(Research) Die Crack Failure of 3500T High Speed Forging Automatic Production Line

3500t high-speed forging automatic production line is a world-class forging automatic production line (Fig. 1) bought by our company from Germany in 2005.

forging automatic production line

Fig. 1 panorama of 3500t high speed forging automatic production line

The design maximum production beat is 25 pieces / min.

Feeding, heating, forging and afterheat treatment are all automatically controlled.

The closed forging process is adopted to realize forging forming in five steps: pre upsetting → upsetting → pre forging → final forging → punching.

As shown in Fig. 2, the cooling mode of the mold is automatic spray, and the medium is a mixture of water and mold release agent in a certain proportion.

(Research) Die Crack Failure of 3500T High Speed Forging Automatic Production Line 1

Fig. 2 automatic spray cooling and lubrication

In actual production, it has been found that the die life of some forging varieties has not reached the normal value, and the main failure form is die crack (Fig. 3).

On average, about 1500 pieces are cracked, about 2500 molds are scrapped, about 300 pieces are cracked in serious cases, and about 1200 molds are scrapped.

position and form of mold crack (crack in red circle)

Fig. 3 position and form of mold crack (crack in red circle)

Failure cause finding

In the first step, the incoming physical and chemical inspection records and heat treatment physical and chemical inspection records of die steel were retrieved, and the hardness of the failed die was verified, and no obvious abnormalities were found.

Second, after careful observation of the shape, size and depth of the crack of the field failure mold, it is found that the height of the crack is suspected to be gradually formed by cutting with high temperature and high pressure gas.

The third step is to simply assemble the upper and lower dies of final forging and pre forged hot blanks with the help of CAD (Fig. 4).

It is found that although there is a closed space with a height of about 1.9mm here, there is an exhaust hole, which is no problem in design, but in actual production, the die crack here is obvious, which needs to be carefully analyzed with the help of deform simulation software.

CAD drawing of final forging upper and lower dies and pre forged hot blanks assembly

Fig. 4 CAD drawing of final forging upper and lower dies and pre forged hot blanks assembly

Fourth, through Deform simulation and careful observation, when the final forging forming simulation runs to step 16, the arcs on both sides of the final forging upper die spokes have first contacted with the hot pre forged blank, and an independent confined space is formed between the two nearest exhaust holes on the left and right, as shown in fig. 5.

(Research) Die Crack Failure of 3500T High Speed Forging Automatic Production Line 2

Fig. 5 failure mold deform simulation (independent confined space in the green circle)

Through the deform simulation forming analysis of several other abnormal crack dies, it is found that during the forging process, there is also the phenomenon of forming an independent confined space between the hot blank and the two nearest exhaust holes on the left and right of the upper and lower dies, as shown in Fig. 6, Fig. 7 and Fig. 8.

deform simulation of failure mold II (independent confined space in the green circle)

Fig. 6 deform simulation of failure mold II (independent confined space in the green circle)

three deform simulation of failure mold (independent confined space in the green circle)

Fig. 7 three deform simulation of failure mold (independent confined space in the green circle)

deform simulation of four failed molds (independent confined space in the green circle)

Fig. 8 deform simulation of four failed molds (independent confined space in the green circle)

The fifth step is to analyze the above information in detail as follows.

Due to the production characteristics of 3500t high-speed forging automatic production line, at the moment when the slider reaches the top dead center, the spray system will quickly stretch into the upper and lower molds of five steps and spray a large amount of mold release agent and water mixture on the upper and lower molds for about 1 second.

The timely spraying of a large amount of coolant (the mixture of mold release agent and water) can not only ensure the timely and stable demoulding and ejection of hot forgings, but also ensure that the working temperature of the die is stable in a certain low temperature range, so as to prevent the die from being scrapped due to serious wear or collapse deformation due to the rapid rise of temperature in a short time.

In short, the timely spraying of a large amount of coolant is an important means to ensure the smooth operation of high-speed automatic forging, which is indispensable.

During forging, the hot blank (temperature about 1150 ~ 1200 ℃) in each step contacts with the mold (temperature about 200 ~ 300 ℃) at high speed, which can instantly convert the coolant into high-temperature and high-pressure gas, especially the high-temperature and high-pressure gas converted by the coolant in an independent confined space cannot be smoothly discharged along the exhaust hole, and finally the high-temperature and high-pressure gas will act on the hot mold, causing cutting damage to it.

Improvement measures

Based on the above analysis results, the improvement measures are formulated.

First, the previous pre forging die of the failed die I is experimentally modified, so that when the new hot pre forging blank is put into the final forging cavity for forging, the spoke surface of the final forging upper die first contacts the hot pre forging blank spoke, and there is no independent confined space between the two nearest exhaust holes on the left and right.

A large amount of high-temperature and high-pressure gas generated during forging can be discharged smoothly and timely from the exhaust hole, the gap between the upper and lower ejector rods and the mold, and the gap between the split molds, as shown in Fig. 9.

The experimental improvement scheme for the failed mold I has been very successful in the practical verification, and the mold life has reached 15000 pieces, and there is no mold crack failure again.

deform simulation of failure mold after improvement

Fig. 9 deform simulation of failure mold after improvement (there is no independent confined space in the green circle)

According to the successful experience of experimental improvement, we have improved other problematic molds one by one, so that when the new hot forging blank enters the problem step, there will be no independent confined space between the new hot forging blank and the two nearest exhaust holes on the left and right of the mold. The improvement method is shown in Fig. 10, fig. 11 and Fig. 12.

deform simulation after improvement of failed mold II (there is no independent confined space in the green circle)

Fig. 10 deform simulation after improvement of failed mold II (there is no independent confined space in the green circle)

deform simulation after improvement of failure mold III

Fig. 11 deform simulation after improvement of failure mold III (there is no independent confined space in the green circle)

deform simulation after improvement of failed mold IV (there is no independent confined space in the green circle)

Fig. 12 deform simulation after improvement of failed mold IV (there is no independent confined space in the green circle)

Improvement effect

The failure molds I, II, III and IV were improved respectively, and the effect was very good.

The average service life of the molds reached 15000 pieces, and there was no crack failure in the molds.

The economic benefit of saving die cost is obvious (Table 1), and the appearance quality of forgings is ensured, and the waste of manpower and material resources during subsequent repair of forgings is avoided.

Table 1 cost saving table of improved mold

NO.

Forging demand (10000 pieces / month)

Mold demand before improvement (piece / month)

Mold demand after improvement (piece / month)

Die saved after improvement (piece / month)

Cost saved after improvement (10000 yuan / month)

Monthly savings (10000 yuan / month)

Annual savings (10000 yuan)

Mold I

3

12~20

3~4

10

4.5

18

216

Mold II

Mold III

Mold IV

Improvement summary

(1) In order to maintain the stable operation of 3500t high-speed forging automatic production line, a large amount of mist coolant will be sprayed, but when the spray device fails, such as air valve failure (upper and lower mold spray air valve has no air or lower mold spray has no air), water valve failure (upper mold spray water valve does not spray, lower mold spray water valve is sprayed for a long time, or both upper and lower mold are sprayed for a long time).

The failure of the release agent valve (the upper mold spray release agent valve is not sprayed, the lower mold spray release agent valve is sprayed for a long time, or both the upper and lower mold are sprayed for a long time, etc.) will cause more water or the mixture of water and release agent to remain in the low concave of the hot blank, and sometimes the upper mold can not be lubricated and cooled in time.

The temperature rises quickly, and the mold before improvement is easier to be cut and damaged by high temperature and high pressure gas.

(2) High temperature and high pressure gas acts on the hot die, causing cutting damage to it, so why not cut the hot forging, but only the die.

Later, it was concluded that the hot forging (temperature about 1150 ~ 1200 ℃) has extremely high plasticity, and the die temperature (working temperature about 200 ~ 300 ℃) does not allow it to have plasticity.

High temperature and high pressure gas acts on hot forgings and hot dies at the same time. Because hot forgings have extremely high plasticity, the cutting trace will repair itself.

In contrast, the die must ensure a relatively low temperature state to ensure its hardness, and it is not allowed to have a certain plasticity due to a sharp rise in temperature.

The small crack of the high temperature and high pressure gas cutting die cannot repair itself.

As the forging and forming work continues, the very small crack will slowly extend and expand until the mold is scrapped.

(3) Practice has proved that this improvement is successful, and the effect is very obvious, which can be said to be “cure the disease”.

After a long time of observation and verification, 3500t high-speed forging automatic production line has no such die crack failure problem.

This improvement has also been recognized by the relevant designers of the forging process room of the hot forming group of the material physical and chemical center of the group company, and the changed drawings have been issued.

Conclusion

The successful solution of the die crack failure problem of the 3500 ton high-speed forging automatic production line provides details for the die design of the 3500t high-speed forging automatic production line.

After improvement, the appearance quality of forgings is improved, and the die cost is greatly saved.

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