free forging

Free forging is a simple and flexible metal forming method. Although free forging is recognized as an outdated and uneconomical forging forming method for small and medium-sized forgings in batch or mass production, it is still a suitable and economical production method for small batch or unit production, especially for large forgings, free forging on hammer or hydraulic press. Free forging is to control the shape and size of forgings by the operation of workers, so the precision of forgings is poor, the forging productivity is low, and the labor intensity is high.

die forging

Die forging is the main forging method for batch or mass production of forgings. Die forging also has disadvantages:(1) large investment in equipment;(2) long production preparation cycle, especially the forging die manufacturing cycle is generally relatively large, the production of small quantities of forgings is not economically cost-effective;(3) forging die cost is high, and low life;(4) process flexibility is not as free forging.

Improper forging process easy to produce defects

1. Large grain

Large grains are usually caused by excessive initial forging temperature and insufficient deformation, or excessive final forging temperature, or the degree of deformation falling into the critical deformation zone. Aluminum alloy deformation degree is too large, the formation of texture; high temperature alloy deformation temperature is too low, the formation of mixed deformation tissue may also cause coarse grains, coarse grains will make the plasticity and toughness of forgings reduced, fatigue performance decreased significantly.

2. Uneven grain

Uneven grain means that the grains of some parts of the forging are particularly coarse, but some parts are smaller. The main reason for the uneven grain is that the uneven deformation of the blank makes the degree of grain crushing different, or the degree of deformation in the local area falls into the critical deformation zone, or the local work hardening of high-temperature alloy, or the local grain coarseness during quenching and heating. Heat-resistant steels and superalloys are particularly sensitive to grain inhomogeneities. Uneven grain will make the durability of forgings, fatigue performance decreased significantly.

3. Cold and hard phenomenon

Deformation due to low temperature or deformation speed is too fast, as well as after forging cooling too fast, may make the recrystallization caused by softening can not keep up with the hardening caused by deformation, so that the forging after hot forging internal still part of the cold deformation tissue. The existence of this structure improves the strength and hardness of the forging, but reduces the plasticity and toughness. Severe chilling may cause cracking.

4. Cracks

Cracks are 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 billet has the greatest stress and the thinnest thickness. If there are micro cracks on the surface and inside of the blank, or there are tissue 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 degree of deformation is too large, exceeding the allowable plastic pointer of the material, etc., cracks may occur in the processes such as roughing, drawing, punching, reaming, bending and extrusion.

5. Cracks

Crack is a shallow turtle crack on the surface of the forging. Such defects are most likely to occur on surfaces that are under tensile stress in forging formation (e. g., under-filled bulges or portions subjected to bending).

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 for a long time at high temperature, the surface of the steel material has copper precipitation, coarse surface grains, decarburization, or the surface after repeated heating;(3) the sulfur content of the fuel is too high, and there is sulfur penetration on the surface of the steel material.

6. Fly edge crack

The flash crack is a crack generated at the parting surface during die forging and trimming.

The causes of flash cracks may be:(1) in the die forging operation due to heavy blow to the strong flow of metal to produce the phenomenon of rib penetration;(2) magnesium alloy die forgings cutting edge temperature is too low; copper alloy die forgings cutting edge temperature is too high.

7. Fracting surface crack

A parting surface crack is a crack that occurs along the parting surface of a forging. There are many non-metallic inclusions in the raw materials, and the flow and concentration to the parting surface during die forging or the residual shrinkage tube often forms cracks on the parting surface after squeezing the flash during die forging.

8. Folding

Folding is formed by the merging 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 to flow together. It can also be formed by bending and reflowing deformed metal. It can also be formed by partial deformation of part of the metal and being pressed into another part of the metal. 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 bearing area of the part, but also becomes the fatigue source due to the stress concentration here.

9. Flow through

Crossflow is a form of improper distribution of streamlines. In the flow-through area, the streamlines originally distributed at a certain angle merge together to form a flow-through, which may make the grain size difference between the flow-through area and the outside. The cause of flow-through is similar to folding, which is formed by two strands of metal or one strand of metal with another strand of metal converging, but the metal in the flow-through part is still a whole, which reduces the mechanical properties of forgings, especially when the grains on both sides of the flow-through belt are quite different, the performance is obviously reduced.

10. Forging streamline distribution is not smooth

The irregular distribution of the forging flow line refers to the flow line disorder phenomenon such as flow line cutting, reflux and eddy current on the low times of the forging. If the mold design is improper or the forging method is unreasonable, the flow line of the prefabricated blank is disordered; the improper operation of the worker and the mold wear and the uneven flow of the metal can make the forging flow line distribution not smooth. Flow line is not smooth will reduce a variety of mechanical properties, so for important forgings, there are flow line distribution requirements.

11. Residual casting structure

The casting structure residue mainly appears in forgings with ingots as billets. The as-cast structure mainly remains in the difficult deformation zone of the forging. Insufficient forging ratio and improper forging method are the main reasons for the residual casting structure. Casting microstructure residue will reduce the performance of forgings, especially the impact toughness and fatigue properties.

12. Carbide segregation level does not meet the requirements

Carbide segregation levels do not meet the requirements mainly in the ledegreete mold steel. The main reason is that the carbide in the forgings is not evenly distributed, with large lumpy concentrated distribution or mesh distribution. The main reason for this defect is that the raw material carbide segregation level is poor, and the forging ratio is not enough or the forging method is improper when changing the forging. Forgings with this defect are prone to local overheating and quenching cracking during heat treatment and quenching, and the cutting tools and dies are easy to collapse when used.

13. banded tissue

The banded tissue is a kind of tissue in which ferrite and pearlite, ferrite and austenite, ferrite and bainite, and ferrite and martensite are banded in forgings, and they appear in sub-common folding steel, austenite steel and semi-martensite steel. This kind of organization is the banded structure produced during forging deformation under the coexistence of two phases, which can reduce the transverse plastic pointer of the material, especially the impact toughness. It is often easy to crack along the ferrite band or the junction of the two phases during forging or part work.

14. Insufficient local filling

Local insufficient filling mainly occurs in rib, convex corner, corner and fillet, and the size does not meet the requirements of the drawing.

The reasons may be:(1) the forging temperature is low, the metal flow is poor;(2) the equipment tonnage is not enough or insufficient hammering force;(3) the blank mold design is unreasonable, the blank volume or section size is not qualified;(4) the accumulation of oxide in the die chamber or welding deformation of metal.

15. Undervoltage

Underpressure refers to a general increase in the dimension perpendicular to the direction of the parting plane.

The reasons may be:(1) low forging temperature;(2) insufficient equipment tonnage, insufficient hammering force or insufficient hammering times.

16. Wrong move

Misplacement is the displacement of the upper half of the forging relative to the lower half along the parting surface.

The reasons may be:(1) the gap between the slider (hammer) and the guide rail is too large;(2) the forging die design is unreasonable, the lack of the lock or guide post to eliminate the wrong movement force;(3) the mold installation is poor.

17. Axis bending

The axis of the forging is bent, and there is an error with the geometric position of the plane.

The reasons may be:(1) the forging out of the mold is not paid attention to;(2) the uneven force when trimming;(3) the cooling speed of each part is different when the forging is cooled;(4) improper cleaning and heat treatment.