Forging Defects: Most Common Ones to Know

Forging is a way to produce parts with high strength and excellent reliability, but a lack of proper knowledge or poor process control can lead to various forging defects.

These defects potentially affect the surface finish and strength of the product.

Common Forging defects include cold shut, cracks, porosity, inclusion, unfilled section, flakes, scale pits, folds, and warping. These defects can affect the surface finish and strength of the material. Internal forging defects like improper grain growth, residual stress, and improper forging penetration can lead to poor mechanical properties.

Forging Defect	Description
Cold Shut	The folding of the metal surface over itself
Unfilled Section	The forged metal does not fill the forging die entirely
Die Shift	Misalignment of forging dies
Folds (Laps)	Deflections at overlapping edges of metal pieces
Scale Pits	Small depressions on the surface of forged metal
Flakes	Ruptures or cracks inside the metal
Improper Gain Growth	Change in the original grain structure of the metal
Improper Forging Penetration	Only the surface of the metal experiences forging, while the interior dendritic structure remains unaltered.
Surface Cracks	Appearance of cracks all over the surface of forged metal
Residual Stress	Persisting internal stress in absence of an external load
Warping	Distortion of the forged metal
Porosity	Presence of small holes or voids in the material
Inclusion	Foreign material trapped in the forged metal

This article discusses forging defects in detail and provides information about the remedies for these defects to enhance the forging quality.

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13 Types of Forging Defects

Forging is a metal-forming process in which the workpiece is shaped into the desired form by applying compressive pressure.

Depending upon the application of pressure and use of dies, there are various types of forging, such as drop forging, upset forging, press forging, etc., and each forging process has its significant application.

Irrespective of the type of forging process used, the part produced by forging has significant advantages when compared to machining and casting.

However, the forging process is also susceptible to certain defects that can degrade the physical and aesthetic qualities of the workpiece.

Making it important to learn about these defects and be wary of them when performing forging operations.

Cold Shut

These are the most common defects that can occur during the forging process and are generally prone in parts with sharp edges or corners.

They occur as lumps or cracks at the corners of the workpiece as a result of metal folding over itself.

Cold shut defects are symptoms of various shortcomings that include improper die design, sharp corners, high friction between the die surface and hot metal, and excessive cooling of the forged metal. 

These defects reduce the strength and stability of the metal, compromising the durability of the forged part.

Furthermore, cold shuts also degrade the visual appearance by producing a lump in the smooth surface of the workpiece.

Unfilled Section

Also known as underfilling, these are the type of defects that occur when the metal does not properly fill the cavities of the forging die, resulting in products with hollow or underfilled areas.

The main causes for these defects include poor die designs, improper heating, insufficient raw material, and inadequate forging pressure.

Like cold shuts, unfilled section defects result in products with weak structures and reduced durability.

Die Shift

It is a type of defect that occurs when the dies used for the process of forging are misaligned or if the dies shift and move during the forging process.

These defects generally result in improper shape and size of the forged metal, but may also lead to surface defects.

Folds 

Also called lap defects, these forging defects occur when compressive stresses cause an abrupt lateral displacement, resulting in the folding of metal over its surface. 

These defects generally occur due to excessive compressive force or due to excessive raw material without the provision for flash formation.

These defects can impair the surface finish, mechanical characteristics, and material efficiency of the forged material.

Scale Pits

The occurrence of depressions or irregularities on the surface of forged metal is related to the scale pit defect.

These defects are more frequent when forging is carried out in an open environment.

Scale pits emerge when there is improper cleaning of the forged surface. A thin layer of oxide, called a scale, is formed on the surface of the forged metal during the process of heating.

The appearance and performance of the workpiece are the primary parameters affected by scale pit defects, but they can also include susceptibility to breaking and cracking.

Flakes

Improper cooling of the workpiece results in flakes. Flakes are internal fractures that occur if there is rapid cooling of the forged metal.

Due to flakes, there is a reduction in the strength of the metal and it becomes more prone to failure.

Improper Gain Growth

When there is an improper grain flow of the forged part, i.e., the grain structure is not aligned with the shape of the forged part, it gives rise to an improper gain growth defect.

The causes of improper gain growth defects are poor die designs.

This defect can induce a variety of problems in the workpiece including, poor surface finish, less fatigue strength, and dimensional instability.

Moreover, grain growth defects can also lead to other types of forging defects due to non-uniform mechanical properties in the material.

Improper Forging Penetration

This defect generally occurs due to inadequate pressing force application. As a result, the forged metal does not fully occupy a die, resulting in incomplete forging. 

In this defect, the compressive forces applied for forging are weak, leading to a deflection on the surface of the workpiece, without any significant change in its internal structure.

When the forging penetration is not deep enough, the center may not be fully consolidated resulting in reduced strength and reliability of the workpiece.

Surface Cracks

Surface cracking is a common defect encountered in forging and arises if the workpiece is forged at lower temperatures or worked on excessively.

As a result, the forged metal develops cracks all over the surface leading to reduced aesthetics, reliability, and a weakened structure that is prone to fail under stress.

Residual Stress

Another temperature-related forging defect is residual stress. 

Residual stress is a type of stress that remains in the forged material in the absence of external load. 

This defect emerges if the workpiece is allowed to cool rapidly (quenching) or is subjected to a non-homogenous deformation.

The presence of residual stress in the workpiece affects its fatigue resistance and reduces its ductility, making it unsuitable for further machining operations.

Warping

This defect induces distortion in the workpiece.

It is caused when the thinner areas of the forged metal cool faster and the thicker areas take longer to cool.

As much as it affects the aesthetics, warping also causes the workpiece to be out of tolerance and makes it difficult to be used for the intended application.

Porosity

As the name suggests, porosity results in the formation of pores or voids in the forged metal.

Porosity occurs due to the entrapment of molten metal oxides as it is solidifying.

These defects are difficult to detect and lead to a loss of the overall strength of the workpiece, enhancing the risk of failure when the workpiece is put under load.

Inclusions

These defects arise when foreign material such as bits of sand or slag gets trapped inside the forged metal.

Inclusions in the surface of the workpiece can be removed by sanding, but the inclusions trapped inside the surface of the material can reduce its strength and affect its reliability.

Things to Improve Forging Quality 

Although proper forging techniques and careful consideration go a long way, it is not always possible to conduct perfect faultless forging. 

In the event of the occurrence of a defect, however, what are the possible ways to get around it? Are there any remedies to make the forging process better? 

Defect-Specific Remedies

The following are the defect-specific remedies that can be incorporated to eliminate or reduce forging defects.

• Cold shuts can be reduced by increasing the fillet radius along the edges of the design.

• A proper die design, sufficient forging pressure, adequate heating, and availability of sufficient raw materials can help prevent defects like unfilled sections, porosity, improper forging penetration, and fold or lap defects.

• The solution to die-shift defects is to use good quality dies, replace worn dies regularly, and ensure that the forging dies are properly aligned before starting the forging process.

• Adequate cleaning and the selection of pure ingots can help avoid defects like scale pits and inclusions.

• Residual stresses and flaking of the workpiece can be eliminated by allowing it to cool gradually, which results in the annealing of the workpiece, thereby enhancing its ductility.

• Improper gain growth can be reduced by ensuring appropriate die designs that force the grain flow to be properly aligned with the forged metal.

• Surface cracking defects are avoided by maintaining sufficient temperatures and by regulating the compressive forces applied to the workpiece.

• Factors such as grain refinement, uniform forging pressure, and gradual cooling can reduce the risk of warping. The choice of metals also governs the amount of warping.

General Guidelines to Improve Forging Quality

Apart from the defect-specific remedies, the following guidelines can be implemented to optimize the forging process and reduce the overall defects from the forged metal. 

Use high-quality raw material

The quality of the raw materials used in forging can have a significant impact on the final product. 

It is thus advised to always use materials that are free of defects and have desired properties, such as high strength, hardness, and ductility.

Follow proper forging techniques

To ensure the success of the forging process, each step of forging should be performed with careful consideration.

These include preheating the metal, applying the proper amount of pressure for deformation, and cooling the parts appropriately. 

Implement quality control measures

Implementation of quality control measures throughout the process is also beneficial. 

This includes inspecting the raw materials, monitoring the forging temperature and other process parameters, and performing regular maintenance on the forging equipment. 

Maintain proper forging temperatures

The forging temperature plays a crucial role in the success of the process. If the temperature is too low, the metal may not be malleable enough to be shaped properly. 

On the other hand, if the temperature is too high, the metal may become brittle and prone to cracking. 

Therefore, it is essential to maintain the proper forging temperature throughout the process.

Use software simulations

The use of software simulations can help in predicting common forging faults like under-filling, and forging laps/folds. 

Apart from that, software simulations also provide the ability to calculate the appropriate force and energy required to perform the forging process.

Final Thoughts

Although forging produces parts with enhanced mechanical properties like high tensile strength, reliability, cost efficiency, etc., it is also prone to various defects.

These defects can easily creep in during the process of forging and result in compromised quality, performance, and overall characteristics of the workpiece.

Defects such as cold shut, unfilled sections, flakes, improper gain growth, and surface cracks lead to a reduction of strength, structural integrity, and loss of reliability in the workpiece.

On the other hand, defects such as scale pits, warping, and die shift primarily result in dimension instability and reduced aesthetics.

However, by employing proper forging techniques and ensuring careful preventative measures, these defects can easily be eliminated or reduced to a large extent. 

Frequently Asked Questions (FAQ)

What are the most common types of forging defects?

The most common types of forging defects include unfilled sections, cold shuts, and flakes. However, other defects can also arise if the proper forging technique is not implemented.

Can forging defects be eliminated by reworking?

Yes, forging defects such as scale pits, flakes, and other surface defects can be eliminated by reworking, i.e., by filling them with additional material or by smoothing them using special grinding machines. However, defects such as unfilled sections, die shifts, and improper gain growth demand reforging of the workpiece.

Does the presence of foreign elements increase the likelihood of defects in forging?

Yes, the presence of foreign elements contributes towards a greater likelihood of forging defects. For example, the presence of a large quantity of hydrogen in the material often leads to flaking.