Defects caused by improper forging process
1. Large grains; large grains are usually caused by excessive initial forging temperature and insufficient degree of deformation, or excessive final forging temperature, or a degree of deformation falling into a critical deformation zone. The deformation degree of aluminum alloy is too large to form texture; the deformation temperature of high-temperature alloy is too low, and coarse grains may be caused when mixed deformation structure is formed; coarse grain will reduce the plasticity and toughness of forgings, and the fatigue performance will be significantly reduced.2. chilling phenomenon; due to low temperature or deformation speed too fast during deformation, and too fast cooling after forging, the softening caused by recrystallization can not keep up with the strengthening (hardening) caused by deformation, so that the forged forging after hot forging The cold deformed structure remains partially inside. The presence of such a structure increases the strength and stiffness of the forging but reduces plasticity and toughness. Severe chills can cause cracking.
3. Grain inhomogeneity; grain inhomogeneity means that the grains of some parts of the forging are particularly coarse, and some parts are small. The main reason for the grain inhomogeneity is that the deformation of the billet is uneven, 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 alloy hardening of the superalloy or the local grain during quenching heating. Coarse. Heat resistant steels and superalloys are particularly sensitive to grain irregularities. The uneven grain size will significantly reduce the long-term performance and fatigue performance of the forging.
4. Cracks; cracks are usually caused by large tensile stresses, shear stresses or additional tensile stresses during forging. The part where the crack occurs is usually the part where the billet has the largest stress and the thinnest part. If there are micro-cracks on the surface and inside of the blank, or there are tissue defects in the blank, or if the hot working temperature is improper, the plasticity of the material is lowered, or the deformation speed is too fast, the deformation degree is too large, and the plastic pointer is allowed to exceed the allowable material. Cracks may occur in processes such as lengthening, punching, reaming, bending, and extrusion.
5. Cracks; cracks are shallow cracks on the surface of forgings. A surface that is subjected to tensile stress in the forming of the forging (for example, an unfilled convex portion or a bent portion) is most likely to cause such a defect. The internal cause of cracking may be multi-faceted: 1 the raw material is too much fusible elements such as Cu and Sn. 2 When the high temperature is heated for a long time, the surface of the steel material has copper precipitated, the surface crystal grains are coarse, decarburized, or the surface is heated several times. 3 The sulfur content of the fuel is too high, and there is sulfur on the surface of the steel.
6. Flying edge cracks; flying edge cracks are cracks generated at the parting surface when die forging and trimming. The cause of the flash crack may be: 1 In the die forging operation, the metal is strongly flowed to cause the ribs due to the heavy blow. 2 The cutting edge temperature of magnesium alloy die forgings is too low; the cutting edge temperature of copper alloy die forgings is too high.
7. Partial surface crack; Partial surface crack refers to the crack generated along the parting surface of the forging. There are many non-metallic inclusions in the raw materials. When the die is forged, the flow to the parting surface and the concentration or shrinkage of the tube are often formed into a parting surface crack after being squeezed by the die.
8. Folding; folding is formed by the combination of oxidized surface metals during metal deformation. It may be formed by the confluence of two (or more) metal convections; it may also be formed by the rapid mass flow of a metal and the flow of the surface metal of the adjacent part, which may be due to deformation; The metal is formed by bending and reflowing; it may also be a partial deformation of a part of the metal, which is formed by being pressed into another part of the metal. Folding is related to the shape of the raw materials and blanks, the design of the mold, the arrangement of the forming process, the lubrication situation and the actual operation of the forging; the folding not only reduces the bearing area of the parts, but also becomes a source of fatigue due to the stress concentration here.
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