger holes are cut out, and portions of the work are cut off with heavy, sharp chisels similar to cold chisels used to cut cold metal. Combinations of several of these operations can produce forgings of a wide variety of shapes.

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III

Machine Forging

The chief difference between hand forging and machine forging is that in the latter technique various types of machine-powered hammers or presses are used instead of hand sledges. These machines enable the operator to strike heavy blows with great rapidity and thus to produce forgings of large size and high quality as swiftly as required by modern production-line methods. Another advantage of machine forging is that the heavier the blows struck during forging, the greater the improvement in the quality of metallic structure. Fine-grain size in the forging, which is particularly desirable for maximum impact resistance, is obtained by working the entire piece. With large, hand-forged metal, only the surface is deformed, whereas the machine hammer or press will deform the metal throughout the entire piece.

A special type of machine forging is drop forging, also called impact-die forging. Drop forging consists of placing soft, hot metal between two shaping dies (see Die). The upper one of these dies is hammered, or dropped, on the lower die, forcing the heated metal into the shaped die cavities, as in coin-making operations.

For reducing part of a piece of metal stock to a predetermined size, forging rolls are sometimes employed. These consist of a pair of grooved, cam-shaped rollers through which the metal is passed. The rollers touch each other and work on the metal during only part of each rotation and therefore reduce only part of the stock that is fed to them.

Machine-forging operations are frequently accomplished by use of a series of dies mounted on the same press or hammer. The dies are arranged in sequence so as to form the finished forging in a series of steps. After the piece has been partially formed by one stroke, it is moved to the next die for further shaping on the next stroke.

See also Metallurgy; Metalwork.

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