Reasons for scrapping vacuum brazing radiators and solutions

Ⅰ. Reasons for Vacuum Brazed Radiator Failure
Radiators that are scrapped after vacuum brazing are primarily due to numerous leaks, corrosion, and cold joints. These reasons can manifest as follows:
1. Breakdown of the thermocouple housing in the vacuum brazing furnace, air leakage in the vacuum brazing furnace, and oxidation of the product.
2. Oil backflow in the vacuum brazing furnace, resulting in product scrapping.
3. High exit temperature and rapid cooling of the product material, resulting in cracking and scrapping.
4. Product sticking to fixtures, resulting in product scrapping.
5. Incomplete brazing and incomplete melting of the brazing material.
6. Parts not cleaned properly, resulting in leaks and scrapping.
7. Excessive clearance between parts, resulting in cold joints and leaks, resulting in product scrapping.

II. Solutions
1. Product scrapping due to thermocouple breakdown in a vacuum brazing furnace: During vacuum brazing, high temperatures break through the thermocouple tube shell. The temperature displayed on the thermocouple conduction display is significantly inaccurate. Air enters the vacuum brazing furnace along with the tube shell, rapidly degrading the furnace’s vacuum level. This leads to product oxidation and scrapping. Generally, when the thermocouple shell breaks down, the product near the thermocouple in the previous batch becomes noticeably foggy or blackened.
Solution: During the vacuum brazing process, the recorded temperature may be unstable, sometimes exceeding or falling below the actual furnace temperature. If these two conditions are observed, the furnace must be shut down and the thermocouple or shell replaced. When the thermocouple breaks down, a small amount of air enters the furnace chamber, causing the product to turn black.
2. Product scrapping due to oil backflow in a vacuum brazing furnace: Oil backflow refers to improper operation of the vacuum brazing furnace, which allows mechanical pump oil to enter the tubes or furnace chamber, causing black spots on the product or oxidation. If oxidation is severe, the product becomes scrapped. Solution: During daily operation, take targeted measures for the following three possible causes of oil backflow. First, the vacuum pressure in the vacuum brazing furnace is above 8.5E², and the mechanical pump’s suction force is less than the vacuum pressure. When the valves are opened, pump oil enters the furnace through the exhaust pipe. This causes the furnace temperature of the product to be below 800°C. Nitrogen can be flushed in to reduce the vacuum pressure to below 8.5E², then the mechanical pump can be started and the valves opened for exhaust. Follow the prescribed procedure.

Second, when the product temperature in the vacuum brazing furnace exceeds the high temperature of the process forced cooling, the vacuum brazing furnace should be restarted. The partial pressure must be increased to 2.5E² before the mechanical pump can be started. After the mechanical pump runs steadily for 3 minutes, the valves can be opened for exhaust. Follow the prescribed procedure.

3. When removing products from the furnace or opening the furnace door when the furnace is empty, check that the furnace pressure is below atmospheric equilibrium. Whether using positive or negative pressure inflation, the bleed valve should be opened to raise the furnace pressure above atmospheric equilibrium. Then, close the bleed valve, start the mechanical pump, open the valves, and release the locking ring. There are two situations in which the vacuum brazing furnace returns oil: (1) Return oil before high temperature. Depending on the amount of returned oil, the product will have fewer convex surfaces after leaving the furnace, and the surface color will be the same as that of normal products. However, the product will bulge and become scrapped when the static pressure is around 5p. Therefore, the operator should pay special attention. (2) Return oil at low temperature refers to returning oil during strong cooling or leaving the furnace. Black spots will appear on the surface of the product, which can be wiped off with a green sand tape, but the internal quality of the product will remain unchanged. 3. The furnace temperature is high, the product material cools too fast, and the product material cracks. (1) Phenomenon and cause: Product material cracks. During strong cooling, the process card stipulates that 304 stainless steel material should be below 930 degrees. 403 material should be below 850 degrees, and 3003 aluminum material should be below 520 degrees. Strong cooling higher than the process card temperature can cause product surface cracks or no obvious difference in the surface after quenching. In fact, the entire furnace has been scrapped. (2) Solution: Employees should strictly follow the regulations during normal operations. Especially the forced cooling switch, when the high temperature insulation ends, nitrogen automatically flushes and the fan automatically starts rapid forced cooling. When welding aluminum, the vacuum pump is directly turned off before the furnace temperature is reached, and the gas is released out of the furnace, causing the entire furnace product to be scrapped.
4. Sticky mold product scrapping
1. Phenomenon and cause: It refers to the situation that when the brazing process is at high temperature, the copper brazing flux overflows and sticks to the welding mold, causing the product to be scrapped. This is called sticky mold product scrapping.
2. Solution: There are three main reasons for the formation of sticky mold:
(1) The high temperature period of the vacuum brazing process is too long and the temperature is too high. The nitrogen partial pressure in the furnace does not meet the requirement, which is 2.0E2.
(2) The alumina powder is not brushed well or the position of the ceramic piece is incorrect.
(3) There is too much copper brazing material
5. Product unmelting
1. Phenomenon and cause: It refers to the situation that during normal process brazing, a small part of the product in the furnace is unmelted, which is called product unmelting. 2. Solution: The main reasons for unmelted products are:
(1) Inaccurate temperature measurement by thermocouple.
(2) Unreasonable vacuum brazing process, unreasonable brazing temperature and holding time.
(3) Too many products are put into the furnace, the temperature of the products in the center of the vacuum brazing furnace does not reach the brazing temperature, and the products are not placed according to the specified requirements.
(4) Thermocouples are not calibrated regularly as required.
6. Parts are not cleaned clean, leakage and scrap phenomenon and reason: Brazed parts are not cleaned clean and do not meet the welding requirements. Solution: After cleaning, the parts are tested with a No. 38 Dyne pen, which should meet the Dyne pen detection and cleaning continuity.
7. The gap between parts is large, and the welding leakage and scrap phenomenon and reason: There is a clear gap in the weld and the brazing material is discontinuous. Solution: The tolerance of brazed parts must be reasonable. Generally, the plate-fin heat exchanger requires the fin height to be 0.02mm higher than the seal height tolerance.

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