What Is the Basic Classification of Forging?

Forging is a fundamental metal forming process that plays a critical role in manufacturing strong and durable components across various industries. From automotive crankshafts and aerospace brackets to construction fasteners and oilfield tools, forged parts are known for their excellent mechanical properties and structural integrity.

Understanding the basic classification of forging helps engineers, designers, and procurement professionals choose the most suitable forging method based on application, part complexity, production volume, and material type. This article explores the main types of forging and their characteristics to help you make informed decisions.

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What Is Forging?

Forging is a manufacturing process that involves shaping metal using localized compressive forces. This can be done through hammering, pressing, or rolling—usually with the metal in a heated (but solid) state. Forging improves the internal grain structure, enhances strength, and eliminates defects like porosity or inclusions.

Forging has evolved into various techniques depending on factors such as temperature, equipment used, and die configuration.

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Basic Classification of Forging

Forging processes can be broadly classified based on two main criteria:

1. Forming Temperature

2. Die Configuration and Tooling

Let’s look at each classification in detail.

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Classification by Forming Temperature

This is the most common way to categorize forging processes. Depending on the temperature at which forging is performed, it is divided into:

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1. Hot Forging

Definition: Performed at high temperatures, typically above the recrystallization temperature of the metal (around 1100–1250°C for steel).

Advantages:

• High ductility and low resistance to deformation

• Enables complex shapes

• Refines grain structure

• Eliminates porosity and defects

Disadvantages:

• Scale formation due to oxidation

• Dimensional accuracy is less than cold forging

• Requires more energy for heating

Applications:

• Automotive parts (crankshafts, gears)

• Heavy machinery components

• Industrial shafts and flanges

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2. Warm Forging

Definition: Performed at intermediate temperatures (between 500°C and 900°C), combining some advantages of hot and cold forging.

Advantages:

• Reduced forming loads

• Improved dimensional control

• Less oxidation compared to hot forging

• Better surface finish

Disadvantages:

• Limited to specific materials

• More complex equipment requirements

Applications:

• Transmission components

• Bearing races

• Gear blanks

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3. Cold Forging

Definition: Performed at or near room temperature without heating the material.

Advantages:

• Excellent surface finish

• Close dimensional tolerance

• Work hardening improves strength

• No oxidation or scaling

Disadvantages:

• High forming forces required

• Limited to simpler shapes and softer materials

• Risk of residual stress

Applications:

• Fasteners (bolts, screws, rivets)

• Shafts

• Small precision components

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Classification by Die Configuration

Forging can also be classified based on the type of dies and equipment used in the process:

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1. Open Die Forging (Free Forging)

Definition: Metal is placed between flat or simple dies that do not enclose the material completely.

Process:

• The workpiece is deformed in multiple steps

• Operator controls the deformation direction

• Ideal for custom or low-volume production

Advantages:

• Suitable for large and simple shapes

• Lower die cost

• Good control over grain flow

Disadvantages:

• Lower dimensional accuracy

• More skilled labor required

• Additional machining may be needed

Applications:

• Large shafts, disks, rings

• Heavy industrial components

• Marine and power generation parts

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2. Closed Die Forging (Impression Die Forging)

Definition: The metal is placed in a die cavity that resembles the desired shape of the part.

Process:

• High-pressure force forces the metal into the die

• Flash is often formed and then trimmed

• Ideal for high-volume production

Advantages:

• Precise, near-net shape components

• High repeatability and efficiency

• Improved mechanical properties due to grain alignment

Disadvantages:

• Higher tooling cost

• Limited to smaller and medium-sized parts

• Requires more sophisticated equipment

Applications:

• Gears

• Connecting rods

• Automotive and aerospace components

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3. Upset Forging

Definition: Involves increasing the diameter of a portion of a metal bar by compressing its length.

Process:

• Typically done in horizontal forging machines

• Used to form heads on bolts, rivets, and fasteners

Advantages:

• Efficient production of symmetrical components

• Good mechanical strength

• High production speed

Applications:

• Bolts

• Screws

• Shafts and valve stems

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4. Seamless Ring Rolling

Definition: A specific type of forging where a ring is formed from a pierced preform and then expanded by rolling.

Advantages:

• Excellent grain orientation

• Precise wall thickness

• Cost-effective for large diameter rings

Applications:

• Bearings

• Flanges

• Gears and pressure vessel components

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Additional Classifications

In modern forging, processes are also classified by:

a. Machine Type

• Hammer forging

• Hydraulic press forging

• Screw press forging

• Mechanical press forging

b. Automation Level

• Manual forging

• Semi-automatic forging

• Fully automatic forging

c. Material Type

• Ferrous (carbon steel, stainless steel)

• Non-ferrous (aluminum, copper, titanium, nickel alloys)

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Forging vs Other Metal Forming Methods