Quality control of vacuum brazing for plate heat exchanger
1. The technology of vacuum brazing has been applied late in China, and great progress has been made in the application and development of vacuum brazing technology since the 1970s. With the development of modern aviation, aerospace and electronics industries, as well as the appearance of many new type of high temperature alloy and application of vacuum brazing technology of its unique advantages will gain more and more widely used in industry. Brazing plate heat exchanger is a new type of special pressure vessel in China. Its design and manufacture are in accordance with the regulations of the United States, Japan and other industrial developed countries, and there are mandatory licensing requirements. In order to connect with the international market, China is also actively doing this work.
2. The structural characteristics of increasing heat transfer area is a major means of strengthening heat transfer effect, but increase the heat transfer area should not increase the overall dimensions of the equipment to achieve alone, and should consider from the structure characteristics of the equipment, improve its compactness, namely the offer as far as possible big heat transfer area per unit volume. In addition, the thinner the steel plate, the better heat transfer effect, the use of ultra-thin plate structure is the most ideal. Obviously, plate heat exchanger is much more advanced in heat transfer efficiency, compactness, unit heat exchange area and metal consumption than tube and shell heat exchanger. General detachable plate heat exchanger in sealing performance and pressure of less than the tube heat exchanger, but in a large vacuum furnace brazing method is used to manufacture welded plate heat exchanger to overcome the detachable shortcomings about the worse performance of plate heat exchanger and sealing pressure, and has all the advantages of the plate heat exchanger. The basic structure of the brazing plate heat exchanger is composed of a number of flow plates with corrugated grooves (constituting the heat transfer medium channel) and two cover plates at the front and back ends which are designed in the opposite direction. Then, the thickness 0, which is consistent with the size of the circulation plate, is placed between the circulation plate and between the circulation plate and the cover plate. 07mm pure copper foil (solder). According to the needs of users, as long as the number of circulation plate can be increased or decreased, the purpose of changing the original heat exchange area can be achieved, thus easy to form a series of products.
3. The process of vacuum brazing in vacuum brazing process is: preparation, assembly, filler metal filler metal, coating flow inhibitor (if necessary), brazing and furnace inspection before brazing of parts. According to the structure of components, filling solder can simultaneously during assembly, the solder can be used as a spare parts and other parts assembled together, as mentioned in this article the brazing of plate heat exchanger assembly is a typical example.
3. 1 parts ready to clean and no oxide surface before soldering is the necessary condition to ensure high quality brazing welding head: remove oxide on the surface of the parts, when mass production, had better use ultrasonic cleaning method or chemical cleaning method, both can get the surface quality of the surface of consistent high quality parts, and high productivity. As for the oil removal of brazing parts, it is usually carried out in trichloroethylene steam oil tank in batch production.
3. When assembling and locating plate heat exchanger of part 2, a layer of copper foil solder shall be inserted between each circulating plate (cover plate) and assembled together, and then clamped with clamping tool. The clamping surface in contact with the upper and lower cover plates shall be coated with a flow inhibitor in advance to prevent the liquid solder from flowing to the joint surface between the two and the clamping device from welding firmly. The components are then fed into the furnace.
3. 3 the principles of choosing brazing process parameters in order to ensure enough qian penetration rate, rational alloying reaction, avoid brazing defects, to obtain the high quality of braze welding head, need to fully consider reasonably choose brazing process parameters. The process parameters of vacuum brazing include: vacuum degree, heating speed, stable temperature and time, brazing temperature and time, cooling speed and method, furnace temperature, etc.
(1) the vacuum degree of vacuum is a kind of precise control of brazing atmosphere more economic method, vacuum can make the surface to achieve the required cleanliness, so that the solder without flux has good wettability and liquidity, in order to prevent be bit parts and furnace components (mainly the heating element and radiation panel, etc.) oxidation discoloration, and avoid the heat directly through the mechanical pump, in front of the stove temperature, furnace chamber should be take in advance to a certain degree of vacuum, the choice of the vacuum degree is mainly according to the nature of the brazing material, for stainless steel parts, should reach (1 ~ 5) x 10-4. Then the furnace can be heated. As a result of the copper vapor pressure is higher (at 946 ℃ for 10-2), so in 10-4 under the condition of pressure and temperature over 1000 ℃, copper will be a large number of volatile. In order to prevent the volatilization of the copper, when the stove heated to above 900 ℃, the purity of inert gas should be to the vacuum furnace filling height, make the pressure in the furnace is up to (2 ~ 3) x 10-2 Joe (the pressure slightly higher than the volatile balancing pressure value), to prevent the volatilization of copper. At the same time the flow of the inert gas filling will impact on the furnace heat convection, make component by the radiant heat is more uniform, vacuum brazing is visible in the furnace pressure for (2 ~ 3) x 10-2 supporting dynamic balance in the flow of the inert gas.
(2) heating speed the heating speed of the furnace is mainly determined by ensuring that the gas precipitated from the parts, fixtures and other parts is fully extracted to maintain the vacuum degree in the furnace; The components should also be heated uniformly to reduce or prevent deformation of the components during heating.
(3) a stable temperature and time stability temperature and time, refers to the heated to solder solidus line under a certain temperature (or some), stop heating and at this temperature, stay for a period of time, the purpose is to minimize the temperature gradient of the components inside and outside, make the temperature of the component parts as far as possible even.
(4) the brazing temperature and holding time on the brazing temperature, on the one hand to make the solder melts, and I will have enough time of heat preservation, the molten solder fill under the action of capillary force, joint clearance, at the same time with the substrate metal alloying; On the other hand, austenite stainless steel materials to complete the process of solid solution treatment. In this way, the brazing and solid solution treatment process is complete, at the same heating cooling cycle of austenitic stainless steel parts by pure copper solder brazing temperature at 1120 ℃ for brazing can at the same time satisfy the above two basic requirements.
(5) in order to prevent air flow from stirring the liquid solder, when the solid phase line temperature of the solder is above, it shall be cooled with the furnace in a vacuum; Such as furnace temperature to 1000 ℃ after solder has been completely frozen, and then fill the high purity of inert gas and start the vacuum chamber of the fan, dropping temperature rapid mixing complete austenitic stainless steel materials of solid solution treatment process.
(6) melt temperature of stainless steel components, it is advisable to take out temperature less than 150 ℃.