Forging is the shaping of metal using localized compression forces. Forging is often classified according to the temperature at which it is performed: ‘”cold,” “warm,” or “hot” forging. Forged parts can range in weight from less than a kilogram to 170 metric tons. Forged parts usually require further processing to achieve a finished part.
Forging is one of the oldest known metalworking processes.
Traditionally, forging was performed 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 or forge has evolved over centuries to become a facility 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 may have reciprocating weights in the thousands of pounds. Smaller power hammers, 500 lb (230 kg) or less reciprocating weight, and hydraulic presses are common in art smithies as well. Some steam hammers remain in use, but they became obsolete with the availability of the other, more convenient, power sources.
Forging can produce a piece that is stronger than an equivalent cast or machined part. As the metal is shaped during the forging process, its internal grain deforms to follow the general shape of the part. As a result, the grain is continuous throughout the part, giving rise to a piece with improved strength characteristics.
Some metals may be forged cold and offer less machine stock and tighter tolerances. Low carbon and alloy steels can be cold forged. Typical parts for cold forging include step shafts of all types , headed pins and many others.
Production forging involves significant capital expenditure for machinery, tooling, facilities and personnel. In the case of hot forging, a high temperature furnace (sometimes referred to as the forge) will be required to heat ingots or billets. Owing to the massiveness of large forging hammers and presses and the parts they can produce, as well as the dangers inherent in working with hot metal, a special building is frequently required to house the operation. In the case of drop forging operations, provisions must be made to absorb the shock and vibration generated by the hammer. Most forging operations will require the use of metal-forming dies, which must be precisely machined and carefully heat treated to correctly shape the work piece, as well as to withstand the tremendous forces involved.
There are many different kinds of forging processes available, however they can be grouped into three main classes:
- Drawn out: length increases, cross-section decreases
- Upset: Length decreases, cross-section increases
- Squeezed in closed compression dies: produces multidirectional flow
Common forging processes include: Open-die forging, impression-die forging, press forging, automatic hot forging and upsetting and cold forging (extruding).
All of the following forging processes can be performed at various temperatures, however they are generally classified by whether the metal temperature is above or below the recrystallization temperature. If the temperature is above the material’s recrystallization temperature it is deemed hot forging; if the temperature is below the material’s recrystallization temperature but above 3/10ths of the recrystallization temperature (on an absolute scale) it is deemed warm forging; if below 3/10ths of the recrystallization temperature (usually room temperature) then it is deemed cold forging. The main advantage of hot forging is that as the metal is deformed work hardening effects are negated by the recrystallization process.
There are two types of drop forging: open-die drop forging and closed-die drop forging. As the names imply, the difference is in the shape of the die, with the former not fully enclosing the work piece, while the latter does. The similarity between the two is that a hammer is raised up and then dropped onto the work piece to deform it according to the shape of the die.
Open-die drop forging
Open-die forging is also known as smith forging. In open-die forging, a hammer strikes and deforms the work piece, which is placed on a stationary anvil. Open-die forging gets its name from the fact that the dies (the surfaces that are in contact with the work piece) do not enclose the work piece, allowing it to flow except where contacted by the dies. Therefore the operator needs to orient and position the work piece to get the desired shape. The dies are usually flat in shape, but some have a specially shaped surface for specialized operations. For example, a die may have a round, concave, or convex surface or be a tool to form holes or be a cut-off tool.
Impression-die drop forging
Impression-die forging is also called closed-die forging. In impression-die work metal is placed in a die resembling a mold, which is attached to the anvil. Usually the hammer die is shaped as well. The hammer is then dropped on the work piece, causing the metal to flow and fill the die cavities. The hammer is generally in contact with the work piece on the scale of milliseconds. Depending on the size and complexity of the part the hammer may be dropped multiple times in quick succession. Excess metal is squeezed out of the die cavities forming what is referred to as flash. The flash cools more rapidly than the rest of the material; this cool metal is stronger than the metal in the die so it helps prevent more flash from forming. This also forces the metal to completely fill the die cavity. After forging the flash is removed by trimming.
In commercial impression-die forging the work piece is usually moved through a series of cavities in a die to get from an ingot to the final form. The first impression is used to distribute the metal into the rough shape in accordance to the needs of later cavities; this impression is called an edging, fullering, or bending impression. The following cavities are called blocking cavities, in which the piece is working into a shape that more closely resembles the final product. These stages usually impart the work piece with generous bends and large fillets. The final shape is forged in a final or finisher impression cavity. If there is only a short run of parts to be done it may be more economical for the die to lack a final impression cavity and instead machine the final features.
Impression-die forging has been further improved in recent years through increased automation which includes induction heating, mechanical feeding, positioning and manipulation, and the direct heat treatment of parts after forging.
Closed-die forging requires the creation of dies and required design work to make working die cavities. Closed die forging has low recurring costs for each part, thus forgings become more economical with more volume. This is one of the major reasons closed-die forgings are often used in the automotive and tool industry. Another reason forgings are common in these industrial sectors is because forgings generally have about a 20 percent higher strength-to-weight ratio compared to cast or machined parts of the same material.
Design of impression-die forgings and tooling
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. In order to produce a better, more economical die the following rules should be followed.
- The dies should part along a single, flat plane if at all possible. If not the parting plan should follow the contour of the part.
- The parting surface should be a plane through the center of the forging and not near an upper or lower edge.
- Adequate draft should be provided; a good guideline is at least 3Â° for aluminum and 5Â° to 7Â° for steel (Depends on the forgers capability) Cold forging requires -0- Draft. Generous fillets and radii should be used Ribs should be low and wide The various sections should be balanced to avoid extreme difference in metal flow Full advantage should be taken of fiber flow lines. Dimensional tolerances should not be closer than necessary.
The dimensional tolerances of a steel part produced using the impression-die forging method are outlined in the table below. It should be noted that the dimensions across the paring plane are affected by the closure of the dies, and are therefore dependent die wear and the thickness of the final flash. Dimensions that are completely contained within a single die segment or half can be maintained at a significantly greater level of accuracy.
|Dimensional tolerances for impression-die forgings|
|Mass [kg (lb)]||Minus tolerance [mm (in)]||Plus tolerance [mm (in)]|
|0.45 (1)||0.15 (0.006)||0.48 (0.018)|
|0.91 (2)||0.20 (0.008)||0.61 (0.024)|
|2.27 (5)||0.25 (0.010)||0.76 (0.030)|
|4.54 (10)||0.28 (0.011)||0.84 (0.033)|
|9.07 (20)||0.33 (0.013)||0.99 (0.039)|
|22.68 (50)||0.48 (0.019)||1.45 (0.057)|
|45.36 (100)||0.74 (0.029)||2.21 (0.087)|
A lubricant is always used when forging to reduce friction and wear. It is also used to as a thermal barrier to restrict heat transfer from the work piece to the die. Finally, the lubricant acts as a parting compound to prevent the part from sticking in one of the dies.
Press forging works slowly by applying continuous pressure or force, which differs from the near-instantaneous impact of drop-hammer forging. The amount of time the dies are in contact with the work piece is measured in seconds (as compared to the milliseconds of drop-hammer forges). The press forging operation can be done either cold or hot.
The main advantage of press forging, as compared to drop-hammer forging, is its ability to deform the complete work piece. Drop-hammer forging usually only deforms the surfaces of the work piece in contact with the dies; the interior of the work piece will stay relatively undeformed. Another advantage to the process includes the knowledge of the new parts strain rate. We specifically know what kind of strain can be put on the part, because the compression rate of the press forging operation is controlled. There are a few disadvantages to this process, most stemming from the work piece being in contact with the dies for such an extended period of time. The operation is a time consuming process due to the amount of steps and how long each of them take. The work piece will cool faster because the dies are in contact with work piece; the dies facilitate drastically more heat transfer than the surrounding atmosphere. As the work piece cools it becomes stronger and less ductile, which may induce cracking if deformation continues. Therefore heated dies are usually used to reduce heat loss, promote surface flow, and enable the production of finer details and closer tolerances. When done in high productivity, press forging is more economical than hammer forging. The operation also creates closer tolerances. In hammer forging a lot of the work is absorbed by the machinery, when in press forging, the greater percentage of work is used in the work piece. Another advantage is that the operation can be used to create any size part because there is no limit to the size of the press forging machine. New press forging techniques have been able to create a higher degree of mechanical and orientation integrity. By the constraint of oxidation to the outer most layers of the part material, reduced levels of micro cracking take place in the finished part.
Press forging can be used to perform all types of forging, including open-die and impression-die forging. Impression-die press forging usually requires less draft than drop forging and has better dimensional accuracy. Also, press forgings can often be done in one closing of the dies, allowing for easy automation.
Net-shape and near-net-shape Cold Forging
See also: Near-net-shape
This process is also known as precision cold forging. This process was developed to minimize cost and waste associated with post forging operations. Therefore, the final product from a precision forging needs little to no final machining. 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. Precision Cold Forging requires No Draft and very tight tolerances not achievable form Hot Forgings. Tooling costs for Cold Forging are extremely affordable and this process is an excellent cost savings process to replace hot up-set forgings and machined bar stock.
The most common type of forging equipment is the hammer and anvil. Principles behind the hammer and anvil are still used today in drop-hammer equipment. The principle behind the machine is very simple – raise the hammer and then drop it or propel it into the work piece, which rests on the anvil. The main variations between drop-hammers are in the way the hammer is powered; the most common being air and steam hammers. Drop-hammers usually operate in a vertical position. The main reason for this is excess energy (energy that isn’t used to deform the work piece) that isn’t released as heat or sound needs to be transmitted to the foundation. Moreover, a large machine base is needed to absorb the impacts.
A forging press, often just called a press, is used for press forging. Mechanical presses function by using cams, cranks and/or toggles to produce a preset (a predetermined force at a certain location in the stroke) and reproducible stroke. Due to the nature of this type of system, different forces are available at different stroke positions. Mechanical presses are faster than their hydraulic counterparts (up to 50 strokes per minute).
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