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Vacuum Heat Treatment Furnace

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Vacuum Brazing Furnace

Diagnosis and Treatment of Discoloration Faults of Vacuum Brazing Furnace Parts

In recent years, vacuum furnaces have been widely used in factories. Some old furnaces that have been used for many years are prone to oxidation and discoloration of parts during processing, which seriously affects the quality and efficiency of parts processing. The following is an introduction to quickly judging the cause of the failure of parts discoloration, and taking corresponding maintenance and preventive measures.

1 Working principle of vacuum brazing furnace

The typical structure of vacuum brazing furnace is shown in Figure 1. When working, first start the mechanical pump, open the bypass valve, and evacuate the vacuum chamber and pipeline. When the vacuum pressure is lower than a specified value of the system (such as 800 Pa), start the Roots pump and open the main circuit valve (vacuum the diffusion pump). When the vacuum pressure is lower than the allowable value of the diffusion pump manufacturer (such as 30 Pa), turn on the heating button to preheat the diffusion pump. When the diffusion pump is preheated to the specified temperature (such as 235°C) and the vacuum pressure is lower than the specified value of the system (such as 5Pa), open the high vacuum valve and pump a high vacuum (such as <10-Pa) to the vacuum chamber.

2 Analysis of the causes of discoloration of parts

The discoloration of the parts is due to the color changes of the parts such as black, blue and gray caused by oxidation of the parts at high temperature during the brazing process, which cannot meet the process inspection requirements. The main reasons are air leakage and water leakage, low pump efficiency, impure argon gas and oil return in the furnace. There are also some other reasons, such as fixtures or parts not cleaned, long-term out of use or not oven-baked before re-moisture processing in the rainy season, solder, operation and other issues.

3 Ways to quickly diagnose discoloration of parts

3.1 Air leakage

Air leakage will reduce the ultimate vacuum degree, and the vacuum degree beating is not obvious (but when the inside of the vacuum gauge ruptures and leaks and the air leaks black, the vacuum degree will also beating, which should be excluded first), and a helium mass spectrometer leak detector can be used for leak detection. Air leakage mainly occurs in the sealing rubber ring at the connection between the furnace door of the furnace body and the outside world. Common parts include the sealing ring of the furnace door rubber ring, high vacuum valve cylinder, water-cooled electrode, temperature measuring coupler seat and other rubber ring seals. Due to the alternating temperature of the vacuum gauge, the glass wall inside the gauge is broken and leaked, resulting in blackening.

3.2 Water leakage

The ultimate vacuum degree does not drop significantly (except when the water leakage is serious and the liquid water is visible), sometimes only half an order of magnitude, it can be seen that the vacuum degree “jumps” (instantly drops and then rises) during the heating process; the inner surface of the furnace wall has top-to-bottom White blots, traces of leaks after evaporation (but not obvious when the leaks are tiny). Common parts are: heat exchanger, heat exchanger connecting pipes and welds, water-cooled electrodes, and the inner wall of the furnace.

3.3 Low pump efficiency

The pumping speed will be slower, and the efficiency of the three-stage pump can be measured by the detection device. The working temperature of the mechanical pump is usually required to be ≤75 °C, and the temperature can be measured with the help of a “portable infrared thermometer”.

3.4 Detection method of argon purity

When using a dew point meter, it is required to be ≤-54°C, preferably ≤-58°C; when using a trace oxygen tester, the oxygen content is required to be ≤2×106; when the argon gas impure causes the parts to change color, the cooling (slow cooling) method can be adopted. , or after the temperature in the furnace is appropriately lowered, argon rapid cooling is started. The fault can be judged by improving the color of the parts.

3.5 Oil return

It is often seen that black solidified or semi-solidified liquid oil is attached to the surface of the radiator, air cooler and cold wall pipeline on the inner side of the furnace wall (near the diffusion pump channel).

(1) The mechanical pump returns oil. When the vacuum degree in the furnace is higher than the vacuum degree of the mechanical pump pipeline, when the bypass valve is not closed tightly or is opened, the mechanical pump will return oil.

(2) The reason for the oil return of the diffusion pump. ①When the mechanical pump Roots pump is turned on only, the high vacuum valve is not closed tightly; ②When the diffusion pump is working, the water cooling capacity of the outer wall of the diffusion pump, the cold well or the cold water baffle is insufficient.

4 Maintenance measures

4.1 Maintenance method of air leakage

(1) A helium mass spectrometer leak detector is recommended. The helium mass spectrometer leak detector (hereinafter referred to as the leak detector) is a mass spectrometer leak detector that uses helium as the leak gas. It is the most sensitive and most commonly used leak detector in vacuum leak detection. The leak detector has two operating modes “Vacuum” and “Sniffer”. In leak detection, vacuum mode is usually used (Figure 2).

4.2 Maintenance method of water leakage

(1) Find the leak point method. When the water leakage is serious, the leakage point can be directly seen, but it is usually necessary to remove the hot zone, heat exchanger, cover plate and other coverings. When checking the heat exchanger, usually put the heat exchanger into the water tank, after inflation and pressure, observe the bubble phenomenon, which is the leak point. For small leaks: blow out the water in the vacuum brazing furnace body interlayer and water pipe with compressed air, and then use a vacuum pump to connect it to the furnace body interlayer and the water inlet or outlet of the cooling water pipe, and vacuum the furnace body interlayer and water cooling pipe. , in order to detect leaks in the furnace interlayer and water pipes, using the sniffer mode of the helium mass spectrometer leak detector (Figure 3). Operation process: The water in the vacuum furnace was stopped, the cooling water in the interlayer was placed, and 10 kg of compressed air was charged, and the leak point was found with a helium mass spectrometer leak detector.

(2) Other methods. Apply a layer of red syrup on the inside of the furnace wall, fill the furnace with positive pressure, wipe off the red syrup on the surface, and observe the cracks.

4.3 Maintenance methods for low pump efficiency

Field measurement method. ①Mechanical pump, Roots pump pumping capacity judgment method: install a “low vacuum gauge” on the pipeline, close the main circuit valve and bypass valve, turn on the mechanical pump and the Roots pump in turn, record the low vacuum count value respectively, compare Factory specification of pump model: ultimate pressure. ②The method of judging the pumping capacity of the diffusion pump. Turn on the front pump, when the vacuum degree meets the starting conditions (usually vacuum degree <5Pa, oil temperature>235℃), start the diffusion pump, within a period of time, the vacuum in the furnace should meet the process requirements or factory specifications.

4.4 Detection method of argon purity

(1) Use a dew point meter. Requirements ≤-54 ℃, preferably ≤-58 ℃.

(2) Trace oxygen tester. The oxygen content is required to be ≤2×10. The operator sometimes blows the dust on the parts with the argon gas in the argon gas tank. After a long time, the argon gas in the tank will be exhausted, but he does not know how to inflate the argon gas tank. At this time, if the inspection door or pipeline of the argon gas tank leaks, the oxygen and water molecules in the air will enter the argon gas tank and be mixed in the argon gas. When the parts are rapidly cooled, the parts will be oxidized and discolored.

(3) Check the argon gas tank and pipeline for leaks. Check with soap bubbles to observe bubbles. (4) The pipeline of the argon gas tank can be modified to increase the function of automatic filling of argon gas. On the one hand, it ensures that the pressure in the argon tank meets the working requirements, and on the other hand, it reduces the oxidation and discoloration of parts during rapid cooling caused by the air entering the argon tank.

4.5 The treatment method of returning oil in the furnace

(1) Manual scrubbing. Gas welding can be used first, and then gasoline, alcohol, acetone, etc. can be used as solvents to remove oil stains on furnace walls, water-cooled baffles, high vacuum valves, and pipes. The radiator can be disassembled and put into the “ultrasonic cleaning tank” for cleaning. However, because the places where the oil accumulates are mostly places where heat cannot be radiated, and also places that are not easy to reach by hand or general tools, such as the cracks between layers of multi-layer reflectors, it is very difficult to clean.

(2) A simple method to deal with oil pollution discovered in the work. After the furnace or brazing is completed, the heat in the furnace is used, and the argon gas cooling process is used to conduct the heat through the convection circulation of argon in the furnace, and the cold components such as the emission plate in the furnace are heated to raise the temperature to 200 ℃ Above, the oil stains adsorbed on its surface are evaporated. Using this method to remove oil stains, the effect is very good, and it is relatively simple to do.