What should we pay attention to when welding copper pipes? Explain from the selection of solder, welding operations, etc.
1 Introduction
The refrigeration system in refrigeration and air conditioning generally uses non-ferrous metal materials such as copper and aluminum. During the manufacturing, installation and maintenance processes, pipe welding is a key link. It not only affects the appearance, but more importantly, affects whether the system can work properly. The lighting problem must be taken seriously.
2. Selection of solder
Most of the pipe fittings of refrigeration and air conditioning are made of copper (purple) pipes. Commonly used solder types include copper-phosphorus solder, silver-copper solder, copper-zinc solder, etc. When welding, we must correctly select the solder and operate it skillfully according to the characteristics of the pipe material to ensure the quality of the welding.
2.1 Welding of similar materials
2.1.1 Brazing of copper to copper
Phosphorus copper solder or phosphorus copper solder with low silver content can be used, such as 2% or 5% silver-based solder. This kind of solder is relatively cheap, has a good solution, uses a gap filling and wetting process, and does not require flux.
2.1.2 Welding of copper and steel
Brass strip solder and appropriate flux can be used. When welding. The solder is heated to a certain temperature and then inserted into the flux, so that the flux melts and adheres to the solder. However, after welding, the residual flux near the welding joint must be scrubbed clean 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 on the surface of the welding joint. During welding, the alcohol evaporates quickly to form a smooth film that is not easy to drain. It also avoids the danger of water immersing in the refrigeration system.
2.2.2 Welding of copper and iron
Phosphor copper solder or brass strip solder can be used, but a corresponding flux is also required, such as borax, boric acid or a mixed flux of boric acid.
3.Welding operation
The size of the welding gun and the flame temperature required for welding different materials and different pipe diameters are different. The size of the flame during welding can be controlled and adjusted through two needle valves. The flame is adjusted according to the oxygen and acetylene gases. Depending on the volume ratio, it can be divided into three types: carbonizing flame, neutral flame and oxidizing flame.
3.1 Types and characteristics of flames
3.1.1 Carbonizing flame
Its characteristics are that the volume ratio of oxygen to acetylene gas is less than 1, and it is slightly oxygen-deficient. It is easy to bring carbon particles into the metal and affect the solder flow. It emits black smoke and the temperature is about 2700. It can be used for baking pipes, etc.
3.1.2 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, and the flow rate of the mixed gas determines the length of the flame core. The length of the neutral flame The flame is divided into three layers. The flame core is pointed and conical, white and bright in color. The inner flame is blue-white, and the outer flame gradually changes from lavender to orange and blue from the inside to the outside. The temperature is about 3000~3500℃, and the oxygen The volume ratio to acetylene gas is 1:1.2. Neutral flame is often used for welding pipe fittings in refrigeration and air conditioning.
3.1.3 Oxidation flame
Its characteristics are that the flame core is conical, the length is obviously shortened, the outline is not clear, the color is dim, the outer flame is also shortened, the flame is blue, the flame is accompanied by noise when burning, the size of the noise depends on the oxygen pressure, and the oxidation flame The temperature is higher than that of neutral flame, and it is suitable for welding brass pipe fittings.
The nature of the flame is selected according to the type and properties of the metal to be welded, and attention should be paid to scientific selection and use.
3.2 Flame adjustment
Before igniting, open the valves of the oxygen cylinder and acetylene cylinder respectively according to the operating procedures, so that the low-pressure oxygen meter indicates around 0.2~0.5Mpa, and the acetylene gas meter indicates around 0.05Mpa. Then slightly open the oxygen valve of the welding gun. Then slightly open the acetylene gas valve on the welding gun, and at the same time, quickly ignite the gas from the back of the welding tip. Never light a fire in front of the welding tip to avoid burning your hands. It can be adjusted after ignition. The adjustment of the two valves is to adjust the ratio of oxygen and acetylene gas immersed in the mixed gas of the welding gun, thereby obtaining different flames.
3.3 Welding
The steps should be strictly followed when welding, otherwise, the quality of the welding will be affected.
3.3.1 Preparation and piping
3.3.1.1 Check whether the oxygen cylinder and acetylene cylinder are sufficient.
3.3.1.2 Check the drawing requirements to ensure that all components are complete and functional.
3.3.1.4 Ensure that the optical path of the pipeline is horizontal and vertical, and pay attention to the directionality of each valve.
3.3.1.5 According to the size and pipe diameter required in the drawing, measure the corresponding length with a tape measure and mark the position with a line marker.
3.3.1.6 The thicker copper pipe should be fixed before being removed with a cutter, ensuring that the cut is flush and not deformed.
3.3.1.7 Use a file to smooth the burrs of the cut and wipe them clean with a rag.
3.3.1.8 The surface of the pipe fittings to be welded should be cleaned or expanded. The expanded bell mouth should be smooth, round, free of burrs and cracks, and uniform in thickness. Use sandpaper to polish the joint of the copper pipe to be welded clean, and finally use a dry cloth. Wipe it clean. Otherwise, the solder flow and welding quality will be affected.
3.3.1.9 Except for the welding of red copper to red copper, all pipe fittings should be cleaned with gauze or stainless steel wire brush before welding to expose the bright metal surface. (The inner surface must be cleaned, and metal chips and sand must be removed)
3.3.1.10 Insert the copper pipes to be welded overlapping each other (pay attention to the size) and align the centers of the circles.
3.3.1.11 The insertion depth and clearance of copper pipe joints and copper pipes are shown in the table. (The insertion depth is approximately equal to the pipe diameter)
3.3.2 Nitrogen-filled shielded welding
3.3.2.1 During welding, low-speed nitrogen should be passed through the pipe to be welded to prevent oxidation.
3.3.2.2 Acetylene gas should pass through the non-oxidation welding generator to prevent oxidation of the outer surface of the welded object.
3.3.3 Copper pipe welding
3.3.3.1 When welding, the parts to be welded must be preheated. Heat the copper pipe welding joint with a flame. When the copper pipe is heated to purple red, remove the flame and place the solder against the welding joint so that the solder melts and flows into the welded copper parts. The temperature after heating can be reflected by the color. The temperature is as follows: dark red: about 600 degrees Celsius; dark red: about 700 degrees Celsius; orange-red: about 1000 degrees Celsius.
3.3.3.2 During welding, the gas welding flame shall not directly heat the electrode.
3.3.3.3 Corresponding protective measures should be taken for parts that are easily deformed or damaged under high temperature conditions. For example, angle valves, evaporators, condensers, etc. must be wrapped with wet gauze before welding. For solenoid valves, expansion valves, liquid mirrors, and four-way valves, those that can be disassembled must be disassembled and then welded. The same applies to those that cannot be disassembled. Take the above measures.
3.3.3.4 When welding, when the copper pipe is annealed after welding, the annealing temperature shall not be lower than 300 degrees Celsius.
3.3.3.5 After welding is completed, cool down and use dry nitrogen to clean the oxides and welding slag in the tube.
3.3.4 For weak welding and trachoma repair welding
3.3.4.1 If welding leakage is found during the pressure test, repair welding should be carried out. During the repair welding, the nitrogen used in the system pressure test should be released, and the pressure should be tested again after welding.
3.3.4.2 Wipe off the oxide layer on the surface of the front waist with gauze for repair welding. After repair welding, the oxide scale should be removed, and after quenching in water, the copper pipe should be dried without any water droplets.
3.3.4.3 After all welding is completed, the system must be purged with nitrogen.
