What Is Forging?- Definition, Process, And Types

Forging is a manufacturing process involving the shaping of a metal through hammering, pressing, or rolling.

These compressive forces are delivered with a hammer or die. Forging is often categorized according to the temperature at which it is performed—cold, warm, or hot forging.

A wide range of metals can be forged. Typical metals used in forging include carbon steel, alloy steel, and stainless steel.

Very soft metals such as aluminum, brass, and copper can also be forged. The forging process can produce parts with superb mechanical properties with minimum waste.

The basic concept is that the original metal is plastically deformed to the desired geometric shape—giving it higher fatigue resistance and strength.

The process is economically sound with the ability to mass-produce parts and achieve specific mechanical properties in the finished product.

What is forging?

Forging is a manufacturing process where metal is pressed, pounded or squeezed under great pressure into high-strength parts known as forgings. The process is normally (but not always) performed hot by preheating the metal to a desired temperature before it is worked.

It is important to note that the forging process is entirely different from the casting (or foundry) process, as metal used to make forged parts is never melted and poured (as in the casting process).

History of forging

Forging is one of the oldest known metalworking processes.

Traditionally, forging was performed by a smith using a hammer and anvil, though introducing water power to the production and working of iron in the 12th century allowed the use of large trip hammers or power hammers that increased the amount and size of iron that could be produced and forged.

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.

Types of Forging Processes

There are several forging methods with different capabilities and benefits.

• Drop forging
• Roll forging
• Press forging
• Upset forging
• Cold Forging
• Precision forging
• Isothermal forging

#1. Drop Forging.

Drop forging derives its name from the process of dropping a hammer onto the metal to mold it into the shape of the die. The die refers to the surfaces that come 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.

Open-die forging (smith forging)

Open-die forging is also known as smith forging. A hammer strikes and deforms a metal on a stationary anvil. In this type of forging, the metal is never completely confined in the dies—allowing it to flow except for the areas where it is in contact with the dies.

It is the operator’s responsibility to orient and position the metal to achieve the desired final shape. Flat dies are used, with some having specially shaped surfaces for specialized operations. Open-die forging is suitable for simple and large parts, as well as customized metal components.

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)

Closed-die forging is also known as impression-die forging. The metal is placed in a die and attached to an anvil. The hammer is dropped onto the metal, causing it to flow and fill the die cavities.

The hammer is timed to come into contact with the metal in quick succession on a scale of milliseconds. Excess metal is pushed out from the die cavities, resulting in a flash.

The flash cools faster than the rest of the material, making it stronger than the metal in the die. After forging, the flash is removed.

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.
• Blocking cavities: 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

#2. Roll forging.

Roll forging consists of two cylindrical or semi-cylindrical horizontal rolls that deform a round or flat bar stock. This works to reduce its thickness and increase its length.

This heated bar is inserted and passed between the two rolls each containing one or more shaped grooves—and is progressively shaped as it is rolled through the machine. This process continues until the desired shape and size are achieved.

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

#3. Press forging.

Press forging uses a slow, continuous pressure or force, instead of the impact used in drop-hammer forging. The slower ram travel means that the deformation reaches deeper so that the entire volume of the metal is uniformly affected.

Contrastingly, in drop-hammer forging, the deformation is often only at the surface level while the metal’s interior stays somewhat undeformed. By controlling the compression rate in press forging, the internal strain can also be controlled.

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 tons

#4. Upset forging.

Upset forging is a manufacturing process that increases the diameter of the metal by compressing its length. Crank presses, a special high-speed machine, are used in upset forging processes.

Crank presses are typically set on a horizontal plane to improve efficiency and the quick exchange of metal from one station to the next. Vertical crank presses or a hydraulic press are also options.

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

#5. Automatic hot forging.

In automatic hot forging, mill-length steel bars are inserted into one end of the forging machine at room temperature, and hot forged products emerge from the other end.

The bar is heated with high-power induction coils to a temperature ranging from 2190–2370°F in under 60 seconds.

The bar is descaled with rollers and shared into blanks. At this point, the metal is transferred through several forming stages that can be coupled with high-speed cold-forming operations.

Typically, the cold-forming operation is left for the finishing stage. By doing so, the benefits of cold-working can be reaped while also maintaining the high speed of automatic hot forging.

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)

#6. Precision forging (net-shape or near-net-shape forging).

Precision forging requires little to no final machining. It is a forging method developed to minimize the cost and waste associated with post-forging operations. Cost savings are achieved from the reduction of material and energy, as well as the reduction of machining.

#7. Isothermal forging.

Isothermal forging is a forging process where the metal and die are heated to the same temperature. Adiabatic heating is used there is no net transfer of mass or thermal exchange between the system and the external environment.

The changes are all due to internal changes resulting in highly controlled strain rates. Due to the lower heat loss, smaller machines may be used for this forging process.

What metals are forged?

Just about any metal can be forged. However, some of the most common metals include: carbon, alloy and stainless steels; very hard tool steels; aluminum; titanium; brass and copper; and high-temperature alloys which contain cobalt, nickel or molybdenum.

Each metal has distinct strength or weight characteristics that best apply to specific parts as determined by the customer.

What kind of equipment is used to make forgings?

There are several primary tools that are used in the metal forging process depending on the exact method being used.

Forge

The forge is the primary heating apparatus used to reach the required temperature for forging. Modern forges utilize gas or electric heat sources.

Hammers

The hammer, or power hammer, is a tool most commonly associated with forging. Whether a hand-held hammer or a massive power hammer, the tool is used to repeatedly hit the metal in order to deform it.

As long as it possesses a 50,000 lbs driving force to deliver high-pressure impact blows, a hammer can pound metal into shape.

Presses

Presses use either mechanical or hydraulic pressure to apply continuous pressure on forging dies.

This kind of equipment requires a 50,000 ton driving force to vertically squeeze metal into die cavities with controlled high pressure. Instead of hitting the metal repeatedly to deform it, the metal is slowly pressed into the dies.

Upsetters

Upsetter forging is similar to press forging, however, the main difference is that an upsetter is a forging press that is used horizontally. Instead of forcing the metal downward into a die, the metal is moved into the die impression in a horizontal direction.

Ring Rollers

Ring rollers are used to produce rings with diameters from just a few inches to over 300 inches. Ring rollers squeeze out a one-piece ring, which removes the need for welding. It turns a hollow round piece of metal under extreme pressure against a rotating roll.

How Does Forging Strengthen Metal?

Compared to other manufacturing methods, metal forging is known to produce some of the strongest manufactured parts available.  As the metal is heated and pressed, minor cracks are sealed and empty spaces found in the metal are closed up.

In addition, the hot forging process breaks up any impurities in the metal and redistributes such material across the metalwork.

This results in vastly reduced inclusions in the forged part. Inclusions are compound materials embedded inside steel throughout manufacturing, causing stress points in the product.

Even though impurities should be managed during the initial casting process, this process will further refine the metal.

Another way that forging strengthens metal is through the altering of its grain structure. This has to do with the material’s grain flow as it deforms. Like other forming processes, a favorable grain structure can be created, making the forged metal sturdier.

Application of Forging

In addition to engine and transmission parts, forgings are used for a wide variety of gears, sprockets, levers, shafts, spindles, ball joints, wheel hubs, rollers, yokes, axle beams, bearing holders, and links.

• Mainly forged products are used in every mechanical industry.
• Turbine rotor, generator rotor, etc. are forged products.
• It gives higher fatigue strength so most of the moving parts like crankshaft, camshaft gears, etc. are made by forging operation.
• Cold forging is used to produce chisel, bolts, etc.
• These are mostly used in hand tools and hardware manufacturing.
• It is used in shipbuilding in various structure works.

Advantages of Forging

Forging provides better mechanical properties, ductility and fatigue, and impact resistance because this process refines and directs the grain flow according to the shape of the piece. Almost all metals, ferrous and non-ferrous can be forged.

• Parts manufactured by forging are stronger.
• It is more reliable and less costly.
• It offers better response to heat treatment.
• It offers more consistent and better metallurgical properties.
• It offers broad size range of products.
• It requires fewer secondary operations.
• It has great design flexibility.

Disadvantages of forging

• In hot forging it is difficult to perform secondary operations.
• Capital cost is more.
• Very high man and material safety procedures need to be followed.

FAQs.