The situation and solution of the scrapped vacuum furnace

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1.The thermocouple breakdown product is scrapped

Phenomenon and reasons: When brazing and heating, the high temperature breaks down the thermocouple tube shell. The thermocouple conduction temperature display record is noticeably inaccurate. The atmosphere enters the furnace with the tube shell, which makes the vacuum of the vacuum furnace drop rapidly. The product is scrapped due to oxidation. Generally, the thermocouple shell is broken down, and the product in the previous furnace is obviously fogged or blackened near the thermocouple.

Solution: During the brazing process, the recorded temperature is unstable, sometimes over-temperature or lower than the actual temperature in the furnace. When the above two points are found, the furnace must be stopped to replace the thermocouple or shell. When the thermocouple breaks down, a small amount of air enters the furnace cavity, and the product turns black, which can be whitened or brazed again. At high temperature, the shell breakdown time is short, and when the cross section is small, it must be brazed again, and the brazing furnace temperature must be 5 degrees higher than the furnace temperature to be whitened. If the breakdown time is more than half an hour, the breakdown section is large, and the products in the furnace are basically scrapped.

2.Scrap oil return

Phenomenon and causes: Oil return refers to improper operation during vacuum in the vacuum furnace, and the mechanical pump oil enters the tube or the furnace to cause black spots or oxidation of the product. If the oxidation is serious, the product is scrapped and other phenomena are called oil return.

Solution: During the daily operation, take specific methods for the oil return that may be caused by the following three situations. First, the vacuum pressure in the vacuum furnace is above 8.5E2, and the pumping force of the mechanical pump is less than the vacuum pressure. When each valve is opened, the pump oil enters the furnace with the suction pipe. Therefore, there are products in the furnace whose furnace temperature is below 800. Nitrogen can be flushed into the vacuum furnace to make the vacuum pressure of the vacuum furnace less than 8.5E2, and then the mechanical pump can be started to open the valves to extract air, and operate in the specified order.

Second, when the product temperature in the furnace is higher than the high temperature of the process forced cooling, the vacuum furnace is restarted. The partial pressure must be opened so that when the pressure in the furnace is 2.5E2, 2 mechanical pumps can be turned on at the same time to make the mechanical pumps run smoothly for 3 minutes, then open each valve to pump air. Operate in the specified order.

Third, when the product is out of the furnace or the furnace door is opened in an empty furnace, check whether the pressure in the furnace is balanced with the atmosphere. Whether it is positive pressure charging or negative pressure charging, the pressure in the furnace should be balanced with the atmosphere after opening the gas valve. Then close the air release valve, you can start the mechanical pump to open each valve and open the locking ring.

There are two cases of oil return:

(1) Oil is returned before high temperature. According to the amount of oil returned, there are fewer bosses than usual after the product is released, and the appearance color is the same as the normal product, but the product will be scrapped when the static pressure is about 5p. (2) Low temperature oil return refers to the oil return during strong cooling or when the product is out of the oven. There are black spots on the surface of the product, which can be wiped off with green sand belt, but the internal quality of the product remains unchanged.

3.Quenching and cracking of product materials

Phenomenon and reasons: The product material is quenched and cracked, and the process card stipulates that the 304 material is below 930 degrees during strong cooling. 403 material below 850 degrees. Strong cooling higher than the temperature of the process card can cause the surface of the product to be quenched and cracked or the surface of the chip has no obvious difference after quenched and cracked, and the whole furnace has actually been scrapped.

Solution: operate in strict accordance with the regulations during normal operation. Especially the strong cooling switch, at the end of the high temperature heat preservation, nitrogen gas is automatically flushed, and the fan automatically starts rapid strong cooling, causing the whole furnace product to be scrapped. Another situation is that in the second half of the night, the operation is more fatigued, and it is easy to misread the furnace temperature, and the whole furnace will be scrapped due to misoperation.

4.Bump scrap

Phenomenon and reasons: bump scrap means that the product is scrapped when it encounters other items after clamping or when it is released from the oven, which is called bump scrap.

Solution: Do not touch other items during operation. Before entering the furnace, check whether the clamping is bumped. Put it carefully in the furnace, and pay attention to it. When out of the oven, the product cannot touch the welding mold. After the product is oiled, it must be balanced when placed in a turnaround. The bottom plate should not touch the vacuum furnace parts when the large bottom plate product is placed in the turnover after oiling.

5.Parts are displaced and scrapped

Phenomenon and reasons: The displacement of vacuum furnace product parts means that after clamping, the brazing worker pushes the clamped product to the designated furnace entry area with a trolley and places it on the furnace entry car. In the process of pushing into the furnace, improper force or encountering other items causes a certain part of the product to be displaced. If it is not found that the product is scrapped after brazing is released, it is called the displacement and scrapping of the other part of the product.

Solution: Pay special attention to the whole process, do it lightly, slow down, and generally solve the problem.

6.Product hidden injury scrap

Phenomenon and reasons: At present, it mainly occurs in 12.2H. During the clamping process, the ceramic pieces are used on the bottom plate. Due to the poor quality, after clamping, the high-temperature brazing ceramics form few lumps after being pressed. Stain on the master or sheet during the oiling process. Caused when product stacking is running.

Solution: pay more attention when it is released, or correct the ceramic sheet or correct the process without ceramic sheet.

7.Claws are scrapped

Phenomenon and causes: Refers to the clamping ring loosening and touching the product sheet or flange during the brazing process, causing the product to be scrapped, which is called scrapping of the claw.

Solution: When entering the furnace, the claws should be opposite to each other, or they can be isolated by small iron bars or tied with iron wires, and the position and method of the clamping ring should be considered in the design requirements.

8.Scrap welding mold products

Phenomenon and causes: It means that when the brazing process is high temperature, the copper flux overflows and sticks to the welding die to scrap the product, which is called the scrapping of the sticky welding die.

Solution: There are three main reasons for the formation of the sticky welding mold: (1) The high temperature period of the brazing process is too long and the temperature is too high. The partial pressure of nitrogen in the furnace cannot meet the requirements, and the partial pressure requirement is 2.0E2. (2) The alumina powder is not brushed properly or the position of the ceramic sheet is incorrect (3) There is too much copper flux.

9.Scrap bullets

Phenomenon and reasons: It means that a certain section of the upper core piece and the lower core piece cannot be combined during the brazing process. Or the phenomenon that the film is seriously ejected and scrapped is called the scrapping of the film.

Solution: For 403 materials, the effective solution is to place the bottom plate upwards, that is, the welding seam faces upwards. Do not interrupt during heating. If the above brazing method cannot be solved, the structural design must be corrected.

10.White border

Phenomenon and causes: Refers to the irregular appearance of the product after brazing without copper in the weld, which is called white edge.

Solution: The white edge is caused by the unreasonable gap between the seams of the film. However, when vacuum brazing, the white edge can be reduced.

Equipment selection: The vacuum furnace provided by SIMUWU is an excellent product for processing the vacuum heat treatment process. It has the characteristics of good temperature uniformity and high temperature control accuracy. SIMUWU provides a professional team of engineers who can solve various problems encountered in the production process and are committed to giving customers the most convenient and efficient experience.