Understanding the Different Metal Forging Methods

Forging is a manufacturing process that many industries rely on. In fact, hundreds of forged components can be found in aircraft, vehicles, farming equipment, trains, mining machinery, and more. When compared to other processes, like casting and fabrication welding, forgings are stronger and more reliable. Additionally, because the metal forging process can produce complex geometries efficiently, it can be a more cost-effective manufacturing process.

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One common misconception is that forging is one universal method; in reality, there are several methods that a forging manufacturer may use. In general, forging can be classified in two ways: by the tooling and by temperature.

In this article, we’ll cover each forging method, as well as highlight the main types of equipment used in forging.

Let’s start with the first classification: forging by tooling

Forging by Tooling

The words “die” and “tooling” are often used interchangeably in the forging industry. They refer to the device that is used to shape a (usually pre-heated) billet, ingot, preform, or bar. When comparing forging by the tooling that is used, there are three primary methods: impression die, open die, and rolled ring.

Impression Die Forging

With impression die forging, metal is squeezed/compressed between two custom dies that each have pre-cut profiles/cavities (or impressions) of the final part shape. 

Then, using various types of equipment, like a mechanical press or a drop hammer, the billet or preform is compressed while it undergoes plastic deformation. In each impression, the workpiece is forced into the cavity as the flash (excess metal) flows out into the die. As the flash cools, pressure builds up in the impression which helps ensure a more uniform component. Then, the flash is removed either manually or with trimming dies.

Impression die forging is primarily used for manufacturing complex symmetrical and asymmetrical forged components. This method can also achieve closer tolerances when compared to open die forging.

Open Die Forging

In open die forging, a workpiece (ingot, bar, billet, or preform) is deformed between an upper and lower flat-faced die, and the movement of the workpiece itself is largely what helps shape the final part. 

In addition to flat-faced dies and hammers (or presses), there are other tools that can be used in open die forging, including saddles, blocks, rings, mandrels, and punches.

The components made in open die forging are usually simple in shape (long and rectangular, flat and round, etc.) and can be further machined after the forging process. 

Open die forging is used to produce components of all sizes, but, when it comes to complexity, impression die forging is usually the preferred method over open die forging.

Rolled Ring Forging

The last method by tooling is rolled ring forging. Unlike impression die and open die, the roll forging method does not utilize dies, and it is used to make very specific shapes: rings. 

This forging process can be used to make all types and sizes of seamless rings. Roll forging is preferred over machining, cutting, and casting because it can produce components to tight tolerances. Additionally, forging eliminates internal porosity, unlike casting, which reduces creep, fatigue, and other issues.

In roll forging, the stock is usually preformed. The preform technique used in roll forging is called punching. As the name implies, a hole is punched into the stock, creating a donut shape. Then, the preform is run through a rolling mill until the final component is produced.

All types of components can be created by roll forging, including engine bearings, wheel bearings, flanges, and gears.

Forging Processes by Temperature

Metal forging processes can also be categorized by the actual temperature of the workpiece. Different temperatures affect the plasticity of the metal, the final quality of the piece, and can determine many of the component’s final mechanical properties, including strength, ductility, and resistance to impact or fatigue.

When categorizing forging by temperature, there are three primary methods: hot, warm, and cold.

Hot Forging

In hot forging, typically the dies/tooling are pre-heated to a specific temperature and then the workpiece itself is heated to a predetermined temperature. The temperature used in hot forging varies based on the raw material being used, the component that is being produced, and any specific application requirements.

In hot forging, heat makes the metal more ductile, and the consistent pressure of the dies squeezing hot metal produces a more refined grain structure and thus stronger, more ductile components.

Warm Forging

In warm forging, a workpiece is heated within a specific range: above work hardening temperature, but below the scaling temperature. 

Warm forging covers a range of forged products that do not require the expense and tight tolerances of cold forging or the higher strength yield of hot forging. Warm metal forging is typically used for parts that are already close to their final shape, such as shafts and gears.

Cold Forging

As the name suggests, the cold forging process is performed at cooler temperatures (anywhere between room temperature to just a few hundred degrees). 

Cold forging also utilizes other processes including bending, coining, rolling, and extruding, depending on the type of component that is being produced. The metal used in the process is also typically annealed or softened during the forging process, and continuous lubrication is required during this process to prevent frictional heat. 

Although cold forging is extremely precise, and often requires little to no finishing work, hot forging is still the preferred method for complex, custom forgings. Additionally, components that are cold-forged are more likely to contain residual stresses, which can eventually lead to cracking, warping, and other issues. 

Equipment Used in Forging

If you ever see or hear terms like press forging or hammer forging, this is referring to the type of equipment being used during a particular forging process.

Presses

During open die or impression die forging, manufacturers may use a press, which allows for the controlled deformation of a workpiece through high pressure. There are several types of presses: screw, hydraulic, and mechanical. 

A screw press drives a power ram up or down on a screw shaft to press-form metal. A hydraulic press uses a cylinder under hydraulic oil pressure, and a mechanical press is actuated by a gear, lever, or other mechanical operator connected to a motor and a crankshaft.

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Drop Hammers

Next, a drop hammer, as the name implies, uses the power of a heavy metal hammer that when dropped from a height (gravity-powered) or power-assisted, will strike the workpiece. The powerful blow releases kinetic energy to deform the metal, and typically about three blows at each stage is enough to shape the molten metal, although this number can vary.

Counterblow Hammers

For very large metal parts, a counterblow hammer imparts high-impact energy of a tonnage hammer onto a die. The hammer moves in opposite directions with each blow and offers more precise formation of large forgings. Counterblow hammers may be operated by hydraulic or pneumatic drive systems.

Ring Rollers/Rolling Mills

A ring roller, rolling mill, or reduction mill is mostly used in seamless rolled ring forging, but can also be used to preform stock during open die or impression die forging. In the rolling mill, a preform is passed through the machine’s rolls, which then shapes the final component (additional machining of the component may be required after it passes through the rolling mill). Some rolling mills even include a manipulator, which is used to punch the stock.

Is One Forging Method Better Than the Other?

There is no particular forging method that is inherently “better” than the other. Each will have unique advantages depending on the size, shape, and quantity of components, the complexity of the forged parts, or any demanding strength or mechanical properties.

Some part geometries are also better suited to certain processes and equipment. In one of our previous articles, we’ve highlighted some of the many benefits of each forging process. Click here to learn more about the advantages of each process.

Have Questions? Contact Trenton Forging Today

Trenton Forging is a U.S.-based forging manufacturer that specializes in both hot forging and impression die forging. We are capable of producing both symmetrical and asymmetrical custom components from steel, stainless steel, and steel alloys. Some of our other capabilities include prototyping and reverse engineering, custom tooling, and machining. 

If you have general questions about our metal forging process, or would like to request a quote, fill out our online form today.

Finding the right forging method for your metals requires knowing your options. Here is the complete guide on open die forging vs. closed die forging.

As of 2022, the global metal market is worth around $3,949 billion. This makes sense considering the number of applications metal has in our daily lives. Metal is a staple of our existence, from movable bridges to aluminum space equipment to consumer electronics.

Open die vs. closed die forging are two metal forming options for those who want to have custom metal parts manufactured. Here, we will discuss these two metal forming processes and how they impact grain flow and utility. Read on to learn which approach best meets your needs.

What Is Open Die Forging?

Open die forging is a metal forming process that uses multiple dies. None of these dies completely enclose the material as the component is being shaped. Instead, the dies hammer the metal over and over until the forger creates the desired end shape.

Before the process begins, the metal is heated in a furnace. This makes it malleable so that dies can alter its shape.

Hammering and pressing the metal lets you achieve any shape. Assuming that the expert making the material understands metalworking, they will know exactly how to change the billet's dimensions without needing to fully enclose the metal. Open dies are simpler than closed dies because they do not require a specific shape to contain and shape the metal.

Open Die Grain Flow, Benefits and Downsides

The grain flow structure is one of the most significant open die forging benefits. As the metal is worked into the desired shape, grain flow is strengthened as it follows the contour of the piece. The grain flow movement is possible because force is applied to the metal during various forging operations, including upsetting, drawing out and/or rolling.

These methods of forging create a final product that is not porous. In fact, the final product will be stronger and more durable due to its even grain flow and improved microstructure.

Some other open die forging benefits include:

• Continuous grain flow and finer grain size
• Consolidation of voids in the material
• Greater fatigue resistance for long-term use
• Eliminates shrinkage and similar defects
• Less material waste (eco-friendly)
• Better production scheduling as different parts can run through the shop at the same time

There are also a few disadvantages:

• Usually requires some machining to create the correct dimensions
• The inability to create shapes with intricate inner passages
• Rough surface finish (unless refined later)

Open die forging is the most conventional metal forging process available. As a result, despite its shortcomings, it is ideal for R&D, prototypes, oversized components or small-run applications.

How Is Closed Die Forging Different?

Closed die forging is a slightly different process than open die alternatives. It is not a free forging process. Instead, it is often called "impression die forging" because it uses a die impression to give the metal its shape.

Like open die forging, the metal for closed die is heated in a medium frequency induction heating furnace. However, it is then placed in an enclosed die molded in the desired shape of the final product. Then, the metal is compressed and formed into the end product through high pressure or multiple hammer impacts.

Molding dies are made with precision machines before a metal preform is placed between the top and bottom dies. Depending on the component type, machining might not be needed after the end product is removed from the die. This is because the closed die process creates intricate components depending on the detail of the dies.

Closed Die Advantages and Disadvantages

Like open die forging, internal grain flow is formed since material reduction is involved in creating the product.

There are some things closed die forging excels at:

• A cost-efficiencies are realized with this method for large production runs
• Little machining is required in most cases (for close tolerances)
• The ability to create tighter tolerances

Still, other areas are lacking compared to open die forging. These downsides include:

• High costs are associated with the manufacturing of the dies making this process too expensive for short production runs (since the die production costs are high)
• The time to produce the dies can push out your deadline
• Constrained capacity in closed die shops (Once a run is booked, they are locked into that one part until production is complete)

Ultimately, certain applications require closed die forging methods. However, it is less conventional than open die methods.

Which Metal Forging Process Should You Choose?

Open die forging is generally used for products manufactured in small quantities, require large sizes, and are critical for the end user or R&D. Generally, open die forging shapes are easy and used for more straightforward shaped products (rather than complex ones).

Some examples of products made from open die forging include:

• Small discs
• Forged and rolled rings
• Forged cylinders
• Domes
• Rollers
• Forged shafts
• Metal sleeves

These products are often used in shipbuilding, mining, defense and energy industries. However, they are also used in the machines that manufacture semiconductors, food/beverage equipment, industrial machinery, paper and more. Furthermore, with a skilled Forge Development team, like the one at Scot Forge, many complex shapes can be manufactured through the open die forging methods.

On the flip side, the closed-die forging process is generally reserved for creating smaller and more complex products and large production runs.

Some items made by closed die forging techniques include:

• Forged fittings
• Lifting hardware
• Rigging hardware
• Small/precise car parts
• Valves

Because of the precision level, closed die forging is essential in industries like commercial aircraft and automotive markets. In general, forgings keep users safe from part malfunction due to the contoured grain flow resulting in higher strength components.

Get Started With Metal Forming Today

While finding the perfect metal piece for your applications can be challenging, it doesn't need to be. Custom forging services can give you tailored parts that meet your precise specifications.

Scot Forge is committed to creating components you can rely on from quality to delivery. Request a quote to learn if forgings fit your project's needs and the best method of manufacture available to you.

Contact us to discuss your requirements of seamless rolled ring forging factories. Our experienced sales team can help you identify the options that best suit your needs.