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Improper forging process usually causes the following defects:
1. The large grains in the forging process
The large grains are usually caused by overhigh initial forging temperature and insufficient deformation degree, overhigh final forging temperature, and the critical deformation. If the deformation degree of aluminum alloy is oversize, texture will be formed; if the deformation temperature of superalloy is too low, it may also cause the coarse grains when forming mixed deformation structure. The coarse grains will reduce the plasticity and toughness of the forgings with metal forging tools, and the fatigue performances will decrease significantly.
2. The uneven grains in the forging process
The uneven grains mean that the grains in some parts of the forging are particularly coarse, while some parts are small. The main reason is that the deformation of the blank is not uniform, so that the grains are broken at different degrees, or the deformation of the local area falls into the critical deformation region, or the local work hardening of the superalloy, or the local grains coarsening during quenching and heating. Heat-resistant steels and superalloys are particularly sensitive to grain inhomogeneity. The uneven grains will significantly reduce the durability and fatigue performances of forgings.
3. The chilling phenomenon in the forging process
The deformation is due to the fact that the temperature is too low or the deformation speed is too fast, and the cooling after the forging process is too fast, which may make the softening caused by recrystallization unable to keep up with the strengthening (hardening) caused by deformation, so that the interior of the forging after hot forging still partially remains cold deformed tissue. The existence of this structure increases the strength and hardness of the forging, but reduces the ductility and toughness. The severe chilling phenomenon may cause the fracture.
4. The crack in the forging process
The crack is usually caused by large tensile stress, shear stress or additional tensile stress during forging. The location where the crack occurs is usually the location where the stress of the blank is the greatest and the thickness is the thinnest. If there are micro-cracks on the surface and inside of the blank, or there are structural defects in the blank, or improper hot working temperature reduces the plasticity of the forging materials list, or the deformation speed is too fast, the degree of deformation is too large, and exceeds the allowable plasticity index of the material, etc. The cracks may occur in processes such as roughening, drawing, punching, reaming, bending and extrusion.
5. The fissure in the forging process
The fissure is a shallow tortoise-like crack on the surface of the forging. Surfaces that are tensile stressed in forging forming (eg, underfilled bulges or bent sections) are most prone to this defect. The internal causes of cracking may be various:
① The raw materials contain too many fusible elements such as Cu and Sn.
② When heated at high temperature for a long time, copper is precipitated on the surface of the steel, the surface grains are coarse, and the decarburized surface has been heated for many times.
③ The sulfur content of the fuel is too high, and sulfur penetrates into the surface of the steel material.
6. The flash crack in the forging process
The flash crack is the crack generated at the parting surface during die hot metal forging and trimming. The cause of flash crack may be:
① In the die forging operation, the strong flow of the metal causes the piercing phenomenon due to the heavy blow.
② The trimming temperature of magnesium alloy die forgings is too low; the trimming temperature of copper alloy die forgings is too high.
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