Hot And Cold Forging Process Pdf
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- Cold and Hot Forging: An Overview
- Understanding the Different Forging Processes
- Cold And Hot Forging: Fundamentals And Applications
Cold and Hot Forging: An Overview
Forging is a manufacturing process involving the shaping of metal using localized compressive forces. The blows are delivered with a hammer often a power hammer or a die. Forging is often classified according to the temperature at which it is performed: cold forging a type of cold working , warm forging, or hot forging a type of hot working. For the latter two, the metal is heated , usually in a forge. Forged parts can range in weight from less than a kilogram to hundreds of metric tons.
Since the Industrial Revolution , forged parts are widely used in mechanisms and machines wherever a component requires high strength ; such forgings usually require further processing such as machining to achieve a finished part. Today, forging is a major worldwide industry.
Forging is one of the oldest known metalworking processes. 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. 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 texture deforms to follow the general shape of the part.
As a result, the texture variation is continuous throughout the part, giving rise to a piece with improved strength characteristics. Considering all the costs that are incurred in a product's life cycle from procurement to lead time to rework, and factoring in the costs of scrap, and downtime and other quality considerations, the long-term benefits of forgings can outweigh the short-term cost savings that castings or fabrications might offer. Some metals may be forged cold, but iron and steel are almost always hot forged.
Hot forging prevents the work hardening that would result from cold forming , which would increase the difficulty of performing secondary machining operations on the piece.
Also, while work hardening may be desirable in some circumstances, other methods of hardening the piece, such as heat treating , are generally more economical and more controllable. Alloys that are amenable to precipitation hardening , such as most aluminium alloys and titanium , can be hot forged, followed by hardening. 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 is required to heat ingots or billets.
Owing to the size of the massive 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 use metal-forming dies, which must be precisely machined and carefully heat-treated to correctly shape the workpiece, 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: . Common forging processes include: roll forging, swaging , cogging , open-die forging, impression-die forging close die forging , press forging, cold forging automatic hot forging and upsetting.
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. 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.
Cold forging typically results in work hardening of the piece. Drop forging is a forging process where a hammer is raised and then "dropped" onto the workpiece to deform it according to the shape of the die. There are two types of drop forging: open-die drop forging and impression-die or closed-die drop forging. As the names imply, the difference is in the shape of the die, with the former not fully enclosing the workpiece, while the latter does.
Open-die forging is also known as smith forging. Open-die forging gets its name from the fact that the dies the surfaces that are in contact with the workpiece do not enclose the workpiece, allowing it to flow except where contacted by the dies. The operator therefore needs to orient and position the workpiece 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. In some cases, open-die forging may be employed to rough-shape ingots to prepare them for subsequent operations. Open-die forging may also orient the grain to increase strength in the required direction. It is commonly used to work a piece of raw material to the proper thickness. Once the proper thickness is achieved the proper width is achieved via "edging".
The process is called "edging" because it is usually carried out on the ends of the workpiece. These processes prepare the workpieces for further forging processes. Impression-die forging is also called "closed-die forging".
In impression-die forging, the metal is placed in a die resembling a mold, which is attached to an anvil. Usually, the hammer die is shaped as well.
The hammer is then dropped on the workpiece, causing the metal to flow and fill the die cavities. The hammer is generally in contact with the workpiece 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. 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 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 workpiece 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, then 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 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. In this type of forging, the die cavities are completely closed, which keeps the workpiece from forming flash. The major advantage to this process is that less metal is lost to flash. The disadvantages of this process include additional cost due to a more complex die design and the need for better lubrication and workpiece placement.
There are other variations of part formation that integrate impression-die forging. One method incorporates casting a forging preform from liquid metal. The casting is removed after it has solidified, but while still hot. It is then finished in a single cavity die. The flash is trimmed, then the part is quench hardened. Another variation follows the same process as outlined above, except the preform is produced by the spraying deposition of metal droplets into shaped collectors similar to the Osprey process.
Closed-die forging has a high initial cost due to the creation of dies and required design work to make working die cavities. However, it has low recurring costs for each part, thus forgings become more economical with greater production volume. This is one of the major reasons closed-die forgings are often used in the automotive and tool industries. Another reason forgings are common in these industrial sectors is that forgings generally have about a 20 percent higher strength-to-weight ratio compared to cast or machined parts of the same material.
Forging dies are usually made of high-alloy or tool steel. Dies must be impact- and wear-resistant, maintain strength at high temperatures, have the ability to withstand cycles of rapid heating and cooling. In order to produce a better, more economical die the following standards are maintained: . Barrelling occurs when, due to friction between the work piece and the die or punch , the work piece bulges at its centre in such a way as to resemble a barrel.
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.
The dimensions across the parting plane are affected by the closure of the dies, and are therefore dependent on 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.
A lubricant is used when forging to reduce friction and wear. It is also used as a thermal barrier to restrict heat transfer from the workpiece to the die. Finally, the lubricant acts as a parting compound to prevent the part from sticking in the dies.
Press forging works by slowly applying a 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 workpiece 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 workpiece.
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. Another advantage to the process includes the knowledge of the new part's strain rate.
By controlling the compression rate of the press forging operation, the internal strain can be controlled. There are a few disadvantages to this process, most stemming from the workpiece being in contact with the dies for such an extended period of time. The operation is a time-consuming process due to the amount and length of steps.
The workpiece will cool faster because the dies are in contact with workpiece; the dies facilitate drastically more heat transfer than the surrounding atmosphere. As the workpiece 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.
The workpiece may also need to be reheated. When done in high productivity, press forging is more economical than hammer forging.
Understanding the Different Forging Processes
Forging is one of the common manufacturing processes, that shapes a metal piece by applying compressive forces on it. Forging could be performed under different temperature conditions, like hot forging, warm forging and cold forging. The forging process uses hammers or presses to squeeze and deform the material into high strength parts. The forging manufacturing process is completely different from the casting one, where the molten material is poured into a mold see difference between forging and casting. This makes forging served in various industrial applications. The hot forging process involves the application of extreme heat, with the desired temperature depending on the type of metal. Hot forging is typically the best choice for steel forgings used in technical applications and for deforming metal that has a high formability ratio.
Forging is the process of deforming metal into a predetermined shape using certain tools and equipment—deformation is accomplished using hot, cold, or even warm forging processes. Ultimately, the manufacturer will look at a number of criteria before choosing which type of forging is best for a particular application. Forging is used where the arrangement of the grain structure imparts directional properties to the part, aligning the grain so that it will resist the highest stress the part will encounter. Casting and machining, in comparison, usually have less control over the arrangement of grain structure. Forging is defined as the forming or deforming of metal in its solid state. Much forging is done by the upsetting process where a hammer or ram moves horizontally to press against the end of a rod or stem to widen and change the shape of the end.
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Cold And Hot Forging: Fundamentals And Applications
Forging is a manufacturing process that uses compressive forces to form metal into the desired shapes and sizes. Industry professionals employ a variety of different forging techniques—depending on the material and the product being forged—each of which carries its own advantages and best uses. The following blog provides an overview of the different types of forging methods available, outlining what they entail, their key advantages, and their typical applications. Forging operations can be classified into four main types:. Open die forging employs the use of two flat or simply shaped dies to apply pressure to the base material from both sides.
The Cold Forging Manufacturing Process. Disadvantages of Cold Forging. The Hot Forging Manufacturing Process. Cost Efficiency in Cold Forging.
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