Forging Steel Explained: How it's Done

Forging of steel produces parts with excellent mechanical properties and high strength, that can withstand comparatively higher loads than standard steel workpieces.

Steel forging is the process of shaping hot steel workpieces by applying strong forces with the help of a hammer, press, or die. Forging steel alters its properties resulting in finer grain structure and excellent machinability while enhancing its mechanical strength and ductility.

This article discusses steel forging by going through its process, techniques, benefits, and applications.

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Steel Forging Explained

Steel forging involves heating the steel workpiece above its recrystallization temperature, typically between 950℃ - 1250℃. 

This enhances the ductility of steel, making it easier to shape the workpiece, thereby reducing the effort required for forging.

After heating, the hot steel is placed in a mold or die and hammered until it takes the shape of the die cavity.

During hammering, metallurgical recrystallization and grain refinement take place, which increases its strength and ductile properties.

Metallurgical recrystallization is a process where the grains dissolve and reform again, eliminating the unwanted stress in the metal. 

On the other hand, grain refinement ensures uniform structure, thereby minimizing the risk of unexpected failures, and increasing the reliability of the part.

Forging steel is preferable over casting or machining steel when excellent load capabilities are needed while maintaining a small cross-section of the workpiece.

Also, forged steel parts are free of any atomic defects or internal gas voids that can lead to fatigue and failure.

Therefore, when comparing forging against machining, forging steel workpieces provide various advantages over other machining processes.

Steps Involved in Forging Steel: The Process

Forging a roll of steel in the steel industry consists of a series of steps that gradually result in the final workpiece.

Step 1: Die design

The die is designed based on the end product. While designing the die, it is important to consider tolerance in the size of the workpiece to account for shrinkage.

Generally, a tolerance of around ±0.003 inches per inch is provided along all the dimensions of the workpiece.

While the external dimensions of the die should be larger by 0.003 inches per inch, the internal dimensions, such as holes, should be smaller by 0.003 inches per inch.

This is because the hot metal shaped into the desired die cavity is susceptible to shrinkage on cooling, which results in overall shrinkage of the dimensions.

Therefore, it is important to consider the tolerances when designing the die for forging.

Step 2: Billet cutting & Heating

After preparing the die, steel is cut into small billets of the required shape and size.

The billets should be small enough to fit the die and have enough volume to completely fill the die cavity.

Apart from that, cutting steel into small billets makes it easy to heat them for the forging process.

Step 3: Selecting the Best Suited Forging Type

Before executing the forging process, it is important to identify the best-suited forging process for your application.

Generally, there are two main types of forging operations, open-die forging, for simple geometries, and closed-die forging for complex shapes.

These two types are further divided into different types of forging, such as drop forging, press forging, roll forging, etc.

Apart from that, the forging process is also categorized by the temperature of the workpiece.

Generally, hot forging is used for producing complex shapes, followed by warm forging, and cold forging to produce workpieces with a high surface finish and excellent strength.

Step 4: Executing the Forging Process

After preparing the die and selecting the appropriate forging process, you can shape the workpiece by applying compressive forces and molding it into the desired shape.

Although forging requires industrial power hammers to shape the steel workpiece, it can also be achieved by using manual hammers for very small and thin steel workpieces.

Step 5: Post-processing

After forging, the steel product can be machined or heat-treated according to your requirements. 

Forged workpieces usually have a high surface finish, but can sometimes require secondary finishing operations to smoothen the surface and produce a high surface finish.

Similarly, heat treatment processes such as annealing, normalizing, tempering, etc can be performed to alter its mechanical properties.

Step 6: Product Inspection and Surface treatment

After completing the process, the end product is inspected for forging defects.

In some cases, the product is surface treated with processes such as galvanizing or anodizing of steel to enhance its corrosion resistance and abrasion resistance properties, while ensuring a smooth finish. 

Steel Forging Techniques

The forging of steel can be classified in two ways: based on the forging temperature and based on the type of die used.

Forging Technique	Overview
Open-die Forging	Suitable for making simple parts with lower tolerance requirements
Closed-die Forging	Suitable for mass-production of complex parts with tight tolerance
Hot Forging	Suitable for forging large workpieces
Warm Forging	Suitable for moderate-sized workpieces, where good strength is desired
Cold Forging	Suitable for applications where excellent strength and a high surface finish are desired.

Based on the Type of Die Used

Based on the type of die used, steel forging can be further classified into two types: open-die forging and closed-die forging.

Open die Forging

Open die forging is a process where the die constantly hammers the steel to alter its shape.

Generally, open-die forging is suitable to form simple shapes by moving the workpiece around and changing its orientation manually to get the desired shape.

The lower part of the die is stationary and can consist of simple shapes and patterns to impart on the workpiece, but the flow of metal is not completely restricted.

Some of the advantages of open-forged steel are:

1. They often possess better fatigue and finer microstructure.
2. Open-forged steel has longer part life.
3. The distribution of grains (grain flow) across the microstructure will be continuous, imparting high strength and ductile properties.

The disadvantages of open-die forging are:

1. This process cannot achieve accurate precision in certain products.
2. Further machining operations are often required to attain industry standards, thus more time needs to be invested.

Closed die Forging

Closed die forging is a process where the die will compress the steel, forcing it to take the shape of the die cavity. 

A closed die consists of two parts, one mounted on the anvil, and the other mounted on the impacting face of the ram.

It has a closed construction which restricts the flow of metal during forging.

Therefore, when the compressive force is applied to the steel workpiece, it is completely enclosed in the die cavity, restricting it to flow out of the die.

As a result, the workpiece produced by this type of forging is comparatively more accurate and has a smoother surface finish.

The advantages of the closed die forging process are:

1. No post-processing is required, so less time is invested.
2. Provides a smooth surface finish.
3. This method is economical for large-scale production of forged steel.

The disadvantages of close die forging are:

1. The dies used in closed-die forging are comparatively more costly than open dies. As a result, it may not be economical for small production runs.
2. As it continuously compresses material in an enclosed die, it creates a high-pressure environment, that potentially can create a dangerous workspace for the operator.

Based on the Forging Temperature

Hot Forging

In hot forging, the steel is heated up to its recrystallization temperature i.e 950℃ - 1250℃.

When forged at high temperatures, it gives high formability and requires less force to shape the workpiece.

As a result, hot forging is suitable for large workpieces that otherwise require strong forces to shape them.

Apart from that, it is also suitable for forging complex and intricate shapes as the high temperature of steel enable it to flow into the die cavity without leaving any voids.

However, the hot forging of steel may form scales on the surface of the finished structure due to inappropriate cooling rates.

Warm Forging

Warm forging is done at a temperature just below the recrystallization temperature of steel. The temperature range will be around 750℃ - 950℃. 

Compared to hot forging, warm forging requires more force to deform the material resulting in reduced formability of the metal. 

This technique requires a moderate amount of pressure and tooling to deform the steel. However, it combines the benefit of both hot and cold forging. 

Warm forged steel, due to its finer structure, does not require further annealing of the workpiece.

So, this process is generally preferred in manufacturing industries almost everywhere.

Cold Forging

Cold forging is done at a very low temperature ~150℃. This results in the least formability of steel, requiring heavy forces to deform the material. 

It can produce parts with tight tolerance due to minimal shrinkage of steel after forging, and eliminates the risk of scale formation, resulting in a smooth surface finish. 

However, cold forging can develop internal stresses in the steel workpiece, making it important to anneal the workpiece to remove any unwanted internal stresses.

Benefits of Forged Steel

Forged steel is preferred over commercially machined or heat-treated steel because it showcases enhanced mechanical properties, making it suitable for different applications.

High Strength

The forged steel will impart high toughness, high strength, and excellent durability.

It can withstand comparatively higher loads without failing. As a result, forged steel is preferred for applications, such as D-clamps on vehicles, that are subjected to extreme loads.

Minimal Internal Stresses

Hot forging of steel involves heating the workpiece above its recrystallization.

This leads to the annealing of the workpiece, which relieves internal stresses and minimizes the risk of cracking on the surface of the workpiece.

Apart from that, it also enhances the ductility of the steel workpiece, making it suitable for further machining processes.

Uniform Grain Structure

Forging involves bending and shaping the workpiece into the desired shape.

Unlike casting and machining, the forging of steel does not break the flow of internal grains of the metal. Instead, it bends the grains along the shape of the workpiece.

As a result, forged steel has uniform mechanical properties throughout its cross-section.

Fewer Defects

Forged steel will have the least gas voids and internal cracks than cast or machined steel.

This ensures a rigid structure and eliminates the risk of failure due to unwanted defects.

Applications of Forged Steel

Forged steel presents superior mechanical properties with high strength and durability, making it ideal for different applications.

Apart from that, its uniform grain structure ensures reliability by eliminating the risk of unexpected failure.

As a result, forged steel is best suited for manufacturing critical components that are subjected to high load conditions.

Forges steel is used for manufacturing pipe fittings and gas pipelines in the oil and gas industry.

It is also used for making automobile components, such as gears and pulleys, that are subjected to high load.

Furthermore, its high reliability makes it ideal for making shackles and D-clamps used on automobiles.

Forged steel is also used to produce hand tools, fasteners, and bolts in the hardware industry.

Final Thoughts

Forging steel provides the ability to shape the workpiece into the desired part with excellent mechanical properties and a good surface finish.

Generally, closed-die forging of steel is recommended for large-scale industries where identical parts are produced repeatedly, whereas, open-die forging is ideal for small-scale applications.

Although warm forging is best suited for steel, if your application requires forging large workpieces, it is advised to use the hot forging technique, and if high strength and tolerance are desired, cold forging is recommended.

Frequently Asked Questions (FAQ)

Which grade of steel is easiest to forge?

Low- and medium-carbon steel is the easiest to forge. This is due to its low carbon and alloy content, which enhances its ductility. 

Can we forge metals like Titanium?

Yes, titanium can be forged. Forged titanium has excellent anti-corrosive properties that can be used in marine applications.

Which metal is most difficult to forge?

Tungsten is the most difficult metal to forge. It requires high temperature, high pressure, and high impact force to forge.