Flame brazing process of copper tube, from copper brazing, fusion welding, flame brazing process
Abstract: Analysis of the weldability of red copper shows that problems such as cracks, incomplete penetration, and pores occur easily during welding of red copper. However, copper brazing can avoid these problems. The flame brazing process of red copper tubes is then introduced.
Keywords: red copper; brazing; fusion welding; flame brazing process
CTL classification number: TG4 Document identification code: A
Preface
Copper is hard, has good thermal conductivity, is not easy to corrode, and is resistant to high temperature and high pressure. Due to these performance characteristics of copper tubes, it can be used in a variety of environments: such as the manufacture of heat exchange equipment (condensers, etc.), oxygen production Low-temperature pipelines in equipment; lubrication systems, oil pressure systems, etc. to transport pressured liquids; due to its good corrosion resistance and hard texture, copper tubes have also become a popular choice for modern contractors in commercial residential water pipes, heating, etc. As for the installation of refrigeration pipelines, galvanized steel pipes were mostly used in residential buildings in the past, which are very easy to rust. After a short period of use, problems such as the tap water turning yellow and the water flow becoming smaller will occur. There are also some materials whose strength will decrease rapidly at high temperatures, making them unsafe when used in hot water pipes. The melting point of copper is as high as 1083°C, making it safe and reliable when used in hot water systems. Due to the increasingly wide application of copper tubes, the welding process and welding quality of copper tubes have attracted more and more attention. This article mainly introduces the flame brazing process of copper tubes.
1. Problems that are likely to occur during copper welding
The welding methods that can be used when welding red copper usually include arc welding, gas welding, inert gas shielded welding and other fusion welding methods. Due to the thermophysical properties of copper itself, it is prone to the following problems during welding.
1. 1 It is easy to form incomplete welding
Although the melting point of red copper (1083°C) is much lower than that of steel, due to its high thermal conductivity, a large amount of heat is dissipated during welding. Therefore, red copper is difficult to melt during welding and is prone to incomplete penetration.
1. 2 It is easy to produce large welding stress and deformation
In order to ensure welding penetration, a large heat input is required, and sufficient preheating is required. Since copper has a large linear expansion coefficient, it is prone to large deformation during welding. In order to prevent deformation, the rigidity of the structure needs to be increased, which will inevitably produce greater welding stress in the workpiece.
1. 3. Easy to form welding cracks
Although red copper does not have a temperature range where solid and liquid can coexist, due to the large welding stress generated during the welding process and the large heat input, it is easy to cause the welding joint to form coarse columnar crystals; in addition, due to the presence of impurities such as lead and bismuth, If it exists, a low melting point eutectic will be formed between the crystals, causing the formation of thermal cracks.
1. 4. Easy to form pores
Due to the high thermal conductivity of copper, the weld crystallizes relatively quickly. At high temperatures, a large amount of hydrogen melted into the molten pool has no time to precipitate, and hydrogen pores are formed in the weld. In addition, copper will be oxidized at high temperatures to form Cu2O, which will react with hydrogen dissolved in liquid copper:
Cu2O + 2H = 2Cu + H2O↑
Since water vapor is insoluble in copper and the crystallization speed of the molten pool is fast, reaction pores will be formed when the water vapor has no time to escape. Therefore, the pore sensitivity of oxygen-containing copper is greater than that of oxygen-free copper. Therefore, when copper is welded, problems such as incomplete penetration, deformation, cracks and pores are likely to occur.
2. Brazing
Brazing uses a metal material with a lower melting point than the base metal as the filler metal. The weldment and the filler metal are heated to a temperature higher than the melting point of the filler metal but lower than the melting point of the base metal. The liquid filler metal is used to wet the base metal and fill the joint. gap and mutual diffusion with the base metal, thereby realizing the welding method of connecting the weldments. Compared with fusion welding, brazing does not melt the base metal during brazing, which reduces the impact of the high thermal conductivity of the base metal on the welding quality, prevents the formation of incomplete penetration, and reduces the occurrence of pores and cracks. tendency to ensure weld performance. And because the base metal does not melt during the welding process, the organization and structural properties of the base metal hardly change, thus ensuring the original performance of the base metal.
3. Brazing process of copper tube
3. 1 Selection of solder material
The brazing material is BCu60Zn Sn-R (Silk 221) in filamentary or rod shape (Ф3. 0 ~ Ф5. 0 mm). The chemical composition is shown in Table 1. The melting temperature of wire 221 is 890 ~ 905 ℃.
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3. 2 Selection of flux
FB101 was selected as the flux, and its composition is shown in Table 2. Its main components are boric acid and potassium fluoborate, and its melting temperature is 550 ~ 850 ℃.
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3. 3 Connector form
The connector is plug-in, as shown in Figure 1.
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3. 4 Cleaning before welding
Correct the edges of the tube opening and remove burrs. There should be no cracks, breaks or other defects in the tube opening. Before assembly, the surface of the joint part where the copper tube is inserted and the surface of the connecting part need to be cleaned. Use a wire brush or sandpaper to remove oxides, and use organic solvents such as acetone to remove oil stains.
3. 5 tack welding
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After assembly, tack welding is used to fix it. The number, size and height of the tack welds are shown in Table 3. The sequence of tack welding is symmetrically and evenly distributed up, down, left, and right along the circumference. During the brazing process, the tack weld should be melted into the weld.
3. 6 welding
Brazing should be carried out in a downward position whenever possible to ensure continuous welding. Choose the appropriate welding gun according to the size of the copper pipe joint
. The brazing flame should be a neutral flame, because the oxidizing flame contains high oxygen, which will oxidize the copper and cause cracks; the carbonizing flame contains free hydrogen, which can cause pores. When heating the pipe, the flame should be perpendicular to the heating surface, and the copper pipe joint to be welded should be heated evenly, and the base metal should be heated as quickly as possible. When the temperature is 650 to 750°C, the solder is fed, and the lower part of the solder is generally heated to make it The molten solder fills the gap. When the solder is completely melted, the heating should be stopped immediately to avoid the temperature of the copper pipe joint being too high and the heating time being too long. During the brazing process, both the brazing seam and the brazing filler metal should be protected by the flame. The brazing seam cools naturally after welding. Note that the workpiece must not be moved before the brazing seam solidifies.
Since the FB101 flux used contains fluoride, which has a certain corrosive effect, and the presence of welding slag will also hinder the inspection of the brazing seam, so after the brazing seam cools, hot water or a wet cloth should be used to wipe the brazing seam and the surrounding area. , remove welding slag to ensure the corrosion resistance of the joint and the reliability of post-weld inspection.
4. Brazing quality
Check the quality of the brazing seam after welding. There are no cracks, pores, lack of fusion and other defects on the surface of the welding seam, and the surface of the welding seam is beautifully formed. Conduct mechanical property tests on the brazed joints, and their tensile strength shall not be lower than that of the base metal.
5 Conclusion
Flame brazing of copper tubes can effectively prevent defects such as pores, cracks, and incomplete penetration that are easily formed during welding. Satisfactory welding can be achieved by using a combination of BCu60Zn Sn-R solder and FB101 flux to weld copper tubes. quality.
