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Vacuum Heat Treatment Furnace

Vacuum Sintering Furnace

Vacuum Brazing Furnace

What Sets Vacuum Brazing Apart from the Rest

Vacuum brazing operation and process

The heating operation process of vacuum brazing is to execute the process parameters and obtain the decisive process of soldering joints. According to the different brazing process methods, it is mainly divided into manual processes that can be easily observed and adjusted (such as flame brazing, soldering iron brazing, etc. ) and automatic brazing processes where the process is difficult to observe and adjust (such as furnace brazing, automatic brazing, etc.). When manual brazing, the quality of the process is closely related to the technical level and proficiency of the operator. When manual brazing, necessary measures should be taken to ensure uniform heating of the brazing part and to prevent excessive oxidation of the base metal and brazing material as much as possible. When flame brazing, the flame should be adjusted to a reducing flame, and the workpiece should be heated with an inner flame or an outer flame. When heating, attention should be paid to moving the flame and focusing on the thicker side of the heating material to ensure uniform heating of the brazing part, and to avoid direct heating of small parts for a long time to avoid local overheating.

When automatically brazing, the key to a successful brazing process lies in the correct furnace loading operation and the accurate execution of process parameters. When brazing and loading the furnace, it is necessary to ensure that the workpiece to be brazed can receive radiation evenly to avoid excessively dense radiation. The brazing process uses qualified and effective monitoring instruments to measure and control major process parameters such as heating temperature, time, and vacuum degree. Due to the influence of workpiece size and uneven radiation, the temperature in the brazing furnace is uneven. Sometimes the temperature difference even reaches several hundred degrees. Therefore, the placement of the thermocouple when measuring temperature is very important. When conditions permit, the thermocouple should be placed Place it in a position with good contact with the workpiece or insert it inside the workpiece. When the temperature of the workpiece cannot be directly measured, the difference between the measured temperature and the actual temperature of the workpiece should be determined through experiments, and the brazing temperature parameters that need to be controlled should be adjusted accordingly.

Vacuum brazing process

In vacuum brazing engineering, the workpiece is mainly heated by thermal radiation. The heater of the vacuum brazing furnace transfers heat through radiation. Radiation heat transfer has its own unique law, namely Stefan Boltzmann’s law, which shows that under the same circumstances, the temperature rise rate in a vacuum brazing furnace is much slower than other heating methods. When vacuum brazing, heating should be as slow as possible to keep the internal and external temperatures of the product requiring vacuum brazing consistent, otherwise the quality of vacuum brazing will be directly affected. Products in industrial production generally preheat several heating curves, maintain heat, stop heating, and then enter the cooling process. Vacuum brazing is an effective process that can not only achieve the above purposes but also improve production efficiency. Among them, vacuum brazing Welding temperature and vacuum brazing holding time are the keys to affecting the quality of vacuum brazing. The main process parameters of vacuum brazing include: vacuum degree, heating rate, stable temperature and time, vacuum brazing temperature, brazing holding time, cooling rate, furnace temperature, etc. They all directly affect the filler gap filling and the interaction process between the solder and the base metal, and play a decisive role in the quality of vacuum brazed joints. Therefore, selection must be made through process testing. It is generally required that the thermal working vacuum degree during aluminum-based solder brazing should not be greater than 3.5×10^-3Pa; aluminum and aluminum alloys, small and thin components are recommended, and the heating rate during low-stress assembly is 6℃/min~8℃/min. , Thick components, high-stress assembly components heating rate 4℃/min ~ 5℃/min; vacuum brazing temperature and holding time are related to component structure, wall thickness, and material heat transfer properties. The vacuum brazing temperature is better when it is 30°C to 80°C higher than the liquidus temperature of the aluminum-based solder. The length of the holding time is affected by the heat capacity of the parts and tooling. The larger the heat capacity, the longer the holding time. Different parts Vacuum brazing and furnace loading require experimental screening of the vacuum brazing holding time. Aluminum and aluminum alloys can be cooled naturally with the furnace or forced air cooling. When filling gas into the furnace to cool the workpiece, the workpiece must first be naturally cooled to below 400°C. The inflation pressure of the vacuum brazing furnace can be slightly higher than that of brazing. The volatilization equilibrium pressure of materials, such as the nitrogen-filled pressure of aluminum and aluminum alloys, is 1.2Pa ~ 1.3Pa.

Main process parameters of vacuum brazing heating and cooling

The vacuum degree, working pressure, heating rate during vacuum brazing, stable temperature and holding time, temperature, holding time, cooling rate, outlet temperature, etc. in the vacuum brazing furnace all have a direct impact on the vacuum brazing process.

(1) Cold vacuum degree The selection of cold vacuum degree is mainly based on the type of base material.

(2)Work pressure

1.The working pressure of the vacuum brazing furnace is also called the thermal vacuum degree, and its selection depends on the type of solder.

2.When steel is heated in vacuum brazing, a large amount of gas will be released on the surface of the product, the surface of the tooling fixture, the solder, and the inner wall of the vacuum brazing furnace, causing the cold vacuum degree in the vacuum brazing furnace to continuously change. At the brazing temperature of vacuum brazing, the time of vacuum brazing increases. The vacuum degree in the vacuum brazing furnace has basically returned to the cold vacuum degree. However, there are exceptions. For example, the vapor pressure of copper in copper-based solder is 1Pa at 940 degrees, so the working pressure is not allowed to be less than 1Pa. To this end, high-purity nitrogen and argon can be introduced into the furnace to maintain the brazing vacuum in the vacuum brazing furnace. The water content of nitrogen or argon is nominally 99.999% for high-purity nitrogen, the oxygen content is less than 2ppm, the water content is less than 5ppm, both carbon monoxide and carbon dioxide are less than 1ppm, and the oxygen content is <0.1ppm. The dew point of nitrogen is below -70°C. Otherwise, the color of the product will not be good. Before the vacuum brazing furnace heats up, the pipes of the vacuum brazing furnace must be evacuated with nitrogen to prevent product oxidation.

3.Heating rate during vacuum brazing

The heating rate of the base metal should be able to ensure that the gas released during vacuum brazing is removed, and at the same time, the components should be heated evenly to reduce or prevent the components from deforming due to stress caused by sudden heating. The main factors that should be considered when determining the heating rate of a vacuum brazing furnace. Materials, shapes, structures and dimensions of vacuum brazed parts. For copper and copper alloys, it is necessary to heat at a faster rate between 250-500°C; for heat-resistant alloys or austenitic stainless steel, it is necessary to heat quickly within the dangerous temperature zone for carbide precipitation; for complex shapes and pre-assembled Components with greater stress should be heated slowly; for thick components, the heating rate should not be too fast.

The type of solder and its crystallization temperature range. If it is a pure metal solder, the heating rate can be faster; alloy solder must be heated faster within the melting temperature range to avoid solder segregation and increase the liquidus temperature; when using paste solder, the solder must be heated in a vacuum. The furnace pre-baking process can be used before welding. The pre-baking temperature is generally 120°C and the time is generally about 45 minutes. The heating rate for vacuum brazing below 500°C should be slower to avoid violent volatilization of the adhesive causing solder spatter. No matter what kind of solder is used during vacuum brazing, the heating rate of the vacuum brazing furnace should not be too fast within the range of 50℃-100℃ below the solder solidus temperature to ensure that the internal and external temperatures of the components are basically the same when the solder melts. , so that capillary action can work well. When brazing thin-walled weldments in a vacuum brazing furnace, in order to prevent the base metal from being corroded by the brazing filler metal, the heating rate can be as high as possible under the premise that deformation occurs during the vacuum brazing process.

4.Vacuum brazing holding temperature and holding time

The holding temperature and holding time refer to the heating to a temperature close to the solidus line of the solder material during brazing in a vacuum brazing furnace, and the heating is suspended. At this temperature, the heat preservation time is maintained for a certain period of time. The purpose is to reduce the temperature gradient of the component and make the temperature of each part of the component uniform. When vacuum brazing stainless steel, heat-resistant alloys and other products with poor thermal conductivity in a vacuum brazing furnace, if the vacuum brazing furnace temperature is heated from room temperature to the brazing temperature, it will cause larger temperatures in various parts of the component. Difference. The size of this temperature difference is related to the component material type, structure and wall thickness. At this time, the outer layer of solder will melt and spread along the higher temperature zone; due to the lower temperature in the joint, the solder cannot be well filled, resulting in incomplete soldering and reducing the quality of the joint. Therefore, the stable temperature and holding time must be correctly selected according to the specific conditions of the product. The brazing temperature of the vacuum brazing furnace should meet two requirements. To melt the brazing material, the brazing material will have a metallurgical effect with the base material under capillary action. Secondly, it can complete the vacuum brazing of the base material and then enter the heat treatment temperature in the heat treatment process. Require. Generally, the vacuum brazing furnace brazing temperature should be 30-100°C higher than the solder liquidus temperature line. However, for different solders, the larger the crystallization temperature range of the solder, the higher the brazing temperature will be higher than the melting point of the solder. For single element solder, 50-70℃ is enough; for multi-element alloy solder, it must be 60-120℃ higher than the liquidus line.

Vacuum brazing holding time

The vacuum brazing holding time is related to the following factors:

① When the interaction between the brazing filler metal and the base metal will cause strong dissolution, produce a brittle phase, and cause intergranular infiltration, the holding time should be shortened as much as possible; on the contrary, the mutual diffusion between the brazing filler metal and the base metal is conducive to eliminating the brazing seam. When the brittle phase and low-melting eutectic structure are present, the vacuum brazing furnace holding time should be increased.

② The size, structure and brazing seam gap of the workpiece affect the heat preservation time. Generally, large and thick parts have a longer heat preservation time than thin and small parts; parts with large brazing seams have a longer heat preservation time; and large furnace loads have a longer heat preservation time.

③ The weldment is not large and the furnace capacity is not large. Generally, the brazing holding time is 25-35 minutes. If the weldment is large and the furnace capacity is large, the general brazing holding time is 45-60 minutes.

After the vacuum brazing insulation is completed and before the cooling process, it is recommended to turn off the pressure dividing process of vacuum brazing, turn on the diffusion pump of the vacuum brazing furnace, and quickly increase the vacuum degree in the vacuum brazing furnace, so that the surface color of the vacuum brazed product will be bright.

Vacuum brazing furnace cooling rate

The cooling rate of vacuum brazing depends on the following factors:

1.When the solder is in liquid state, do not ventilate or turn on the fan for cooling;

2.Meet the heat treatment requirements of the base metal, such as 1 Crl8Ni9. In order to prevent the precipitation of carbides at the grain boundary, the cooling rate should be faster. Use nitrogen gas and turn on the cooling fan for forced cooling;

3.For thin, long and complex components, the cooling rate after vacuum brazing is slower. Furnace nitrogen cooling or vacuum cooling can be used at any time.

Vacuum brazing furnace product discharge temperature

Generally, the temperature of stainless steel and heat-resistant alloys is lower than 120°C; if it exceeds 120°C, the surface of stainless steel products is prone to oxidation and the products become gray. Aluminum and aluminum alloys are below 400°C; carbon steel and alloy structural steel are below 80°C. After opening the vacuum brazing furnace door, you can use a high-power electric fan to quickly reduce the surface temperature of the product to prevent surface oxidation and discoloration of the product after it comes out of the furnace.

After the product is released, the vacuum brazing furnace door needs to be closed as soon as possible and the vacuum must be evacuated. Prevent moist air from entering the vacuum brazing furnace and adsorbing on the insulation layer of the vacuum brazing furnace.