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Forging, a metal shaping technique using compressive, localized forces, has been a staple metal fabrication technique since the time of the ancient Mesopotamians. Since its origins in the fertile crescent, forging has experienced significant changes, resulting in a more efficient, faster, and more durable process. This is because today, forging is most commonly performed with the use of forging presses or hammering tools that are powered by electricity, hydraulics or compressed air. Some of the common materials used for forging are carbon steel, alloy steel, microalloy steel, stainless steel, aluminum, and titanium.
A traditional hammer and anvil are used for forging. Image credit: Shutterstock.com/Drpixel
The purpose of forging is to create metal parts. Compared to other manufacturing methods, metal forging produces some of the sturdiest manufactured parts available. As metal is heated and pressed, minor cracks are sealed, and any empty spaces in the metal close.
The hot forging process also breaks up impurities in the metal and redistributes such material across the metalwork. This vastly reduces inclusions in the forged part. Inclusions are compound materials implanted inside steel throughout manufacturing that cause stress points in the final forged parts.
While impurities should be managed during the initial casting process, forging further refines the metal.
Another way that forging strengthens metal is by alternating its grain structure, which is the metal material's grain flow as it deforms. Through forging, a favorable grain structure can be created, making the forged metal sturdier.
The forging process is highly multipurpose and can be used on small parts just a few inches in size to large components that weigh up to 700,000 lbs. It is used to produce critical aircraft parts and transportation equipment. Forging is also used to fortify hand tools such as chisels, rivets, screws, and bolts.
The pounding action of forging deforms and shapes the metal, which results in unbroken grain flow. This causes the metal to retain its strength. Ancillary effects of this unique grain flow include the elimination of defects, inclusions, and porosity in the product. Another advantage of forging is the relatively low costs associated with moderate and long production runs. Once the forging tools have been created, products can be manufactured at relatively high speeds with minimal downtime.There are two main types of forging: hot and cold.
Hot forging requires the metal to be heated above its recrystallization temperature. This can mean heating metals up to 2,300 degrees Fahrenheit. The main benefit of hot forging is the decrease in energy required to form the metal properly. This is because excessive heat decreases yield strength and improves ductility. Hot forged products also benefit from the elimination of chemical inconsistencies.
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Cold forging typically refers to forging a metal at room temperature, though any temperature below recrystallization is possible. Many metals, such as steel high in carbon, are simply too strong for cold forging. Despite this hindrance, cold forging does edge out its warmer equivalent when it comes to standards of dimensional control, product uniformity, surface finish, and contamination. Cold forging encompasses numerous forging techniques, including bending, extruding, cold drawing, coining, and cold heading. However, this increased versatility comes at a cost, because cold forging requires more powerful equipment and may call for the use of intermediate anneals.
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For more detailed info on either of these processes please read our Full Guide to Hot Forging and Cold Forging.
Hot forging being used to create automotive parts. Image credit: Shutterstock.com/Aumm graphixphoto
Beyond basic hot and cold forging, many specific processes exist. This broad range of processes can be grouped into three primary umbrella groups:
Draw forming decreases the width of
the product and increases length. Upset forging increases the width of the products and decreases length. Compression forming provides forging flow in multiple or customized directions.
These three categories entail many different specific types of metal forging methods.
Drop forging gets its name from the process of dropping a hammer onto the metal to mold it into the shape of the die. The die is the surface that comes into contact with the metal. There are two types of drop forging: open-die and closed-die forging. Dies are typically flat in shape with some having distinctively shaped surfaces for specialized operations.
When flat dies that have no precut profiles engage in forging, the forge process is called open die forging (or smith forging). The open design allows the metal to flow everywhere except where it touches the die. To achieve maximum results, correct movement of the workpiece, which should be over 200,000 lbs. in weight and 80 feet long, is essential. It is useful for short-run art smithing or for shaping ingots prior to secondary shaping measures. Open die forging creates pieces with better fatigue resistance and strength and reduces the chance of error or holes. It can also be used for a finer grain size than other processes.
Closed die forging, sometimes called impression die forging, employs the use of molds. These molds are attached to an anvil while a hammer forces molten metal to flow into the cavities of the die. Multiple strikes and/or die cavities are often used when forging complex geometries. High initial tooling costs make closed die forging expensive for short-run operations, but the forging process becomes cost-effective as parts produced increases. Closed die forging also provides exceptional strength over alternative methods. Common applications of closed die forging include the production of automobile components and hardware tools.
In press forging, the main forming factor is compression. The metal sits on a stationary die while a compression die applies continuous pressure, achieving the desired shape. The metal's contact time with the dies is considerably longer than other types of forging, but the forging process benefits from being able to simultaneously deform the entire product, as opposed to a localized section. Another benefit of press forging is the ability of the manufacturer to monitor and control the specific compression rate. Applications of press forging are numerous, as there are relatively no limits to the size of product that can be created. Press forging can be hot or cold forged.
Roll forging is the process of increasing rods or wires in length. The manufacturer places heated metal bars between two cylindrical rolls with grooves, which rotate and apply progressive pressure to shape the metal. The precisely shaped geometry of these grooves forges the metal part to the desired shape. The benefits of this forging method include the elimination of flashing and a favorable grain structure. While roll forging uses rolls to produce parts and components, it is still considered a metal forging process and not a rolling process. Roll forging is frequently used to make parts for the automotive industry. It is also used to forge things like knives and hand tools.
Upset forging is a forging process that increases the diameter of the metal through compression. Crank presses, a particular high-speed machine, are used in upset forging processes. Crank presses are characteristically set on a horizontal plane to improve efficiency and the quick metal exchange from one station to the next. Vertical crank presses or hydraulic presses are also used. The advantages of this process are that it enables a high production rate of up to 4500 parts per hour and full automation is possible. It also produces little to no waste.
Isothermal forging is a forging process where the materials and the die are heated to the same temperature. The name comes from [iso" which means "equal." This forging method is commonly used for forging aluminium, which has a lower forging temperature than other metals such as steel. Forging temperatures for aluminum are around 430 °C, while steels and super alloys can be 930 to 1,260 °C. The benefits are the near net shapes lead to lower machining requirements and, therefore, lower scrap rates, and the metal part is highly reproducible. Another advantage is that smaller machines can be used to make the forging due to the lower heat loss. A few disadvantages are the higher die material costs to handle temperatures and pressures and the required uniform heating systems. It also has a low production rate.
The most popular type of forging equipment is the hammer and anvil. The idea behind the hammer and anvil is still used today in drop hammer forging equipment. The hammer is raised and then dropped or propelled into the workpiece, which rests on the anvil. The main variations between drop hammers are how the hammer is powered, the most common being air and steam hammers. Drop hammers typically operate in a vertical position. This is because the excess energy that isn't released as heat or sound, meaning energy that isn't used to shape the workpiece, needs to be conveyed to the foundation. A large machine base is also required to absorb the impacts.
To overcome some shortcomings of the drop hammer, the counterblow machine or impactor is used. Both the hammer and anvil move in a counterblow machine, with the workpiece held between them. Here, excess energy becomes recoil, allowing the machine to work horizontally and have a smaller base. This creates less noise, heat, and vibration. It also creates a distinctly different flow pattern. These machines are used for open die or closed die forging.
A press is used for press forging. The two main types are mechanical and hydraulic presses. Mechanical presses function using cams, cranks, and toggles to make preset and reproducible hammer strikes. Because of the characteristics of this type of system, different forces are available at different stroke positions. As a result, these presses are faster than their hydraulic counterparts by 50 strokes per minute. Their capacities range from three to 160 MN. Hydraulic presses use fluid pressure and a piston to produce force. The advantages of a hydraulic over a mechanical are its flexibility and superior capacity. The disadvantages are that it is a slower, larger, and costlier machine to operate.
The roll forging, automatic hot forging, and upsetting processes all use specialized machinery.
This guide provides a basic understanding of what forging is and the different forging processes. To find out more about other methods of metal fabrication, read our guide here. For more information on related services consult our other product guides or visit the Thomas Supplier Discovery Platform to locate potential sources or view details on specific products.
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