Blog 29: What is forging?
Metal is shaped by compressive, localized forces during the forging process, a manufacturing method for metal shaping. Forging presses or hammering tools with electrical, hydraulic, or compressed air power are most frequently used nowadays to execute forging. A hammer or die is used to deliver these compressive forces. Cold, warm, or hot forging are the common classifications used to describe forging based on the temperature at which it is carried out.
Since the earliest Mesopotamians, it has been a method for fabricating metal. Forging has undergone major improvements since it first emerged in the fertile crescent, making it a more effective, quick, and durable technique.
Metals of all types can be forged. Stainless steel, alloy steel, and carbon steel are frequently used metals in forging. Aluminum, brass, titanium, and copper are examples of very soft metals that can be forged. With the least amount of waste, the forging process may create products with exceptional mechanical qualities. The main idea is to plastically deform the original metal to the required geometric shape, increasing its strength and fatigue resistance. The method can make parts in large quantityes and obtain precise mechanical qualities in the final product, making it economically sound.
History of forging
Forging has been a skill used byblack smiths for a very long time. In the Bronze Age, bronze and copper were the most frequently used forging metals; later, iron replaced them as the most popular forging metal as temperature control and the method of smelting iron were found.
Kitchenware, hardware, hand tools, and edged weapons are examples of traditional items. Forging was able to develop into a more productive, mass-production process because to the Industrial Revolution. Since then, equipment, robots, electronic controls, and automation have all advanced along with forging. Modern forging facilities produce high-quality metal parts in a broad variety of sizes, shapes, materials, and finishes, making forging a global industry today.
Types of forging depending upon temperature?
Unbroken grain flow is produced as a result of the metal’s deformation and shaping during forging. The metal maintains its strength as a result. This particular grain flow has additional benefits, such as removing product flaws, inclusions, and porosity. Forging can be divided into two categories with respect to temperature hot and cold forging.
The metal must be heated above its recrystallization temperature in order to be hot forged. This could involve heating metals to as much as 2,300 degrees Fahrenheit. The reduction in energy needed to form the metal properly is the main advantage of hot forging. This is due to the fact that extreme heat reduces yield strength while increasing ductility. Chemical irregularities are also removed, which benefits hot forged items.
Although any temperature below that of recrystallization is feasible, cold forging normally refers to the forging of a metal at ambient temperature. Cold forging simply cannot be done with many metals, such as steel with a high carbon content. Despite this obstacle, cold forging consistently outperforms its warmer counterpart in terms of contamination, surface polish, homogeneity, and dimensional control. Numerous forging processes are included in cold forging, such as bending, extruding, cold drawing, coining, and cold heading. The downside to this greater adaptability is that cold forging often necessitates the use of intermediate anneals and more powerful equipment.
There are several forging methods with different capabilities and benefits. Some of the basic methods of forging are mentioned below:
Drop forging gets its name from the technique of striking metal with a hammer before shaping it to fit a die. The metal-contacting surfaces are referred to as the die. Drop forging includes two types: open-die and closed-die forging. Die shapes are normally flat, while some have surfaces that are specifically formed for certain activities.
Open-die forging (smith forging)
Smith forging, also referred to as open-die forging, is the process of forging using flat dies without precut profiles. On a stationary anvil, a metal is struck with a hammer and bent. The metal can flow throughout the process of forging like manner, with the exception of the locations where it comes into contact with the dies. The metal must be oriented and placed by the operator to take the final shape they want. Flat dies are employed, some of which have surfaces that have been particularly designed for certain processes. Simple, massive, and unique metal products can all be produced with open-die forging.
Correct movement of the workpiece, which should weigh more than 200,000 lbs. and be 80 feet long, is crucial for getting the best results. Open die forging decreases the possibility of inaccuracy or holes while producing products with improved fatigue resistance and strength. It can also be used to produce grains with a finer texture than other methods.
Advantages of open-die forging:
- Better fatigue resistance and strength
- Reduces chance of error and/or holes
- Improves microstructure
- Continuous grain flow
- Finer grain size
Closed-die forging (impression-die)
Forging with dies is referred to as closed-die forging or impression-die forging. The metal is put into a die, which is then fastened to an anvil. The metal flows and fills the die cavities as a result of the hammer being dropped onto it. On a millisecond time scale, the hammer is timed to strike the metal multiple times in rapid succession. Flash occurs when too much metal is forced out of the die cavities. The flash is stronger than the metal in the die because it cools more quickly than the remainder of the material. Afterwards, the flash is eliminated.
Closed die forging is expensive for short-run operations because to high initial tooling costs, but as part production increases, the forging process becomes more affordable. Compared to other techniques, closed die forging also offers outstanding strength. Closed die forging is frequently used to produce hardware tools and parts for automobiles.
In order for the metal to reach the final stage, it is moved through a series of cavities in a die:
Edging impression (also known as fullering or bending)
The first impression used to mold the metal into a rough shape.
The metal is worked into a shape that more closely resembles the final product. The metal is shaped with generous bends and fillets.
Final impression cavity
Final stage of finishing and detailing the metal into the desired shape.
Advantages of closed-die forging:
- Produces parts up to 25 tons
- Produces near net shapes that require only a small amount of finishing
- Economic for heavy production
Increasing the length of rods or wires involves roll forging. A round or flat bar stock is deformed by the manufacturer by sandwiching hot metal bars between two horizontal cylindrical or semi-cylindrical rollers. This helps to lengthen it while decreasing its thickness. This heated bar is rolled through the apparatus between two rolls that each have one or more formed grooves, where it is gradually sculpted. This procedure is repeated until the required size and form are obtained.
The elimination of flashing and the good grain structure with this forging technique are advantages. Roll forging is a metal forging method rather than a rolling technique, despite the fact that it uses rolls to create parts and components. The automotive sector regularly uses roll forging to create parts. Additionally, it is employed in the forging of hand tools and knives.
Advantages of automatic roll forging:
- Produces little to no material waste
- Creates a favorable grain structure in the metal
- Reduces the cross-sectional area of the metal
- Produces taper ends
Instead of using an impact like drop-hammer forging does, press forging uses a slow, continuous pressure or force on the metal that is resting on a stationary die while a compression die delivers constant pressure to get the required shape. Due to the deeper deformation caused by the slower ram passage, the entire volume of the metal is uniformly impacted. Contrarily, with drop-hammer forging, the deformation frequently only occurs at the surface level while the inside of the metal maintains some of its original shape. In press forging, the internal strain can also be adjusted by adjusting the compression rate.
The manufacturer’s capacity to track and manage the particular compression rate is another advantage of press forging. Press forging has several uses because there are few restrictions on the size of the products that can be produced. Cold or hot forging can be used in press forging.
Advantages of press forging:
- Economic for heavy production
- Greater accuracy in tolerances within 0.01–0.02 inch
- Dies have less draft allowing for better dimensional accuracy
- Speed, pressure, and travel of the die are automatically controlled
- Process automation is possible
- Capacity of presses range from 500–9000
In the manufacturing process known as upset forging, the length of the metal is compressed, increasing the diameter of the metal. In upset forging procedures, unique high-speed machines called crank presses are employed. To increase productivity and facilitate the swift transfer of metal from one station to the next, crank presses are often mounted on a horizontal plane. Other alternatives include hydraulic presses or vertical crank presses.
Advantages of upset forging:
- High production rate of up to 4500 parts per hour
- Full automation is possible
- Elimination of the forging draft and flash
- Produces little to no waste
Automatic hot forging
When using an automatic hot forging machine, mill-length steel bars are placed into one end at room temperature, and hot forged products come out of the other. High-power induction coils quickly heat the bar to a temperature between 2190 and 2370 °F. The bar is shared into blanks after being descaled with rollers. The metal is now moved through a number of shaping steps that can be combined with quick cold-forming procedures. The cold-forming process is often saved for the finishing phase. By doing this, it is possible to maintain the high speed of automatic hot forging while also enjoying the advantages of cold-working.
Advantages of automatic hot forging:
- High output rate
- Acceptance of low-cost materials
- Minimal labor required to operate machinery
- Produces little to no material waste (material savings between 20–30% over conventional forging)
Precision forging (net-shape or near-net-shape forging)
Little to no finishing machining is necessary for precision forging. It is a forging technique created to reduce waste and expenses related to post-forging processes. The decrease of machining as well as the reduction of material and energy consumption result in cost savings.
The materials and the die are both heated to the same temperature during the isothermal forging process. The word “iso” means “equal,” hence the name. With a lower forging temperature than other metals like steel, such as aluminium, this forging technique is frequently employed to forge aluminium. Forging temperatures for steels and super alloys can range from 930 to 1,260 °C, whereas those for aluminium are at 430 °C. Benefits include cheaper machining costs and decreased scrap rates due to near net forms and excellent reproducibility of the metal product. Due to the reduced heat loss, smaller machinery can be employed to produce the forging, which is an additional benefit. 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.