Common Defects in Forging Technology
作者：admin 来源：本站 发布时间：2019-03-08 06:10:40 浏览量：
Defects caused by improper forging process are usually as follows:
1. Large grain size
Large grains are usually caused by excessive initial forging temperature and insufficient deformation degree, or excessive final forging temperature, or the degree of deformation falling into the critical deformation zone. The deformation degree of aluminium alloy is too large to form texture; the deformation temperature of superalloy is too low, and the formation of mixed deformation structure may also cause coarse grains, which will reduce the plasticity and toughness of forgings and significantly reduce the fatigue properties.
2. Non-uniform grain size
Grain inhomogeneity refers to the fact that the grains in some parts of forgings are especially coarse, but smaller in some parts. The main reason for the uneven grain size is that the uneven deformation around the billet causes the different degree of grain breakage, or the degree of local deformation falls into the critical deformation zone, or the local work hardening of superalloy, or the local grain size is large when quenched and heated. Heat-resistant steels and superalloys are particularly sensitive to grain inhomogeneity. The inhomogeneous grain size will obviously decrease the durability and fatigue properties of forgings.
3. Hardness
The softening caused by recrystallization may not keep up with the strengthening (hardening) caused by deformation due to low temperature or too fast deformation speed and too fast cooling after forging, so that the cold deformation structure remains partly in the forging after hot forging. The existence of this structure improves the strength and hardness of forgings, but reduces the plasticity and toughness. Severe cold hardening may cause forging cracks.
4. Cracks
Cracks are usually caused by large tensile stress, shear stress or additional tensile stress during forging. Cracks usually occur in the parts with the greatest stress and the thinnest thickness. If there are micro-cracks on the surface and inside of the billet, or there are structural defects in the billet, or if the plasticity of the material is reduced due to improper hot working temperature, or if the deformation speed is too fast and the deformation degree is too large, which exceeds the allowable plastic pointer of the material, cracks may occur in the processes of roughening, drawing, punching, expanding, bending and extrusion.
5. Crack
Crack is a shallow tortoise-shaped crack on the surface of forgings. This defect is most likely to occur on the surface of the tension stress in forging forming (e.g., the unfilled protruding part or the bending part). The internal causes of cracking may be multifaceted: (1) excessive amount of fusible elements such as copper and Sn in raw materials. (2) When the steel is heated for a long time at high temperature, copper precipitation, coarse grain size, decarbonization, or multiple heating surfaces occur on the steel surface. (3) The sulphur content of fuel is too high and there is sulphurizing on the surface of steel.
6. Flying edge crack
Flash crack is a kind of crack on the parting surface during die forging and trimming. The causes of flash cracks may be as follows: 1. During die forging operation, the phenomenon of steel bar piercing is caused by 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.
7. Parting surface crack
Parting surface crack refers to the crack along the parting surface of forgings. There are many non-metallic inclusions in the raw materials, and the cracks on the parting surface are often formed when the flow and concentration of the raw materials to the parting surface or the residual shrinkage pipe are squeezed to the flying edge during the die forging process.
8. fold
Folding is formed by the combination of oxidized surface metals during metal deformation. It can be formed by the convection of two (or more) strands of metal; it can also be formed by the rapid and large flow of one metal bringing the surface metal of the adjacent part with it to flow; it can also be formed by the bending and reflux of the deformed metal; it can also be formed by partial deformation of part of the metal and by the compression of another part of the metal. Folding is related to the shape of raw material and blank, the design of die, the arrangement of forming process, lubrication and the actual operation of forging. Folding not only reduces the bearing area of parts, but also often becomes the source of fatigue because of the stress concentration here.
9. through flow
Cross-flow is a form of improper streamline distribution. In the cross-flow zone, streamlines originally distributed at a certain angle converge to form cross-flow, and the grain sizes inside and outside the cross-flow zone may be quite different. The reason of penetration is similar to folding. It is formed by the confluence of two metals or one metal with another metal, but the metal in the penetration part is still a whole. The penetration reduces the mechanical properties of forgings, especially when the grain on both sides of the penetration zone is quite different.
10. Incorrect streamline distribution of forgings
The irregular distribution of streamlines in forgings refers to the disorder of streamlines such as streamline cut-off, backflow and eddy current occurring on forgings at low magnitude. If the die design is improper or the forging method is unreasonable, the flow line of prefabricated blank is disordered; if the worker operates improperly and the die wears and tears, the metal will flow unevenly, the flow line of forgings will not be distributed smoothly. Streamline irregularity will reduce the mechanical properties of various kinds of forgings, so there is a requirement of streamline distribution for important forgings.
11. Cast Tissue Residues
The residual casting structure mainly occurs in Forgings Using ingots as blanks. As-cast structure mainly resides in the difficult deformation zone of forgings. Inadequate forging ratio and improper forging method are the main reasons for residual casting structure. Residual casting structure will degrade the properties of forgings, especially impact toughness and fatigue properties.
12. Carbide segregation level does not meet the requirements
Carbide segregation grade does not meet the requirements mainly occur in ledeburite tool and die steels. The main reason is that the distribution of carbides in forgings is not uniform, and the carbides are concentrated in large blocks or distributed in a network. The main reasons for this defect are poor carbide segregation grade of raw materials, inadequate forging ratio or improper forging method. Forgings with this defect are easy to overheat and crack locally during heat treatment and quenching, and the edges of the tools and dies are easy to collapse when they are used.
13. Banded tissue
Band structure is a kind of structure in which ferrite and pearlite, ferrite and austenite, ferrite and bainite, ferrite and martensite distribute bandwise in forgings. They mostly occur in sub-eutectic, austenitic and semi-martensitic steels. This kind of structure is the banded structure produced during forging deformation under the condition of two-phase coexistence, which can reduce the transverse plastic index of materials, especially impact toughness. It is easy to crack along the ferrite band or the intersection of two phases when forging or parts are working.
14. Insufficient local filling
Local filling deficiencies mainly occur in ribs, convex corners, corners and rounded corners, and the size does not meet the requirements of the pattern. The causes may be as follows: 1) low forging temperature, poor metal fluidity; 2) insufficient equipment tonnage or hammering force; 3) unreasonable design of blank die, unqualified blank volume or section size; 4) accumulation of oxide scale or welded deformed metal in the chamber.
15. under voltage
Underpressure means that the dimension perpendicular to the parting surface generally increases, which may be caused by: (1) low forging temperature. (2) Insufficient tonnage of equipment, insufficient hammering force or insufficient hammering times.
16. dislocation
The offset is the displacement of the forging along the upper part of the parting surface relative to the lower part. The reasons may be as follows: 1) the gap between slider (hammer head) and guide rail is too large; 2) the design of forging die is unreasonable, lacking of locks or guide pillars to eliminate the misalignment force; 3) the installation of die is not good.
