One of the oldest known metalworking techniques is forging, in which metal is shaped by impact deformation. Signs of this type of metalworking are seen as early as 4500 BC in the Tigris-Euphrates valley. Forging was originally carried out in a blacksmith's shop using hammer, anvil, and, more often than not, fire. Surprisingly, very similar techniques are still widely used more than 6500 years later!
The forging process can create parts that are stronger than those manufactured by any other metalworking process. But forgings are only rarely seen, as most forgings are components hidden inside assembled items. Forged parts range from a simple hammer to precision aerospace components. In fact, over 18,000 forgings are contained in a 747. Metal forgings are particular important for the aerospace, automotive, and heavy machinery industries.
A famous historical forged product which receives universal respect is the katana, more familiarly known in the US as a samurai sword. In order to illustrate the potential of the forging process, we will briefly examine the manufacture of this legendary sword.
An authentic katana begins with the production of a specialized steel called tamahagane. Tamahagane is produced by charcoal reduction of a special type of black sand, which was thought to have fewer impurities than conventional iron ores. This results in a matrix of high (hard) and low (malleable) concentration carbon steels. This matrix is broken apart, and then stacked to form, via forging, a blade blank having alternating steel alloys with high and low carbon concentrations.
The blade blank is forged thin, then folded in half and rewelded by forging into a new blade blank. The new blade blank has twice the number of steel layers and has a substantially reduced number of impurities. This process is repeated from 18 to 20 times, yielding a blank with 2-4000 alternating layers of hard and soft steel. This blank is then forged into the rough shape of the final sword, and heat treated to form precipitates of iron carbide in the high carbon steel layers, so that the blade displays both enormous hardness and is strengthened by thermal stresses frozen within the forged metal. After final shaping, sharpening, and polishing, the katana is tested via a process called tameshigiri. This involves slicing large rolls of bamboo stalks in a single blow. Traditionally, condemned criminals were used as targets, but this practice is now frowned upon.
Many distinct varieties of forging have been developed, each offering different potentials and difficulties for the production of technologically sophisticated products. These techniques can roughly be classified by temperature as well as how the metal is deformed.
Metals can be cold forged or hot forged, where the process temperature is defined relative to an annealing temperature (typically 50-65% of the alloy melting temperature) for the metal being processed.
Cold forging is defined as working a metal at a low enough temperature that most of the structural damage (e.g., grain boundaries, dislocations, and the like) are retained in the final product. Cold forging, usually carried out at or near room temperature, typically results in work hardening, and a strong and hard product. With most metals, however, cold forging must be taken slowly, to avoid fracture of the work piece, and the hardness of the forged piece can make any final machining steps quite difficult.
By contrast, in hot forging a metal is worked while it is hotter than its annealing temperature. Most metals in this temperature range are easier to shape, and less likely to fracture. In addition, as the metal is forged the work hardening effects encountered during cold forging are largely removed by in-situ annealing.
Common forging processes include: open-die forging, closed-die forging, and press forging. Each of these will be briefly described.
Open-die forging is essentially the same process used by traditional blacksmiths. The work piece is placed on a stationary anvil, and is deformed by repeated blows from a hammer. The name 'open-die' comes from the fact that the surfaces which contact the work piece do not surround the work piece. This allows the metal to flow except where contacted by the dies. Open-die forging thus requires the smith to orient and position the work piece so as to attain the desired shape. The striking surfaces are usually flat and often parallel, but can be concave or convex, or have special functions to form holes or serve to cut the work piece. Open-die forging is perhaps best suited to production of a small number of specialty parts. As a result, it is often used for art and custom work.
In closed-die forging, a metal blank is placed in a die resembling a mold. Between the die and the hammer, the metal blank is completely surrounded. Usually the impact face of the hammer is specially shaped as well. In the forging process, the work piece is struck by the hammer, which causes the metal to flow and fill the die cavities. This usually requires only a few milliseconds, but it may require multiple impacts to forge large or complex parts.
To prepare for production of a new part using closed-die forging is expensive, as new and often complex dies and hammers must be designed and fabricated. Once proper dies are made, however, production of large volumes of pieces can be quite economical. The inexpensive production of large numbers of parts, combined with the higher strength-to-weight ratio characteristic of forged parts compared to cast or machined parts are major reasons that closed-die forging is a staple process in transportation and tool manufacture.
Forging dies are usually made of high-alloy or tool steel. Dies must be impact resistant, wear resistant, maintain strength at high temperatures, and have the ability to withstand cycles of rapid heating and cooling. A lubricant is used during closed-die forging to reduce friction and wear, and to help the forged part release from the die.
In press forging, continuous force is applied to a set of dies containing a billet of metal. The die surfaces are in contact with the work piece for seconds, in contrast to the millisecond period during which impact forging takes place. Press forging can be used to perform all types of forging, including open-die and impression-die forging. Press forging deforms the entire work piece, and also allows control over the strain rate experienced during the forging process, which allows delicate control over the metallurgical properties of the finished part. Press forging can be a very economical manufacturing technique once the dies have been produced.
In conclusion, metal forging is a very flexible process, offering a great deal of control over the mechanical properties of the final product. The forged product is often less expensive and stronger than an equivalent cast or machined part. Driven by this potential, a vast array of improved and modified forging techniques have been developed to meet modern manufacturing requirements. There is every reason to believe that forging will continue to be an important manufacturing process.