Forging

[1][2] Forging has been done by smiths for millennia; the traditional products were kitchenware, hardware, hand tools, edged weapons, cymbals, and jewellery.

In modern times, industrial forging is done either with presses or with hammers powered by compressed air, electricity, hydraulics or steam.

Smaller power hammers, 500 lb (230 kg) or less reciprocating weight, and hydraulic presses are common in art smithies as well.

The main advantage of hot forging is that it can be done more quickly and precisely, and as the metal is deformed work hardening effects are negated by the recrystallization process.

[10] Open-die forging lends itself to short runs and is appropriate for art smithing and custom work.

Depending on the size and complexity of the part, the hammer may be dropped multiple times in quick succession.

[8][15] In commercial impression-die forging, the workpiece is usually moved through a series of cavities in a die to get from an ingot to the final form.

The disadvantages of this process include additional cost due to a more complex die design and the need for better lubrication and workpiece placement.

Dies must be impact- and wear-resistant, maintain strength at high temperatures, and have the ability to withstand cycles of rapid heating and cooling.

This leads to the central part of the work piece to come in contact with the sides of the die sooner than if there were no friction present, creating a much greater increase in the pressure required for the punch to finish the forging.

The dimensional tolerances of a steel part produced using the impression-die forging method are outlined in the table below.

Dimensions that are completely contained within a single die segment or half can be maintained at a significantly greater level of accuracy.

The amount of time the dies are in contact with the workpiece is measured in seconds (as compared to the milliseconds of drop-hammer forges).

Drop-hammer forging usually only deforms the surfaces of the work piece in contact with the hammer and anvil; the interior of the workpiece will stay relatively undeformed.

[18] A few examples of common parts produced using the upset forging process are engine valves, couplings, bolts, screws, and other fasteners.

The initial workpiece is usually wire or rod, but some machines can accept bars up to 25 cm (9.8 in) in diameter and a capacity of over 1000 tons.

The main advantages to this process are its high output rate and ability to accept low-cost materials.

The final product is a consistent 1,050 °C (1,920 °F) so air cooling will result in a part that is still easily machinable (the advantage being the lack of annealing required after forging).

It is then descaled with rollers, sheared into blanks, and transferred through several successive forming stages, during which it is upset, preformed, final forged, and pierced (if necessary).

Generally, the cold forming operation will do the finishing stage so that the advantages of cold-working can be obtained, while maintaining the high speed of automatic hot forging.

[21] Examples of parts made by this process are: wheel hub unit bearings, transmission gears, tapered roller bearing races, stainless steel coupling flanges, and neck rings for liquid propane (LP) gas cylinders.

[22] Manual transmission gears are an example of automatic hot forging used in conjunction with cold working.

[23] Roll forging is a process where round or flat bar stock is reduced in thickness and increased in length.

[24] Examples of products produced using this method include axles, tapered levers and leaf springs.

Cost savings are gained from the use of less material, and thus less scrap, the overall decrease in energy used, and the reduction or elimination of machining.

[26] Isothermal forging is a process by which the materials and the die are heated to the same temperature (iso- meaning "equal").

Adiabatic heating is used to assist in the deformation of the material, meaning the strain rates are highly controlled.

Due to the narrow temperature range and high thermal conductivity, aluminium forging can only be realized in a particular process window.

[32][33] This forging method has shown to improve tensile properties but lacks uniform grain size.

The principle behind the machine is simple: raise the hammer and drop it or propel it into the workpiece, which rests on the anvil.