Defects caused by improper forging process usually include the following:
1. Large grains
Large grains are usually caused by too high initial forging temperature and insufficient deformation degree, or too high final forging temperature, or deformation degree falling into the critical deformation zone. The deformation degree of aluminum alloy is too large, forming texture; If the deformation temperature of superalloy is too low, coarse grains may also be caused when the mixed deformation structure is formed. The coarse grains will reduce the plasticity and toughness of forgings, and the fatigue property will decrease significantly.
2. Uneven grains
The non-uniform grain refers to that the grain in some parts of the forging is especially coarse, while that in some parts is small. The main reason for the uneven grain is that the uneven deformation of the billet makes the grain broken differently, or the deformation degree of the local area falls into the critical deformation area, or the high-temperature alloy is locally work hardened, or the local grain is coarse during quenching and heating. Heat resistant steels and superalloys are particularly sensitive to grain inhomogeneity. Uneven grain will significantly reduce the endurance and fatigue properties of forgings.
3. Cold and hard phenomenon
During deformation, due to low temperature or too fast deformation speed, as well as too fast cooling after forging, the softening caused by recrystallization may not keep up with the strengthening (hardening) caused by deformation, so that the cold deformed structure remains partially in the forging after hot forging. The existence of this structure improves the strength and hardness of the forging, 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. The place where the crack occurs is usually the place where the billet has the maximum stress and the thinnest thickness. If there are microcracks on the surface and inside of the blank, or there are organizational defects in the blank, or the plasticity of the material is reduced due to improper hot working temperature, or the deformation speed is too fast, the deformation degree is too large, and the allowable plastic pointer of the material is exceeded, cracks may occur in such processes as roughening, elongation, punching, reaming, bending and extrusion.
5. Cracking
Cracking is a shallow turtle shaped crack on the surface of the forging. This kind of defect is most likely to occur on the surface subject to tensile stress during forging forming (for example, the unfilled protruding part or the part subject to bending). The internal causes of cracking may be multifaceted: ① Too many fusible elements such as Cu and Sn in raw materials. ② When heated at high temperature for a long time, the steel surface has copper precipitation, surface grain coarseness, decarburization, or the surface that has been heated for many times. ③ The sulfur content of the fuel is too high, and the sulfur seeps into the steel surface.
6. Flaps and cracks
The flash crack is the crack generated at the parting surface during die forging and trimming. The causes of flash cracks may be: ① The strong metal flow during the die forging operation resulted in the phenomenon of threading. ② The cutting temperature of magnesium alloy die forgings is too low; The trimming temperature of copper alloy die forgings is too high.
7. Cracks on parting surface
The crack on parting surface refers to the crack generated along the parting surface of forgings. There are many non-metallic inclusions in the raw materials. During die forging, the flow to the parting surface and the concentrated or shrunk pipe residues are squeezed into the flash and then the normal component cracks on the die surface.
8. Folding
Folding is formed by the confluence of oxidized surface metals during metal deformation. It can be formed by the confluence of two (or more) metal convection; It can also be formed by the rapid mass flow of a stream of metal to bring the surface metal of adjacent parts to flow, and the two converge; It can also be formed due to bending and backflow of deformed metal; It can also be formed by partial deformation of some metals and being pressed into other metals. Folding is related to the shape of raw materials and blanks, the design of dies, the arrangement of forming processes, lubrication and the actual operation of forging. Folding not only reduces the load-bearing area of the parts, but also becomes the fatigue source due to the stress concentration here.
9. Flow through
Flow crossing is a form of improper streamline distribution. In the cross flow area, the flow lines originally distributed at a certain angle converge to form the cross flow, which may cause a large difference in the grain size inside and outside the cross flow area. The cause of flow through is similar to folding. It is formed by the confluence of two strands of metal or one strand of metal with another strand of metal. However, the metal in the flow through part is still a whole. The flow through reduces the mechanical properties of the forging, especially when the grain difference on both sides of the flow through belt is large, the performance degradation is obvious.
10. Forging streamline distribution is not smooth
Unsmooth streamline distribution of forgings refers to streamline interruption, backflow, eddy current and other streamline disorders at low magnification of forgings. If the die design is improper or the forging method is unreasonable, the flow line of the prefabricated blank is disordered; Uneven metal flow caused by improper operation of workers and wear of dies can lead to uneven distribution of forging streamline. Unsmooth streamline will reduce various mechanical properties. Therefore, streamline distribution is required for important forgings.
11. Residual casting structure
The residual casting structure mainly occurs in the forgings with ingots as billets. As cast microstructure mainly remains in the difficult deformation area of forgings. Insufficient forging ratio and improper forging method are the main reasons for residual casting structure. The residual casting structure will reduce the properties of forgings, especially the impact toughness and fatigue properties.
12. The carbide segregation level does not meet the requirements
The inconformity of carbide segregation grade mainly occurs in ledeburite tool and die steel. It is mainly due to the uneven distribution of carbides in forgings, which are concentrated in large blocks or distributed in networks. The main reasons for this defect are the poor level of carbide segregation of raw materials, insufficient forging ratio or improper forging method during forging modification. Forgings with this defect are prone to local overheating and quenching cracks during heat treatment and quenching, and the cutting tools and dies made are prone to edge collapse when used.
13. Banded tissue
Banded structure is a kind of structure in which ferrite and pearlite, ferrite and austenite, ferrite and bainite, and ferrite and martensite are banded in forgings. They are mostly found in hypoeutectic steel, austenitic steel and semi martensitic steel. This kind of structure is a banded structure produced during forging deformation under the condition of coexistence of two phases, which can reduce the transverse plastic index of materials, especially the impact toughness. It is easy to crack along the ferrite belt or the junction of two phases when forging or parts are working.
14. Insufficient local filling
Local insufficient filling mainly occurs in rib, convex corner, corner and fillet, and the size does not meet the drawing requirements. The reasons may be: ① low forging temperature and poor metal fluidity; ② Insufficient equipment tonnage or hammering force; ③ The design of the blank making die is unreasonable, and the volume or section size of the blank is unqualified; ④ The oxide scale or welding deformed metal is accumulated in the mold chamber.
15. Undervoltage
Undervoltage means that the size perpendicular to the parting surface generally increases, which may be caused by: ① low forging temperature. ② Insufficient equipment tonnage, hammering force or hammering times.
16. Dislocation
The dislocation is the displacement of the forging along the upper half of the parting surface relative to the lower half. The possible causes are: ① The clearance between the slider (hammer head) and the guide rail is too large; ② The design of forging die is unreasonable, and there is no lock or guide pillar to eliminate the displacement force; ③ Poor mold installation.
