Possible scrapping of vacuum brazing furnace products and Solutions – 1

Possible scrapping of vacuum brazing furnace products and Solutions
1、 Vacuum brazing furnace thermocouple shell breakdown, vacuum brazing furnace air leakage, product oxidation scrap
2、 Scrapping of vacuum brazing furnace oil return products
3、 Product material cooling speed too fast: crack scrap
4、 Product scrapping
5、 Parts displaced and scrapped

1、 Scrap of thermocouple breakdown products in vacuum brazing furnace
1. Phenomenon and reason: in vacuum brazing heating, high temperature breakdown thermocouple tube shell. The display screen of thermocouple conduction temperature is obviously inaccurate. As the atmosphere enters the vacuum brazing furnace, the vacuum degree of the vacuum brazing furnace decreases rapidly. The product is oxidized and scrapped. Generally, the thermocouple shell is broken down, and the product in the last furnace is obviously fogged or blackened near the thermocouple.
2. Solution: in the vacuum brazing process, the record temperature is not stable, sometimes over temperature or lower than the actual temperature in the furnace. When the above two points are found, the furnace must be shut down to replace the thermocouple or shell. When the thermocouple breaks down, a small amount of air enters the furnace chamber, and the product turns black, which can be bleached or brazed again. At high temperature, when the breakdown time is short and the cross-section is small, the vacuum brazing furnace temperature must be 5 degrees higher than the upper furnace temperature to whiten. If the breakdown time is more than half an hour and the breakdown section is large, the products in vacuum brazing furnace are basically scrapped;

2、 Scrap of vacuum brazing furnace return oil
1. Phenomenon and reason: oil return refers to the phenomenon that when vacuum brazing furnace is operated improperly, mechanical pump oil enters into the pipe or furnace liner to cause black spots or oxidation of products. If the oxidation is serious, the product is scrapped.
2. Solution: in the process of daily operation, the following three situations may cause oil return to take targeted methods.
Firstly, the vacuum pressure in the vacuum brazing furnace is above 8.5e2, and the pumping force of mechanical pump is less than the vacuum pressure. When the valves are opened, the pump oil enters the furnace along with the suction pipe. Therefore, there are products in the furnace, and the furnace temperature is below 800 ℃. When the vacuum pump is less than 2.5 e, the vacuum pump can be operated in the following order.
Second, when the product temperature in the vacuum brazing furnace is higher than the process strong cold high temperature, the vacuum brazing furnace will restart. When the pressure in the furnace is 2.5e2, the mechanical pump can be started. After the mechanical pump runs smoothly for 3 minutes, open the valves for air extraction. Operate according to the specified sequence.
Third, when the product is out of the furnace or the furnace door is opened when the furnace is empty, check whether the pressure in the furnace is below the atmospheric balance. Whether it is positive pressure or negative pressure, the pressure in the furnace should be higher than the atmospheric balance after opening the vent valve. Then close the vent valve, you can start the mechanical pump, open the valves, open the locking ring.

There are two kinds of oil return from vacuum brazing furnace
(1) According to the amount of oil returned before high temperature, the boss after the product is discharged from the furnace is less than usual, and the appearance color is the same as the normal product, but the product will bulge and be scrapped when the static pressure is about 5p. So the operator pays special attention.
(2) Low temperature oil return refers to the oil returning at the time of strong cooling or discharging. Black spots on the surface of the product can be wiped off with green sand belt, but the internal quality of the product remains unchanged.

3、 Quenching crack of product material
1. Phenomenon and reason: the product material is quenched and cracked, and the process card stipulates that 304 stainless steel material is below 930 degree when it is strongly cooled. 403 materials below 850 degrees. If the forced cooling temperature is higher than the process card temperature, the product surface will be quenched and cracked or there will be no obvious difference in the surface after the sheet is quenched.
2. Solution: employees should operate in strict accordance with the regulations. Especially the forced cooling switch, at the end of high temperature insulation, the nitrogen gas is automatically flushed, and the fan is automatically started, which causes the whole furnace products to be scrapped. Another situation is that we are tired of operation in the second half of the night. It is easy to misjudge the furnace temperature and cause the whole furnace to be scrapped due to misoperation.

4、 Product scrapping
1. Phenomenon and reason: collision scrapping refers to the scrapping of products caused by other items after clamping or discharging.
2. Solution: we can’t touch other objects while we are running. Before entering the furnace, check whether there is collision between the clamps. The furnace should be placed carefully and handled with care. The product shall not touch the welding die when it is discharged from the furnace. After oiling, the product must be balanced when put into circulation. When the large bottom plate products are put into circulation after oiling, the bottom plate shall not touch the vacuum furnace parts; the staff’s sense of responsibility shall be trained.

5、 Product displacement and scrapping
1. Phenomenon and reason: the shift of product parts in vacuum brazing furnace refers to that after clamping, the brazer pushes the clamped product to the designated furnace feeding area with a trolley and puts it on the furnace charging car. Improper force or touching other objects in the process of pushing the furnace will cause a part of the product to shift. If the product is not found to be scrapped after brazing out of the furnace, the product is called another part of the product displaced and scrapped.
2. Solution: special attention should be paid to the whole process, so that the problem can be solved by putting it lightly and pushing slowly.