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16 Common Defects in Forging Processes

Views: 0     Author: Site Editor     Publish Time: 2023-06-12      Origin: Site


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Forging is a process where pressure is applied to metal workpieces using forging machinery, causing plastic deformation to obtain forgings with specific mechanical properties, shapes, and sizes. There are various surface treatment techniques in forging, including surface hardening and surface chemical heat treatment. Surface hardening involves quickly heating the workpiece's surface to the quenching temperature, then rapidly cooling it, so that only the surface layer obtains a quenched structure, while the core retains its pre-quenching structure. Surface chemical heat treatment creates a layer on the workpiece surface with uniform chemical composition, high hardness, and good wear resistance, thus improving the surface hardness and wear resistance. Another decorative technique is surface forging, which is done by using hammers of different shapes to create various dotted textures on the metal surface.


Let's explore the common defects that can arise from improper forging processes:

1、Large Grain Size

Large grains are often caused by excessively high starting forging temperatures, inadequate deformation, excessively high final forging temperatures, or deformation within the critical deformation zone. Excessive deformation in aluminum alloys can lead to texture formation; low deformation temperatures in high-temperature alloys can cause coarse grains, reducing plasticity, toughness, and fatigue performance of the forgings.

2、Uneven Grain Size

This occurs when certain areas of the forging have particularly large grains while others are smaller. Causes include uneven deformation across the workpiece, causing varying degrees of grain fragmentation, or local areas deforming within the critical deformation zone, or localized work hardening in high-temperature alloys, or local grain growth during quenching heating. Heat-resistant steels and high-temperature alloys are particularly sensitive to uneven grain size, significantly reducing the endurance and fatigue performance of the forgings.

3、Cold Hardening

Cold hardening occurs when deformation is done at too low a temperature or too quickly, or when the forging cools too rapidly after forging. This prevents softening caused by recrystallization from keeping pace with the strengthening (hardening) caused by deformation, leaving part of the cold deformation structure in the forging after hot forging. This structure increases strength and hardness but reduces plasticity and toughness. Severe cold hardening can lead to forging cracks.


Cracks often result from large tensile, shear, or additional tensile stresses during forging. They usually occur where the workpiece is under the most stress and is thinnest. Microcracks on the surface or inside the billet, structural defects within the billet, improper thermal processing temperatures reducing material plasticity, excessive deformation speed or degree exceeding the material's plasticity, can all cause cracks during various processes like coarsening, elongation, punching, hole expansion, bending, and extrusion.


Cracking appears as shallow, tortoise-like cracks on the forging surface. Surfaces under tensile stress during forging (e.g., unfilled protrusions or bent parts) are most prone to this defect. Internal causes may include: excessive easily fusible elements like Cu, Sn in the raw material; copper precipitation, coarse grain growth, decarburization on the steel surface after long, high-temperature heating, or surfaces reheated multiple times; high sulfur content in fuel causing sulfur to penetrate the steel surface.

6、Flash Cracks

Flash cracks occur at the parting line during die forging and trimming. Causes may include intense metal flow due to heavy blows during die forging, producing flash; low trimming temperatures in magnesium alloy die forgings; high trimming temperatures in copper alloy die forgings.

7、Parting Line Cracks

These are cracks that form along the parting line of the forging. Non-metallic inclusions in the raw material, flow and concentration towards the parting line during die forging, or shrinkage cavities remaining in the die forging and then being squeezed into the flash often form parting line cracks.



Folding occurs when oxidized surface metal layers come together during the metal deformation process. It can result from the convergence of two (or more) metal streams, or from rapid, extensive flow of a single metal stream carrying adjacent surface metal with it, or from bending and backflow of the deforming metal, or partial deformation of the metal being pressed into another part. Folding is related to the raw material and billet shape, die design, forming process arrangement, lubrication, and actual forging operations. Folding not only reduces the load-bearing area of the part but also often becomes a source of fatigue due to stress concentration during operation.


Flow-through is a form of improper streamline distribution. In the flow-through area, streamlines that were originally distributed at a certain angle converge, possibly causing significant differences in grain size inside and outside the flow-through area. The causes of flow-through are similar to folding, formed by the convergence of two metal streams or a single stream carrying another with it. Flow-through, where the metal is still a whole, reduces the mechanical properties of the forging, especially when there is a significant difference in grain size on either side of the flow-through band.

10、Poor Streamline Distribution in Forgings

This refers to phenomena like streamline cutting, backflow, and vortexes on low magnification in forgings. Improper die design or forging method selection causing disordered streamline in prefabricated billets; improper operation by workers or die wear causing uneven metal flow, can all lead to poor streamline distribution in forgings. Disordered streamlines reduce various mechanical properties, so important forgings often have requirements for streamline distribution.

11、Residual Casting Structure

This mainly occurs in forgings made from cast ingots. The casting structure mainly remains in hard-to-deform areas of the forging. Insufficient forging ratio and improper forging methods are the main causes of residual casting structure. This can degrade the properties of the forging, especially impact toughness and fatigue performance.

12、Carbide Segregation Not Meeting Requirements

This primarily occurs in ledeburitic tool and die steels. It is mainly due to uneven distribution of carbides in the forging, either in large blocks or in a network. The main causes are poor carbide segregation in the raw material, combined with insufficient forging ratio or improper forging method during modification. Forgings with this defect are prone to local overheating and quench cracking during heat treatment, and tools and dies made from them are likely to chip in use.

13、Banded Structure

Banded structure is where ferrite and pearlite, ferrite and austenite, ferrite and bainite, or ferrite and martensite are distributed in bands in the forging. They often appear in hypo-eutectoid steels, austenitic steels, and semi-martensitic steels. This structure, formed during deformation in a two-phase coexistence situation, reduces the material's transverse plasticity, especially impact toughness. Forgings or parts often crack along the ferrite band or the interface between the two phases during forging or operation.

14、Inadequate Local Filling

This primarily occurs in ribs, corners, angles, and rounded corners, where dimensions do not meet the requirements of the drawing. Causes may include: low forging temperature, poor metal flow; insufficient tonnage of the equipment or insufficient hammering force; poor design of the blank die, non-conforming volume or cross-sectional dimensions of the blank; accumulation of oxide skin or deformed metal in the die cavity.



Under-forging refers to a general increase in dimensions perpendicular to the parting line. Possible causes include: low forging temperature; insufficient tonnage of the equipment, hammering force, or number of hammering strokes.


Misalignment is when the upper half of the forging is displaced relative to the lower half along the parting line. Possible causes include: excessive gap between the slider (hammer head) and the guide; poor die design lacking a lock mouth or guide post to eliminate misalignment forces; poor die installation.


Forging is a molding manufacturing process with high demands on the internal structure of the part body. ZONZE is ready to help you achieve better results by controlling the cost of parts to ensure cost-effective production and shorter cycle times.

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