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Study on annealing process of red copper


Cracking analysis of pressure type of copper tube

Case description

The product annealing equipment for vacuum annealing furnace, annealing temperature is 620 ~ 650 ℃, heat preservation, 120 minutes, furnace charging way as the instrumentation, single furnace number about 300. The picture is as follows:

Study on annealing process of red copper

Subsequent we adopted box-type annealing furnace, annealing temperature set at 700 ℃, the heat preservation for 90 minutes, about 50 or so, single furnace annealing flattening are some cracking after annealing. After annealing, the surface of this batch of copper copper was seriously oxidized, and the cracked copper pipe was mostly covered with air bubbles, rough stretch and large grains. These symptoms are common after the copper tube is annealed in a reducing gas, called "hydrogen disease", the chemical reaction equation is as follows

Write Cu20 + CO and cu 2 + CO2 (1)

Cu20 + H2 - cu 2 + H2O write (2)

CO2 and H2O generated in the reaction accumulate at the grain boundary. When there are more O elements in copper, the gas pressure generated exceeds the grain boundary strength, resulting in crystal cracking.

 

Cause analysis:

Vacuum annealing or box annealing are all used in the previous process. The setting of annealing temperature, heat preservation time and cooling method are all ok, and this batch of copper pipe material happens to be new material, which is not the same batch as before. We can preliminarily determine that this batch of materials has higher O element. That led to hydrogen disease.

Question 1: how can vacuum furnace annealing produce hydrogen disease?

The operating principle of vacuum annealing furnace is that copper tube is annealed in vacuum to prevent the reaction of O element in copper tube with H element and C element in air.

However, we observed that there is a large amount of metal rust on the seal groove of the vacuum annealing furnace, and there are gaps, scratches and burrs on the sealing washer, which will seriously affect the air tightness of the vacuum furnace. The effect of vacuum annealing cannot be achieved.

In the following work, we replaced the sealing tape, removed the rust in the sealing groove, and reannealed and pressed a batch of materials by filling nitrogen process, as shown in the following figure.

Study on annealing process of red copper

 

It can be seen from the figure that after the vacuum furnace annealing, there is no cracking and no black oxide on the surface.

Question 2: why is the case annealing cracking less than the vacuum annealing?

The box-type annealing furnace does not pump vacuum, nor is the seal unable to prevent (1) and (2) chemical reactions. Theoretically, the cracking should be more serious. However, the tempering time of the box-type annealing is 90 minutes, and quick water cooling is adopted after the heat preservation. The thermal insulation time of the vacuum annealing furnace is 120 minutes. After the thermal insulation, it will be naturally cooled for 4 hours and the total time is 6 hours.

In the case of vacuum furnace leakage, the time of box-type annealing is much less than that of "vacuum furnace" annealing, which greatly shortens the time of (1) and (2) reactions. As a result, the box-type annealing effect is better than that of the air-leakage vacuum furnace annealing effect.

conclusion

Through various comparative tests, we have demonstrated the problems of pressure cracking of copper pipe:

1. There is a large amount of metal rust on the seal groove of the vacuum annealing furnace, and there are gaps, scratches and burrs on the sealing washer, which seriously affect the air tightness of the vacuum furnace. The effect of vacuum annealing cannot be achieved.

2. The high oxygen content in this batch of materials promoted the production of "hydrogen disease" in annealing.

Improvement measures:

1. Improvement of vacuum furnace (solved)

Control from materials. Negotiate with suppliers to replace the previous t2-y with anoxic copper.

 

Optimization of vacuum annealing process of two copper tube

Case description

In our analysis of pressure cracking of copper tube, we found an incidental problem. After annealing at high temperature, the surface of the product appeared grains, which were very rough. This not only affects the appearance of the product, but also causes the appearance of skin and copper dew after silver plating. As shown in the following figure

Study on annealing process of red copper

After silver plating, peeling, copper exposure

 

Theoretical analysis and test

The optimized process we need is that after annealing, the internal grain structure of the copper pipe is small (the internal grain structure is small, the appearance is smooth, and it is difficult to peel and expose copper after silver plating) and the hardness is lower than 60HB(low hardness is suitable for pressing). Due to different annealing temperatures, the microstructure and hardness of copper are different. High annealing temperature of low hardness, coarse and large structure, easy to press, but affect the appearance. Our aim is to find the best annealing temperature to ensure the grains are small and of suitable hardness. To do this, we did the following three experiments with the material of coat 37x4.5

1. Vacuum annealing furnace, annealing temperature is 620 ℃ ~ 650 ℃, the heat preservation for 120 minutes, furnace charging instrumentation, this paper is about 6 pieces of single furnace. Test for 54 hb hardness, test results are as follows:

Study on annealing process of red copper

620 ℃ ~ 650 ℃, 120 minutes of hot annealing effect

 

2. The vacuum annealing furnace, annealing temperature is 520 ~ 550 ℃, the heat preservation for 90 minutes, furnace charging way as the instrumentation, about 6 single furnace. Test hardness of 39 hb. The test results are shown in the following figure

Study on annealing process of red copper

520 ℃ ~ 550 ℃, 90 minutes of hot annealing effect

 

3. Vacuum annealing furnace, annealing temperature is 450 ℃ ~ 480 ℃, the heat preservation for 90 minutes, furnace charging instrumentation, this paper is about 6 pieces of single furnace. Test the hardness of 37 hb. The test results are shown in the following figure

Study on annealing process of red copper

450 ℃ ~ 480 ℃, 90 minutes of hot annealing effect

 

conclusion

It can be concluded from the test that the hardness measured in the three groups is 39HB, 37HB and 54HB, which can meet the hardness of no more than 60HB, and the lower the temperature, the smoother the surface. Therefore, in the future annealing process, we can properly reduce the annealing temperature and heat preservation time, so as to reduce the occurrence of surface oxides during annealing and the occurrence of copper exposure after peeling and silver plating.

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