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Introduction of Common defects in Forging


Forging is defined as a metal working process in which the useful shape of work piece is obtained in solid state by compressive forces applied through the use of dies and tools. Forging process is accomplished by hammering or pressing the metal. It is one of the oldest known metalworking process. Defects occur on forging despite. These defects need to be examined and can be prevented. Forging defects can be controlled by careful consideration of work material volume, good designing of the forging die, and the process.

Below are the various types of forging defects that might occur during the process:

1.Unfilled Section (Underfilling): This defect is geometrical defect commonly caused by poor design of the die, less raw material, poor forging techniques and poor heating. In this some section of the die cavity are not completely filled by the flowing metal. To avoid the occurrence of the defect, proper care must be taken on the design, heating and there should be enough raw materials.

2.Large grain: Large grains are usually caused by high initial forging temperature and insufficient deformation degree, or high final forging temperature, or the deformation degree falls into the critical deformation zone. Aluminum alloy deformation degree is too large; Superalloy deformation temperature is too low, may cause coarse grain, coarse grain will reduce the plasticity and toughness of the forging, fatigue performance decreased significantly.

3.Uneven grains: Uneven grains mean that the grains in some parts of the forging are particularly coarse, while some parts are small. The main reason for the uneven grains is that the deformation of the billet is not uniform, so that the degree of grain breakage is different, or the degree of deformation of the local area falls into the critical deformation zone, or the local work hardening of the superalloy, or the local grain during quenching and heating. Heat-resistant steels and superalloys are particularly sensitive to grain inhomogeneity. Uneven grains will significantly reduce the durability and fatigue properties of forgings.

4.Crack: Cracks are usually caused by tensile stress, shear stress or additional tensile stress during forging. The crack occurs at the place where the stress is the greatest and the thickness is the thinnest. If there are micro cracks on the surface and inside of the billet, or there are organizational defects in the billet, the reason is that improper thermal processing temperature reduces the plasticity of the material, or the deformation speed is too fast, the deformation degree is too large, and exceeds the allowable plasticity index of the materials.

5.Crack at the flash: This crack is generated at the parting surface during the forging and trimming. The reason for this problem may be: In the die forging operation, the strong flow of the metal caused the piercing phenomenon due to the heavy blow; or the trimming temperature is too low or too high.

6.Fold: Fold is formed when oxidized surface metals join together during metal deformation. It may be formed by the confluence of two or more strands of metal. It can also be formed by a rapid flow of a large amount of metal carrying the surface metal of the adjacent part of the flow, and the two meet; It can also be formed by bending the reflux of deformed metal; Can also be part of the metal local deformation, was pressed into another part of the metal and formed. Folding is related to the shape of raw materials and billets, die design, forming process arrangement, lubrication and the actual operation of forging. Folding not only reduces the bearing area of parts, but also becomes a source of fatigue due to the stress concentration here.

7.Die shift (Mismatch): Mismatch is the displacement of the forging along the upper half of the parting surface relative to the lower half. The reasons may be: 

a.The gap between the slider and the guide rail is too large

b.The design of the forging mold is not reasonable, the lack of lock or guide pillar to eliminate the wrong shift force.

c.The mold is not properly installed.

8.Scale pits: Scale pits are also some of the most common types of forging defects. They are small cracks or holes in the metal that occur due to improper cleaning of forged surfaces. Scale pits are a common defect in forging operations that occur in an open environment. To prevent them, take care to clean the surface thoroughly.

9.Residual stresses in forging: This defect occurs due to improper cooling of forged part. Too much rapid cooling is main causes of this types of defects. This can be removed by slow cooling of the forging in a furnace or under ash cover a period of time.

The knowledge of forging defects and causes is key to managing forging quality. In Stone Industrial we offer our customers always the best quality of the forgings.