Forge
From Free net encyclopedia
Image:Australian blacksmith.jpg
- For other uses, see Forge (disambiguation).
The forge or smithy is the workplace of a smith or a blacksmith. "Forging" is the term for shaping metal by use of heat and hammer.
A basic smithy contains a forge, sometimes called a hearth for heating the metals (commonly iron or steel) to a temperature where the metal becomes malleable (typically red hot), or to a temperature where work hardening ceases to accumulate, an anvil (to lay the metal pieces on while hammering), and a slack tub (to rapidly cool, and thus harden, forged metal pieces in). Tools include tongs to hold the hot metal, and hammers to strike the hot metal.
Once the final shape has been forged, iron and steel in particular often get some type of heat treatment. This can result in various degrees of hardening or softening depending on the details of the treatment.
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Forging
Forging is the working of metal by plastic deformation. It is distinguished from machining, the shaping of metal by removing material (drilling, sawing, milling, turning, grinding, etc.), and from casting, wherein metal in its molten state is poured into a mold, whose form it retains on solidifying. The processes of raising, rolling, swaging, and drawing are essentially forging operations although they are not commonly so called because of the special techniques and tooling they require. Some of these techniques are shown in this animation of the forging of simple flat firetongs.
Forging results in metal that is stronger than cast or machined metal parts. This is because during forging the metal's grain flow changes, making it stronger and more ductile. Image:ForgedConrodShowingEtchedSection-s.jpg
Many metals are typically forged cold but iron and its alloys are almost always forged hot. This is for two reasons: firstly, if work hardening were allowed to progress, hard materials such as iron and steel would become extremely difficult to work with; secondly, most steel alloys can be hardened by heat treatments (i.e. by the formation of Martensite) rather than cold forging. Alloys that are amenable to precipitation hardening (such as most structural alloys of aluminium and titanium) can also be forged hot, then made strong once they achieve their final shape. Other materials must be strengthened by the forging process itself.
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 twelfth century CE the hammer and anvil are by no means obsolete.
In modern times, industrial forging is commonly done either with machine presses or with hammers powered by steam or compressed air. These hammers are very 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.
In industry a distinction is made between open- and closed-die forging. In open-die work the metal is free to move except where contacted by the hammer, anvil, or other (often hand-held) tooling. In closed-die work the material is placed in a die resembling a mold, which it is forced to fill by the application of pressure. A great many common objects (wrenches, crankshafts...) are produced by closed-die forging, which is well suited to mass production. Open-die forging lends itself to very short runs and is appropriate for art smithing and custom work.
Closed-die forging is more expensive for mass production than is casting, but produces a much stronger part, and is therefore used for tools, high-strength machine parts and the like. One particular variant, drop forging, is often used to mass produce flat wrenches and other household tools.
Types of forges
Coal/coke/charcoal forge
A forge which typically uses bituminous coal, industrial coke or charcoal as the fuel to heat metal.
The designs of these forges have varied over time and circumstances for thousands of years but whether the fuel is coal, coke or charcoal the basic design has remained the same. The illustration at the beginning of this article shows a coal forge in operation.
A forge of this type is essentially a hearth or fireplace designed to allow a fire to be controlled such that metal introduced to the fire may be brought to a malleable state or to bring about other metallurgical effects (hardening, annealling, and drawing temper as examples).
The forge fire in this type of forge is controlled in three primary ways: 1) the amount of air, 2) the volume of fuel, and 3) the shape of the fuel/fire.
Over the thousands of years of forging, these devices have evolved in one form or another as the essential features of this type of forge:
- Tuyere -- a pipe through which air can be forced into the fire
- Bellows or Blower -- a means for air to be forced into the tuyere
- Firepot or Hearth -- a place where the burning fuel can be contained over or against the tuyere opening.
In practice, fuel is placed in or on the hearth and ignited. The bellows are worked to introduce additional air (oxygen) into the fire through the tuyere. With the additional oxygen, the fire can consume more fuel and burn hotter.
In practice, a blacksmith will balance the fuel and oxygen in the fire to suit particular kinds of work. Often this involves adjusting and maintaining the shape of the fire.
In a typical (but by no means universal) coal forge, a firepot will be centered in a flat hearth. The tuyere will enter the firepot at the bottom. When in operation, the hot core of the fire will be a ball of burning coke in and above the firepot. The heart of the fire will be surrounded by a layer of hot but not burning coke. Around the unburnt coke will be a transitional layer of coal being transformed into coke by the heat of the fire, and surrounding all will be a ring or horseshoe shaped layer of raw coal usually kept damp and tightly packed to both maintain the shape of the fire's heart and to keep the coal from burning directly so that it "cooks" into coke first.
If a larger fire is necessary, the smith will increase the air flowing into the fire as well as feed and deepen the coke heart. The smith can also adjust the length and width of the fire in such a forge to accommodate different shapes of work.
The major variation from the forge and fire just described is a 'back draft' where there is no fire pot, and the tuyere enters the hearth horizontally from the back wall.
Coke and charcoal may be burned in the same forges that coal is used in, although since there is no need to convert the raw fuel into something more refined at the heart of the fire as with coal, the fire is handled differently.
Individual smiths and specialized applications and needs have led to the development of a variety of forges of this type, from the commercially available coal forge described above to simpler constructions amounting to little more than a hole in the ground with a pipe leading into it.
Gas forge
A forge which typically uses propane or natural gas as the fuel to heat metal. One common, efficient design uses a cylindrical forge chamber and a burner tube mounted at a right angle to the body. The chamber is typically lined with refractory materials, preferably a hard castable refractory ceramic. The burner mixes fuel and air which are ignited at the tip, which protrudes a short way into the chamber lining. The air pressure (and therefore heat) can be increased through use of a mechanical blower or by taking advantage the Venturi effect.
Gas forges vary widely in size and construction, from very large forges using a big burner with a blower or several atmospheric burners to forges built out of a coffee can utilizing a cheap, simple propane torch. A small forge can even be carved out of a single soft firebrick.
The primary advantage of a gas-powered forge lies in its ease of use, particularly for a novice smith. A gas forge is very simple to operate compared to coal forges, and the fire produced will be clean and consistent. They are however less versatile as the fire cannot be reshaped to accommodate large or unusually shaped pieces; it is also difficult to heat a small section of a piece. A common misconception is that gas forges cannot produce enough heat to ennable forge-welding, but a well designed forge will easily run hot enough for any task.
Drop forge
Image:Forging shop-Gesenkschmiede 1.JPG The workpiece, say a spanner, is created by hammering a piece of hot metal into an appropriately shaped die. The metal (in some default, easily produced shape like a rod or brick) is heated and placed on the bottom part of a die. The top part of the die then drops onto the piece. The die may drop under gravity or be powered, but in all cases drop forging involves impact. The force of the impact causes the heated metal to flow into the shape of the die, with some metal squirting out of the thin seams between the die covers. This thin metal is called flash and has to be cut away in the next stage of processing. The drop-forged pieces usually also need further processing (like machining and polishing of working surfaces) in order to comply with stricter tolerances than forging alone can provide, and to present a good finish.
Drop forging: From BYU, with a nice crosssectional diagram.
Hydraulic Press Forge
In hydraulic press forging the work piece is pressed against the two die halves with a gradually increasing force, over a period of a few seconds. The quality of the pieces is better than drop forging as there is more control over metal flow, but the process takes longer and requires more energy.
Finery Forge
A finery forge was water powered mill, where pig iron was fined to produce bar iron - see article on finery forges.
See also
- solar forge
- Manufacturing techniques: University of Washington site.
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