Optimization of cold stamping die forging process

2022-07-26
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Optimization of forging process of Cr12 Steel cold stamping die Abstract: the causes of cracks in the working process of Cr12 Steel cold stamping die are analyzed, and the optimization forging method and process measures to prevent cracks are put forward

key words: cold stamping die; crackle; Forging process; Optimization

I. overview

cr12 steel is a typical cold work die steel, which is widely used in cold stamping dies, drawing dies, screw rolling dies, etc

the silicon steel sheet die for the fan rotor of Xiangtan household appliances factory is made of Cr12 Steel. The die is mainly composed of male and female dies, which are installed on a 600kN stamping machine tool, and the silicon steel sheet with D21 material and 0.5mm thickness is stamped into a hanging fan rotor sheet

the designed hardness of the die is 58 ~ 62Hrc, and the actually measured hardness is 60 ~ 62Hrc, which meets the design requirements. Under normal conditions, the die can punch more than 200000 pieces. However, the die was used less than 9000 times after it was put on the machine, and the die was brought out by the punch. After it was removed from the machine, the die was sharpened and put on the machine again. The die continued to produce the broken block, and cracks appeared on the outer edge of the die. During the continuous punching process, the cracks expanded rapidly, and the die became invalid and unusable less than 20000 times

II. Stamping die heat treatment and forging production process

1 Heat treatment process

during annealing, heat to 850 ~ 870 ℃ for 4h, cool to 730 ~ 740 ℃ with the furnace, and cool to 500 ℃ with the furnace for 4 ~ 5h. Quenching, heating to 980 ℃, oil quenching after heat preservation, and then tempering at 180 ℃ for 3h

2. Forging process

selection of forging blank φ 120mm rolled stock shall be carried out on 500kg air hammer with initial forging temperature of 1050 ℃ and final forging temperature of 820 ℃

axial upsetting and drawing method is adopted in forging production, that is, reciprocating upsetting and drawing without changing the direction along the axial direction of steel material

III. mechanism analysis of crack generation

metallographic microscopic analysis was carried out on samples taken at the front, middle and end of the crack (see Fig. 3, FIG. 4 and Fig. 5 respectively for the microstructure). It was found that the microstructure of the material was not ideal, and the carbide with uneven thickness was distributed in strips. It is this banded carbide that affects the mechanical properties of the material. First of all, the banded carbide zone is a fragile zone with very low strength and poor plastic toughness. It can not withstand the impact force at a reasonable price, and cracks are easy to occur here. Secondly, once the crack appears, it is easy to propagate along the banded carbide zone, because this zone is brittle and easy to produce stress concentration, so this banded carbide zone is the root of crack propagation. This kind of crack propagation is periodic. When the generated crack surface becomes a fatigue crack due to slip, the front end of the crack will become sharp again and continue to expand at the next load. In this way, the continuous loading and crack propagation will eventually lead to the scrapping of the die

the reason for this kind of banded carbides is that Cr12 Steel is a ledeburite steel to avoid wasting power and rotating parts in the pump. It has a high carbon content and contains a large amount of alloy carbides. After rolling in the steel rolling plant, the carbides become banded distribution. The larger the diameter of the rolled profile, the coarser the carbides and the more serious the banded distribution. Obviously, in the process of die manufacturing, the forging process plays a decisive role in improving the banded structure. The quenching of heat treatment adopts one-time hardening method (i.e. low-temperature quenching and low-temperature tempering). At the quenching heating temperature, a large number of carbides cannot be dissolved in austenite, which basically retains the distribution characteristics after forging. Therefore, the heat treatment process cannot eliminate the banded tissue, which accounts for 1/3 of the total population of Mongolia

according to the analysis of forging process, firstly, the tonnage of forging equipment is insufficient, and secondly, the forging method is unreasonable.? φ For 120mm diameter billets, it is difficult to forge through with 500kg air hammer, because Cr12 Steel contains a large number of alloy elements, high deformation temperature and large deformation resistance. Generally, forging hammer equivalent to twice the tonnage of structural steel shall be used for forging. If the tonnage of the forging hammer is too small and the strike force is not enough, the deformation can only occur on the surface, and the carbides in the central part cannot be broken. The forging method adopts the axial upsetting and drawing method. The main disadvantage of this upsetting and drawing method is that the end tends to crack. During repeated upsetting, the contact time between the end face and the anvil surface is long, the temperature drops quickly, and it is easy to crack during drawing (if the crack is not found at this time, it may become the crack source for the future mold cracking). However, the metal deformation in the center is small, and the structure in the center is not much improved. Therefore, The carbides in the core structure could not be redistributed in the forging process, but remained in the state of distribution during rolling, which was the fundamental cause of die cracking

IV. optimization of forging process

1 Select appropriate billet diameter and forging hammer tonnage

the larger the billet diameter, the smaller the deformation during rolling, the more serious the carbide segregation and the coarser the carbide particles, so the original φ 120mm blank φ 80mm, so as to obtain the blank with uniform carbide distribution, which is also the initials of COMAC

the tonnage of the original air hammer is too small, so the deformation will be limited to the surface, and the internal carbides will not be crushed. Therefore, the tonnage of the air hammer should be appropriately increased, and 750KG air hammer can be used to forge through, so as to break the central carbides

2. Multidirectional upsetting and drawing method

multidirectional upsetting and drawing method is shown in Figure 6. It is a good forging method to obtain high-quality die blanks. The forging deformation is uniform, easy to forge through, the structure can be comprehensively improved, the carbide can be finely broken and evenly distributed, and the banded structure formed during rolling is completely eliminated

3. Properly increasing the forging ratio

properly increasing the forging ratio can reduce the degree of carbide unevenness. Using the multi-directional upsetting and drawing method, the total upsetting and drawing times should be 6 ~ 8 times. The total forging ratio shall not be less than 15

4. Avoid forging cracks

it should not be too heavy during forging to avoid forging cracks and crack sources during forging deformation. Chamfer frequently to avoid angular crack caused by temperature difference and additional stress. In addition, the forging shall be carefully observed. If cracks are found, they shall be removed in time to eliminate the crack source

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