Imperfections caused by improper forging process
Large grains are usually caused by high initial forging temperatures and insufficient deformation, or high final forging temperatures, or deformation levels that fall into critical deformation zones. The degree of deformation of the aluminum alloy is too large to form a texture; the deformation temperature of the high-temperature alloy is too low, and coarse grains may also be caused when a mixed deformed structure is formed. The coarseness of the crystal grain will reduce the plasticity and toughness of the forging, and the fatigue performance will obviously decrease.
2. Non-uniform grain
Non-uniform grain refers to the fact that the crystal grains in some parts of the forging are particularly thick and some parts are smaller. The main cause of grain non-uniformity is that the uneven deformation of the billet causes the grain to be broken to varying degrees, or the deformation of the local area falls into the critical deformation zone, or the local processing and hardening of the superalloy, or the local grain during quenching and heating. Thick. Heat-resistant steels and superalloys are particularly sensitive to grain inhomogeneities. Non-uniform grain will significantly reduce the permanent properties and fatigue properties of forgings.
3. Chilling phenomenon
Deformation due to low temperature or deformation speed is too fast, and cooling too fast after forging, may cause the softening caused by recrystallization can not keep up with the deformation caused by hardening, so that the forgings after hot forging still retain some of the cold deformation of the organization. The presence of this structure increases the strength and hardness of the forgings, but reduces the plasticity and toughness. Severe chilling may cause cracking.
Cracks are usually caused by large tensile stress, shear stress, or additional tensile stress during forging. The location where cracks occur is usually at the location where the billet has the greatest stress and the thinnest thickness. If there is micro-cracks on the surface and inside of the billet, or there are structural defects in the billet, or if the thermal processing temperature is not appropriate, the plasticity of the material is reduced, or the deformation speed is too fast, the deformation degree is too large, and the plasticity pointer beyond the material is allowed to be removed, Cracks may occur in the process of drawing, punching, reaming, bending, and squeezing.
Cracking presents shallow, crack-like cracks on the surface of the forging. Surfaces that are subject to tensile stress during forging forming (eg, underfilled or bent portions) are most likely to cause such defects. The internal causes of cracking may be numerous:
1) The raw materials are too many fusible elements such as Cu and Sn.
2) When heated at high temperature for a long period of time, the surface of the steel material is precipitated with copper, coarse grains on the surface, decarburization, or surfaces that have been heated many times.
3) The sulphur content of the fuel is too high and sulfur penetrates the surface of the steel material.
6. Flying edge crack
The flash crack is a crack that occurs at the parting surface when die forging and trimming.
The reason for the crack in the flash may be:
1) The threading phenomenon occurs due to the strong flow of metal due to a heavy blow in the swaging operation.
2) The trimming temperature of magnesium alloy die forgings is too low; the cut temperature of copper alloy die forgings is too high.
7. Split surface crack
Parting surface cracking refers to cracking along the die parting surface of a forging. There are many non-metallic inclusions in the raw material, flow to the parting surface during die forging and residual or shrinkage tube residuals often form cracks on the parting surface after crowding the flashes during die forging.
Folding occurs when the oxidized superficial layers of metal are brought together during metal deformation. It may be formed by the confluence of two or more strands of metal convection; it may also be caused by the rapid and massive flow of one metal, which involves the confluence of the surface metal of the adjacent parts, and the confluence of the two; or it may be due to deformation. The metal is bent and reflowed; it is also possible that some of the metal is locally deformed and pressed into another part of the metal. The folding is related to the shape of the raw material and the blank, the design of the mold, the arrangement of the forming process, the lubrication conditions, and the actual operation of the forging. Folding not only reduces the load-bearing area of the part, but it also tends to be a source of fatigue due to stress concentration at work.
9. Through the flow
Through-flow is a form of improper distribution of streamlines. In the flow zone, the flow lines originally distributed at an angle merge to form a flow, and the size of the grains in the flow zone can be quite different. The reason for the flow through is similar to that of folding. It is formed by two metals or one metal with another metal confluence, but the metal in the flow-through part is still a whole, and the through-flow causes the mechanical properties of the forging to decrease, especially When the crystal grains on the two sides of the flow zone are different from each other, the performance is significantly reduced.
10. Forging flow distribution is not smooth
The unfavorable distribution of the flow lines of the forgings means that the streamline cutting, backflow, eddy currents and other streamline disturbances occur at a low magnification of the forgings. If the mold design is not proper or the forging method is not selected properly, the streamline of the preform is disordered; the improper flow of the metal due to the improper operation of the worker and the wear of the mold can make the streamline distribution of the forging unfavorable. Disadvantages in flow lines can cause various mechanical properties to be reduced. Therefore, flow distribution is required for important forgings.
11. Foundry tissue residue Casting
tissue residues mainly occur in forgings with ingots as a blank. The as-cast microstructure mainly remains in the hard deformed area of the forging. Insufficient forging ratio and improper forging method are the main reasons for the formation of residual cast tissue. Casting tissue residue can degrade the properties of the forgings, especially the impact toughness and fatigue properties.
12. Carbide segregation level does not meet the requirements
The level of carbide segregation does not meet the requirements mainly in the Layman tool steel. The carbides in the forgings are mainly distributed unevenly, and are distributed in a large block or distributed in a network. The main cause of this defect is the poor level of segregation of carbides in raw materials, combined with inadequate forging ratio during forging, or improper forging methods. Forgings with this type of defects are easily overheated and quenched during heat treatment and quenching, resulting in cutting tools and dies. Easy to use when broken.
Illustration: Cracks evolved and precipitated carbides
13. Ribbon organization
The banded structure is a kind of organization in which ferrite and pearlite, ferrite and austenite, ferrite and bainite, and ferrite and martensite are distributed in strips in forgings. Asiatic steel, austenitic steel and semi-martensite steel. This kind of organization is the banded structure produced when forging deformation in the coexistence of two phases can reduce the material's transverse plasticity index, especially the impact toughness. When forging or parts work, it is often easy to crack along the boundary of ferrite or two phases.
14. Insufficient local filling
Insufficient local filling mainly occurs in ribs, lobes, corners, and rounded corners, and the dimensions do not meet the pattern requirements.
The reason may be:
1) Low forging temperature and poor metal flow.
2) The tonnage of the equipment is not enough or the hammering force is insufficient.
3) The blank mold design is unreasonable, and the blank volume or cross-section size is not qualified.
4) Stack oxide or weld deformed metal in the mold.
Underpressure is generally increased in dimension perpendicular to the parting plane. The reason for this may be: 1 Low forging temperature. 2 Insufficient tonnage of equipment, insufficient hammering force or insufficient number of hammer blows.
Misalignment is the displacement of the forging along the upper half of the parting surface relative to the lower half.
The reason may be:
1) The gap between the slider (hammer head) and the guide rail is too large.
2) The design of the forging die is irrational, and there is a lack of locks or guide pillars to eliminate misalignment forces.
3) Poor mold installation.
17. Axis bending
Forging axis bending, and the geometric position of the plane error.
The reason may be:
1) The forgings do not pay attention when they are ejected.
2) Uneven stress when trimming.
3) The cooling rate of various parts during cooling is different.
4) Improper cleaning and heat treatment.