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Vacuum Brazing Furnace

Vacuum Brazing of Stainless Steel Plate-fin Radiator

Brazing is to use a filler material with a lower melting point than the base metal as the solder, heat the weldment and the solder to a temperature higher than the melting point of the solder and lower than the melting point of the base metal, and fill the gap between the base metals by melting the solder. A welding method that achieves a connection by interdiffusion with the base metal. In recent years, vacuum brazing technology has occupied an increasingly important position in various industrial sectors, especially in the electrical instrumentation and national defense industries, and has become an irreplaceable process method. Vacuum brazing is generally used when brazing aerospace alloys such as stainless steel, heat-resistant alloys, titanium, niobium, tantalum and other high melting point metals. Vacuum brazing is a method of brazing in a vacuum atmosphere without flux. The advantage is that the corrosion resistance of the product is significantly improved because no flux is used, and the controllability of the process is good.

Quality assurance and quality control are implemented in all links of stainless steel plate-fin radiator manufacturing (the size of the parts is inspected to make the indicators of the parts meet the requirements of the drawing), assembly clearance, and finished product inspection. The quality control of welded joints includes improving the technical equipment of the enterprise, improving the quality of operators and strict management of the production process. The purpose is to obtain a defect-free welded structure and meet the requirements for the use of welded products in actual work.

In the production process of stainless steel plate-fin automobile oil cooler, vacuum brazing is the core work in the whole production process. The welding quality control of vacuum brazing should run through the whole production process, that is, the vacuum process control, which is the welding production process. Indispensable from beginning to end, it is an important measure to ensure the final welding quality of the product.

The main points of vacuum process control are as follows:

Cleaning before welding: ultrasonic cleaning, steam degreasing, trial assembly, solder processing, ultrasonic cleaning of solder, steam degreasing, assembly (solder assembly), pre-drying of parts to relieve stress.

Vacuum brazing process: ensure the uniformity of temperature in the vacuum welding furnace, the temperature uniformity of the vacuum brazing furnace itself and the vacuum brazing time. Since in the process of vacuum heating and welding, the heat transfer method is only heat radiation without conduction and convection, so the amount of furnace loading and the way of furnishing in each batch will directly affect the uniformity of the temperature in the furnace. In order to ensure the uniformity of temperature throughout the vacuum welding furnace and reduce the welding thermal stress caused by uneven furnace temperature, strict process specifications should be formulated according to the product. For example: large products are placed outside, and small products are placed in the middle of the furnace and monitored.

The whole vacuum brazing process is divided into 7 steps, the specific operation is as follows:

(1) Vacuuming stage: the degree of vacuum is 0.01 Pa. Its purpose is to effectively remove the oxide film on the metal surface, improve surface wettability, and reduce joint pores, inclusions and other defects.

(2)Heating stage: In order to reduce thermal stress, slowly heat to 960°C for 50 minutes. Minimize part deformation as much as possible.

(3)Heat preservation stage: heat preservation at 960°C for 40 minutes to fully remove the impurities of the parts volatilized during the vacuum brazing process of the plate-fin structure in the furnace and the residual gas of the parts and materials themselves.

(4) Heating stage: Continue to heat slowly to Tb = 1060~1250°C for 30 minutes.

(5) Heat preservation stage: heat preservation at Tb=1120~1125°C for 60 minutes to fully diffuse the solder elements. With the increase of brazing temperature, the physical and chemical reaction between the brazing filler metal and the base metal becomes very intense, and the copper brazing filler metal element diffuses very fully. When the vacuum brazing temperature is low, there are more brittle phases in the brazed joint, which makes the strength of the welded joint lower; when the vacuum brazing temperature rises to 1125 °C, a complete solid solution is formed in the middle region of the joint, and The structure in the fillet area becomes more homogeneous, thus increasing the strength of the welded structure.

(6) Vacuum slow cooling: from 1125 to 900 °C, vacuum slow self-cooling is adopted, the purpose is to release part of the residual stress of the brazed joint at high temperature by using the creep relaxation effect, prevent the generation of cracks, and improve the strength of the plate-fin structure .

(7) Nitrogen-filled quick cooling: From 900°C, fill the furnace with N₂, and start the fan of the vacuum brazing furnace at the same time, so that the stainless steel plate-fin automobile oil cooler can be quickly cooled to 80°C before being released from the furnace. After the product is out of the oven, it needs to carry out static pressure leak detection, anti-rust process, hydraulic test, product drying, machining of product joints, final inspection, and product packaging.

Using a vacuum brazing furnace with guaranteed quality can make the processing of the product more satisfactory. SIMUWU high temperature vacuum brazing furnace is used for vacuum brazing process of copper alloy, stainless steel, high temperature alloy and heat-resistant alloy, and also for vacuum brazing process of refractory alloy and dissimilar metals containing Ti, Zr, niobium and Mo elements. Widely used in the brazing process of EGR coolers, diamond tools, iron-chromium-aluminum alloys, plate-fin heat exchangers, and oil coolers.

During the brazing process, the workpiece has the advantages of no oxidation, small deformation, no porosity, no flux slag inclusion, bright surface and no pollution under the condition of vacuum atmosphere or protective atmosphere. The vacuum furnace control system is controlled by PLC; the temperature is controlled by an intelligent temperature controller, with accurate control and high degree of automation. The user can choose automatic or manual switching operation without disturbance; the furnace has an abnormal state alarm function and is easy to operate.