Understanding the basics and precautions of copper pipe welding in one article

March 28, 2024

Understanding the basics and precautions of copper pipe welding in one article



Without rules, there is no square, and the same goes for central air conditioning construction. Also, as the editor received a request from readers to share copper pipe welding technology, we are collecting this article to jointly understand the specific content of the welding operation process.


We hope it can be helpful to everyone!


Firstly, you must know the necessary tools and materials for copper pipe welding.

① Phosphorus copper welding rod;

② The fuel is liquefied gas (coal gas, natural gas, butane, etc.);

③ Oxygen as a combustion aid;

④ Welding torch.


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1、 Selection and Use of Solder Materials

The pipe fittings for refrigeration and air conditioning are mostly made of copper (purple) pipes, and commonly used types of solder include copper phosphorus solder, silver copper solder, copper zinc solder, etc. When welding, it is necessary to select the correct welding material and operate proficiently according to the characteristics of the pipeline material to ensure the quality of welding.


Welding of similar materials

Copper to copper brazing: Phosphorus copper solder or copper phosphorus solder with low silver content can be selected. This type of solder is relatively inexpensive and has good melt. It adopts filling and wetting processes and does not require solder.


Welding of steel: Brass strip welding material and appropriate welding flux can be selected. During welding, the welding material is heated to a certain temperature and inserted into the welding flux to melt and adhere to the welding material. However, the residual welding flux near the weld must be brushed clean after welding to prevent corrosion.


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Welding of different materials

Welding of copper to steel or copper to aluminum: Silver copper solder and appropriate flux can be used. After welding, residual flux near the weld must be cleaned with hot water or steam to prevent corrosion. When using flux, it is best to dilute it with alcohol into a paste and apply it to the surface of the weld. During welding, the alcohol quickly evaporates and forms a smooth film that is not easy to lose. At the same time, it can also avoid the danger of water immersion in the refrigeration system.


Welding of copper and iron: Phosphorus copper solder or brass strip solder can be selected, but corresponding fluxes such as borax, boric acid, or a mixture of boric acid fluxes need to be used


2、 Specific welding operations

The required welding gun size and flame temperature for welding different materials and pipe diameters vary. The flame size during welding can be controlled and adjusted through two needle valves. The flame adjustment can be divided into three types: carbonization flame, neutral flame, and oxidation flame based on the different volume ratios of oxygen and acetylene gas.


Charcoal flame: Its characteristic is that the volume ratio of oxygen to acetylene gas is less than 1, slightly lacking oxygen, easily bringing carbon particles into the metal and affecting the flow of solder, emitting black smoke, and the temperature is about 2700 ℃. It can be used for baking pipelines, etc.


Neutral flame: Its characteristic is that the size of the flame core depends on the composition, consumption, and flow rate of the combustion gas. The diameter of the welding torch nozzle hole determines the diameter of the flame core, while the flow rate of the mixed gas determines the length of the flame core. The flame of a neutral flame is divided into three layers. The flame core is pointed and conical, white and bright, with the inner flame being blue white. The outer flame gradually changes from light purple to orange and blue from the inside out, with a temperature of about 3000-3500 ℃. The volume ratio of oxygen to acetylene gas is 1:1.2. Neutral flames are commonly used for welding pipes in refrigeration and air conditioning systems.



Oxidation flame: Its characteristic is that the flame core is conical, the length is significantly shortened, the contour is not clear, the color is dim, and the outer flame is also shortened. The flame is blue, and there is a sound when the flame burns. The size of the sound depends on the oxygen pressure. The temperature of the oxidation flame is higher than that of the neutral flame, and it is suitable for welding brass pipe fittings. The nature of the flame depends on the being welded.



3、 Adjustment of flames

Before ignition, open the valves of the oxygen cylinder and the acetylene cylinder according to the operating procedures, so that the low-pressure oxygen gauge indicates around 0.2-0.5MPa and the acetylene gas pressure gauge indicates around 0.05MPa. Then slightly open the oxygen valve of the welding gun. Slightly open the acetylene valve on the welding gun, and at the same time, quickly ignite from behind the welding nozzle. Do not ignite on the front of the welding nozzle to avoid burning hands with flames. After ignition, it can be adjusted. The adjustment of the two valves is to adjust the ratio of oxygen and acetylene gas entering the welding gun mixture, thereby obtaining different flames.


Specific welding: When welding, the operation should be strictly carried out according to the steps, otherwise it will affect the quality of welding. The surface of the pipe fittings to be welded should be cleaned or flared, and the flared mouth should be smooth, round, free of burrs and cracks, with a uniform thickness. The copper pipe joints to be welded should be polished clean with sandpaper, and finally wiped clean with a dry cloth. Otherwise, it will affect the flow of solder and welding quality


Insert the copper pipes to be welded overlapping each other and align the centers of the circles. When welding, it is necessary to preheat the welded part. Heat the welding area of the copper tube with flame, and when the copper tube is heated to a purple red color, use a silver welding rod for welding. After removing the flame, place the solder against the weld joint, allowing it to melt and flow into the welded copper part. The temperature after heating can be reflected by the color.


It is best to use strong flame rapid welding to shorten the welding time as much as possible to prevent excessive oxide formation in the pipeline. Oxides will cause dirt and blockage along with the flow surface of the refrigerant, and even cause serious damage to the compressor.



When welding, when the solder is not completely solidified, the copper tube must not be shaken or vibrated, otherwise cracks may occur in the welding part, leading to leakage. After welding is completed, let the welded part cool naturally. Do not use water or low-temperature substances to quickly cool the welded part.


For refrigeration systems filled with refrigerant, welding should not be carried out without being completely discharged, and welding repairs should not be carried out while the refrigeration system is still leaking, in order to prevent the refrigerant from encountering open flames and producing toxic phosgene, which can harm human health and even explode!


4、 Inspection of welding quality

Check if the sealing of the welding joint is good. After adding refrigerant or nitrogen to stabilize for a certain period of time, soap water or other methods can be used to check the absolute absence of leakage in the welding area. After welding, necessary inspections should be carried out.


During the operation of refrigeration and air conditioning, there should be no cracks (seams) at the welding joints due to vibration. The pipeline should not be blocked due to debris entering during welding, nor should water enter due to improper operation. During the operation of refrigeration and air conditioning, the surface of the welding area should be clean and free of oil stains.


In summary, during the welding process of copper pipes, improper operation can easily lead to defects such as virtual welding, corrosion and penetration, overburning and burning through. Therefore, it is necessary to strengthen process control and master the process requirements during use.







Shanghai KUB Refrigeration Equipment Co., Ltd.
Address : No. 328 on the 4th plant hengyong Road, Jiading District, Shanghai
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Mr mick Cai
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