Overview of Common Defects of Large Forgings
Forging is a manufacturing process where metal is shaped by hammering and pressing into high-strength parts. Forgings are generally used in the manufacturing of automotive components and heavy machinery such as crankshafts, gears, shafts, and axles. Large forgings are components are typically made from steel, iron, or titanium for a range of industrial applications.
Like any manufacturing process, forging has the potential for defects to occur during the manufacturing process. Defects can be divided into two general categories: surface and subsurface.
Surface defects are those that can be detected immediately by visual inspection and can often be fixed with minimal effort. The most common surface defects in forging are flash, crevice corrosion, scale, and pockmarks.
Flash is an excess material along the edge of the forged part. It is caused by flash coming away from tooling or the die. Flash has been known to cause a number of issues such as reduced fatigue life, stress corrosion cracking, and the potential for the part to be rejected due to cosmetic issues.
Crevice corrosion can occur when the part is exposed to a corrosive environment, such as salt water. The corrosion will attack the surface of the forged part, and can lead to rust and pitting which can cause the part to be structurally weakened and fail prematurely.
Scale is a layer of oxide that forms on the surface of the forged part. Scale provides a protective layer, however, it can also cause dimensional issues if the surface is not machined off before the part is assembled.
Pockmarks are surface defects caused by shrinkage or gases within the material that causes indentations on the surface of the forged part. These defects can occur from improper alloying, cooling, or pressing and must be addressed, as they can cause failure down the line.
Subsurface defects are those that are not visible to the eye, but have a major impact on the strength and durability of the forged part. The most common subsurface defects in forging are porosity, inclusions, cracks, and voids.
Porosity is the presence of tiny gas bubbles or voids within the material. Porosity reduces the strength and toughness of a forged part and can cause failure at the microscopic level.
Inclusions are foreign particles, such as oxides or silicates that are unintentionally trapped within the material during the forging process. Inclusions can cause localized stress concentrations that can lead to premature failure of the forged part.
Cracks are a type of subsurface defect that can occur due to thermal shock or vibration, improper heating, alloying, and many other factors. Cracks are more likely to occur in larger forgings and can lead to catastrophic failure of the part.
Voids occur when the material fails to fill the die completely. This is most common when the material is too cold, not hot enough, or the forging process is rushed. Voids can lead to major issues such as reduction in tensile and fatigue strength, surface porosity, and poor impact resistance.
In order to ensure that a forged part is free of defects, it is important to ensure that quality is built into the process. Quality assurance measures should be put in place to ensure that all materials, equipment, and processes are governed and fall within established guidelines.
These measures will help ensure that defects are detected early, and any necessary corrective actions are taken before the part is shipped. It is also important to have a rigorous inspection process in place to detect any surface or subsurface defects.
By taking the proper steps in quality assurance, large forgings can be made to exacting standards with minimal defects. This will help ensure that the finished product is of the highest quality and will meet the needs of the customer.
