Forging – Process, Classification & Methods

Forging is a manufacturing process that shapes metals through hammering, pressing or rolling. Its origins can be traced back to Mesopotamia as early as 4000 BC. The process naturally evolved from simple hammering, as early humans discovered that heating metal made it easier to shape. Blacksmiths used furnaces to heat metals, enabling them to craft weapons and tools such as swords and agricultural implements.

Over the centuries, forging has undergone significant advancements to meet the demands of modern industries. Let’s get into it.

What Is Forging?

Forging is a manufacturing process that uses heat and high pressure to shape metal into the desired form. When heated to elevated temperatures, raw metal becomes malleable. Manufacturers may employ manual force, hydraulic presses and other specialised equipment to shape the metal with relative ease.

Most forged products today are manufactured using electrically powered presses. However, certain specialty items, such as Damascus steel knives, are still crafted using traditional manual methods.

Some of the popular metals used in forging include alloy steel, aluminium, brass, carbon steel, copper, duplex steel, nickel, stainless steel, titanium, and tool steel. Generally, metals that are not highly brittle can undergo forging processes. On the other hand, materials like cast iron, certain high-carbon steels, and other brittle alloys are unsuitable for forging due to their inability to withstand impact loads.

Forged parts provide numerous advantages, including a refined grain structure, higher fatigue resistance and strength. Additionally, the final product is free from defects such as porosity, cracks and blowholes.

Forging can be engineered to mass-produce parts without using excess metal. It is widely used in safety and performance-oriented industries, including aerospace, automotive, and oil and gas, for critical components such as crankshafts, high-pressure valves, ball joints, cams and gears.

Forging Process Steps

Heat and high pressure soften most metals, allowing forging to be effective with a wide range of materials. However, because the properties of different metals vary, the forging process must be adapted accordingly. Let’s explore a typical forging process to understand its main steps:

Die Design and Manufacturing

Modern forging processes use dies to enhance the accuracy, precision and speed of parts produced. Die design and manufacturing is the first step in the forging process. A good die can provide advantages such as better grain flow, good surface finish, less wastage, improved dimensional accuracy and consistent production quality.

Billet/Ingot Preparation

Billets or ingots with varying cross-sections serve as raw materials for the forging process. Depending on the product specifications, the required lengths of the appropriate cross-sections are cut and incorporated into the forging production line.

Heating

There’s no point beating on a metal that is cold. In this step, the metal is heated, typically in a furnace, to its forging temperature. The forging temperature for steel ranges from 850 to 1150 degrees Celsius, while the ideal forging temperature for aluminum is up to 500 degrees Celsius. The specific heating and soaking temperatures for the forging process are determined based on the type of metal being used.

Forging

The heated metal is moved to the die, where it is pressed into shape. Multiple passes through various dies may be necessary to achieve the final form. Additionally, the part may require reheating between presses.

Heat Treatment

Most components are subjected to heat treatment following forging to enhance specific mechanical properties, including strength and hardness. The heat treatment processes that are normally used are annealing, tempering, quenching, normalising, solution treatment and case hardening.

Cooling

Cooling presents an excellent opportunity to enhance properties such as strength and grain structure. By employing various cooling mechanisms and rates, the development of desirable characteristics in forged components can be promoted.

Finishing

The component may require finishing operations, such as machining, trimming and surface treatment, before it is fit for use. Additionally, properties like corrosion resistance and aesthetic appeal can be enhanced by applying specific coatings during this stage.

How to Classify Forging Processes

Constant research and advancements in metallurgy have led to the development of several variations of the forging process. Today, a wide range of forging techniques are used to achieve different combinations of properties in the final product. To better understand the various types of forging processes, we will employ two primary classification methods. Forging processes can be classified by –

• Temperature

• Tooling and Types of Equipment

Classification by Temperature

There are three primary types of forging processes categorised by temperature: cold forging, warm or hot forging.

Hot Forging

Hot forging is the original forging procedure where the metal is heated to a temperature that exceeds its recrystallisation temperature. This temperature is typically between 0.3 to 0.4 times the melting point of pure metals, while it is approximately halfway for alloys. At this elevated temperature, the metal’s microstructure alleviates any internal stresses and forms new grains, thereby enhancing mechanical properties such as strength and ductility.

Once the metal reaches this temperature, a compressive force is applied to mould it into the desired shape. Hot-forged products require lower levels of compressive force, as the material deforms more easily at higher temperatures.

However, hot forging has several disadvantages. It offers lower precision, and the final surface finish is rougher compared to other processes. Additionally, the workpiece is susceptible to defects such as oxidation, decarburisation and burn damage.

Warm Forging

In warm forging, the metal is heated above room temperature while still remaining significantly below the recrystallisation temperature. The heat makes the metal more pliable. Compared to hot forging, warm forging results in a smoother surface and greater precision, but it also presents higher deformation resistance. Consequently, greater compressive forces are necessary to shape the metal in warm forging than in hot forging.

Cold Forging

A forging process conducted at room temperature is referred to as cold forging. In this process, the metal is not heated; instead, compressive forces are applied to shape the material directly. This technique yields superior surface finishes, high dimensional accuracy, and parts that are ready for immediate use. Additionally, cold forging is one of the simplest processes to automate.

However, the process requires high-tonnage presses, as significant compressive forces are necessary. The part is also prone to cracking due to its low plasticity at low temperatures.

By Tooling and Equipment Used

We can also use tooling and equipment to classify forging processes. Traditionally, blacksmiths used rudimentary equipment such as hammers and anvils to forge metals. Over time, water wheels powered large bellows, allowing bloomeries to reach higher furnace temperatures.

Then, with the Industrial Revolution, mechanical and hydraulic-powered presses and upsetters were introduced, using dies to produce more complex shapes through forging. Additionally, roll forging mechanisation was developed during this period, employing robust rollers to create elongated components with varying cross-sections.

Types of Forging Methods

The forging process branches out into various methods depending on factors such as process layout, types of parts produced, level of automation, and the type of equipment used. In this section, we will integrate these factors to provide an overview of some of the most popular forging processes used today. The methods we will cover include the following:

• Drop forging (open die forging and closed die )

• Press forging

• Upset forging 

• Automatic hot forging

• Roll forging

• Rolled ring forging

• Precision forging (cold forging)

• Isothermal forging

• Induction forging

• Swaging

Drop Forging

Open Die Forging

Drop forging is a manufacturing process in which a drop hammer is released onto a workpiece from a specific height. The workpiece is positioned on a stationary anvil. The forces generated by multiple impacts gradually compress the workpiece into the desired shape. 

Drop forging can be categorised into two types: open-die forging and closed-die forging. In open die forging, there is no die positioned above the workpiece; instead, the hammer makes direct contact with the surface of the workpiece. A variation of the open die forging process is known as cogging where the material is secured in place using metal claws, while the manipulators (metal dies) approach from either two or four directions to alter its shape.

On the contrary, closed-die forging encapsulates the workpiece within the die. Closed-die forging is also known as impression-die forging. The desired final shape can be cast into one or both of the dies (top and bottom). As the hammer strikes the top die, it forces the metal being forged to fill the die cavity.

The counterblow process is a variation of the drop forging technique. Unlike the drop forging process, which relies on gravity, the counterblow process uses high-pressure air or oil to accelerate the forging die cavities toward one another. The metal stock positioned between the dies absorbs the intense impact forces generated by the accelerating dies, allowing it to conform to the shape of the dies.

Press Forging

Press forging uses a principle similar to drop forging; however, instead of relying on impact, it uses a continuous force that lasts longer. It’s a controlled squeezing rather than a sudden impact.

The impact of drop forging does not deform the core of the metal workpiece; the effects are limited to the surface layer. In contrast, the continuous pressure applied in press forging results in plastic deformation throughout the entire volume of the part. Forging presses can use either a cold or hot forging process.

Upset Forging

Upset Forging

Upset forging, also known as heading, uses a series of dies to deform or upset one end of a workpiece. This process is commonly used to increase the cross-section of round bars at one end to form articles such as nails, bolts and screws.

The round bar is secured using gripping dies, while another die advances toward the exposed end, compresses it, and alters its shape.

Automatic Hot Forging

Automatic Hot Forging

The automatic hot forging method provides a quick and efficient means of mass-producing blanks or finished products from steel bars. On one side of the automatic hot forging mill, steel bars with specified lengths are fed into the machine. These bars are induction-heated, descaled, cut, and forged into finished products within seconds in the forging machine.

The parts may undergo several forming stages while inside the machine. The output from the forging machine is sometimes sent for cold or precision forging to obtain its benefits in the finished product.

Roll Forging

Fully Automated Roll Forging

Roll forging, similar to automatic forging, is a specialised forging process that can produce parts with a unique cross-section at a large scale.

Roll forging involves two opposing rolls that receive long bars with either a flat or round cross-section. As the bar passes through the gap between the rolls, it is compressed, resulting in a reduction of thickness and an increase in the length of the material.

Once it is through, the part may be reversed and passed again through the same or a different set of rollers if the final product requires multiple stages.

Roll forging can be of hot or cold type. Hot forging requires lighter compressive forces. The roll forging process results in an excellent grain structure for the workpiece.

Rolled Ring Forging

Ring Roll Forging

The rolled ring forging process produces seamless metal rings from the forging process. The process begins by piercing a cylindrical stock from the centre to create a donut shape. An idler roller is positioned in this hollow center. The workpiece is then rotated and manipulated using two different sets of rollers.

A driving roller presses the workpiece between itself and the idler roller, gradually increasing the size of the donut hole. Axial rollers, located at the top and bottom, compress the surfaces of the donut to reduce its thickness (or cylindrical height).

The final product is a ring-shaped component that has no seams and an excellent grain structure derived from the original workpiece. Rolled ring forging is typically used for critical components, such as gears, clutches and bearings, which require seamless parts for both functional and safety purposes.

Precision Forging

Precision forging, also known as net-shape or near-net shape forging, is a forging process conducted at significantly lower temperatures than those used in hot forging, allowing for enhanced precision. For steel, the temperature range in precision forging is between 750 and 950 degrees Celsius (1380 to 1740 degrees Fahrenheit).

Precision forging is primarily a finishing process rather than a standalone operation. Components produced from other forging methods are sent for precision forging before being put to use or undergoing additional manufacturing processes.

Cold Forging

Precision forging can also be performed at near-ambient temperatures, a process commonly referred to as cold forging. This method yields highly accurate final dimensions and typically eliminates the need for additional finishing operations. However, it is not suitable for all types of metals, and lubrication plays a crucial role in preserving the mating surfaces of the dies. Cold forging is frequently employed for soft metals such as aluminium.

Isothermal Forging

As the name suggests, isothermal forging is conducted at a fixed temperature. Both the blank and the die are kept at a uniform temperature throughout the entire forging process.

These specific conditions allow to fully use the plasticity of a metal at a designated temperature. Isothermal forging is commonly used to forge aluminum which has a relatively lower forging temperature of 430 °C (806 °F) compared to other metals.

Isothermal forging offers several advantages, including increased precision, reduced defect rates, lower compressive forces and enhanced material plasticity.

Induction Forging

Induction forging is not a distinct metal forging process but rather a heating method used to reach the forging temperature. High-power induction coils facilitate a rapid increase in temperature and are used in mass-production environments to reduce cycle times.

We can use it across a range of processes by adjusting the heating setup to suit the process.

Swaging

Rotary Hot Swaging

Swaging is a specialised metal forging process used to create tapers or pointed edges on hollow cross-sections, such as pipes. Swaging is used to seal open ends or to enlarge the circumference of a pipe’s end, facilitating the connection with other pipe sections.

In this process, the material is first heated to its forging temperature and then pressed against a die to reshape the pipe end. Imagine forcing a flexible garden hose on a water outlet connection. Swaging looks similar to that visually except that the metal pipe retains its shape upon separation from the die. This is just one specific example. The process parameters can be manipulated to produce a range of outcomes.

Conclusion

Forging offers distinct advantages in metalworking. It is a cost-effective and versatile method for producing high-strength, reliable products suitable for critical applications. The refined grain structure resulting from forging imparts exceptional directional strength and structural integrity.

As a result, this process is used for crucial components such as crankshafts, railroad wheels and tracks, vehicle axles, connecting rods, suspensions, gears, shafts and bearings.

Forging has been steadily gaining popularity, particularly with metals such as titanium and superalloys. This process is anticipated to become more mainstream in the coming decades as it becomes increasingly accurate and refined through the use of advanced materials and simulation techniques.