Prevention and treatment of welding bead, trachoma, burn-through, welding leakage and other phenomena in welded copper pipe

【Summary Description】 Selection of 1. Solder Copper pipe is an important raw material for refrigeration equipment, which is mainly used to make heat exchangers and connecting pipes and fittings. Refrigeration pipe fittings are mostly copper (purple) pipes, and commonly used solder types are copper-phosphorus solder, silver-copper solder, copper-zinc solder, etc. When welding, according to the characteristics of the pipe material, the correct choice of solder and skilled operation, to ensure the quality of welding. Copper and copper soldering: phosphorus copper solder or phosphorus copper solder with low silver content, such as 2% or 5% silver-based solder. This kind of solder price is relatively cheap, and has a good solution, uses the filling and wetting process, does not need the flux. Welding of copper and iron: phosphor copper solder or brass bar solder can be used, but corresponding fluxes, such as borax, boric acid or boric acid mixed flux, are also required. 2. welding operation Use sandpaper to remove grease, oxides, stains and dust from the surface of the copper pipe inserted into the joint. The copper pipe is inserted into the welded part. Pipeline welding requirements of the length of the neutral flame is generally 20~30cm, gas output is greater than the oxygen output will produce not suitable for welding operation of the carbonized flame, the use of welding torch on the copper pipe interface heating. Then evenly heat the pipe to be welded until the welding joint is dark red, put in the welding rod and make the melted welding rod wrap around the welding joint for more than 0.8cm, then remove the welding rod, and stop heating when the brazing filler metal is completely melted. Wipe the connection with a damp cloth after several minutes after welding. In this way, the welding part can be stabilized, and the slag on the welding surface can be removed to prevent corrosion. Normal welds after welding shall be free of porosity, cracks and incomplete fusion. Analysis of Common Defects in 3. Welding 1. Welding bead Features: The brazing filler metal flows out of the brazing seam and forms an accumulation. Cause of formation: 1. The welding thermal efficiency is too large; 2, the amount of solder applied too much or improper application; 3. The welding seam is not at the same level during welding; Precautions: 1. Swing the welding gun back and forth during heating to prevent local overheating; 2. Add solder to slide along the weld from the back of the heating position; 3. Stable positioning of weldment. 2. Undercut Features: The edge of the weld is burned into a rotten shape by the flame, but it is not completely burned through, and the pipe wall itself is burned. Cause of formation: 1. The welding thermal efficiency is too large 2. The heating method is wrong. Precautions: 1. The flame is neutral welding. 2. The flame swings the welding torch back and forth to avoid centralized heating. 3. Over-burning or corrosion Features: The oxide on the surface of the joint naturally falls off after welding. After the joint is wiped clean, black pitting is inlaid on the surface of the joint. Cause of formation: 1, welding thermal efficiency is too large (temperature is too high); 2. The welding time is too long. 3, the use of oxidation flame welding. 4. Repeat welding (repair welding) Precautions: 1. Neutral flame welding; 2, do not repeat welding. 4. trachoma Features: The weld has impurities, resulting in deep holes on the surface of the weld. Cause of formation: 1. The brazing filler metal or the base metal itself has oil pollution, impurities or oxide film. 2. The weld temperature is too high, causing the phosphorus in the solder to volatilize. 3. The heating speed is too fast. Precautions: 1. Ensure the cleanliness of the solder and base metal; 2. The welding efficiency is appropriate, so that the gas dissolved in the high-temperature molten pool can be effectively discharged when the weld solidifies. 5. Air hole Features: Insufficient heating time, the gas in the liquid seam metal fails to overflow, forming holes on the surface or inside. Cause of formation: 1. The fiber material or base material itself has oil pollution or oxidation; 2. The welding speed is too fast, and the gas in the welding is effectively discharged; 3. The flame is a reducing flame, producing reducing hydrogen holes; Precautions: 1, to ensure the cleanliness of the base material fiber; 2. Select neutral flame welding with moderate speed. 6. Burn through Features: The weldment is burned and pierced near the seam. Cause of formation: 1. The operation is not skilled and the welding torch is not swung; 2. Improper flame adjustment and uneven temperature control; Precautions: 1. Swing the welding torch back and forth during welding to prevent overheating; 2. Select neutral flame connection. 7. Cracks Features: cracks in brazed joints include cracks in parent metal and brazed joints. The mechanical properties of the base metal and brazing seam decrease, and cracks occur under the action of residual stress. Cause of formation: 1. The phosphorus content of the solder is too high, and the toughness of the material is weakened; 2. The cold shrinkage rate of copper is obvious, and the residual thermal stress causes cracks; Precautions: 1, switch to better welding performance; 2. Tempering treatment to eliminate residual stress. 8. Welding leakage Features: The weld is incomplete, part of the weld is not completely fused to form a whole weld, and there is a leakage channel in the weld. Cause of formation: 1, flame brazing welding heat input is too small; 2. The temperature is uneven when applying the solder; 3. There are oxide skin, oil and other impurities on the surface of the weld. Precautions: 1. Uniform heating; 2. Use flame to heat the workpiece and add it when the workpiece is dark red. 9. Preventive measures In welding: 1. In the process of brazing heating, the welding torch swings back and forth at the part to be heated, and fixed-point heating is not allowed; 2. When adding solder, solder must be added from the back of the flame heating direction; 3. When welding, the nozzle should not be too close to the workpiece, otherwise it is easy to temper; 4. When brazing copper, generally preheat the inserted tube first to make the tube fit tightly, and then focus on heating the outer piping. Welding completed: The thermal expansion and contraction ratios between the mating workpieces are different, and the internal stress between the mating workpieces is generated during the welding process. After welding, tempering is required to eliminate the internal stress between the welding workpieces. The operation method of tempering: After the workpiece is welded, the temperature is reduced to 200 ℃ ~ 300 ℃, and the welding edge is tempered within 30mm. During tempering, the outer flame is used to quickly sweep the welding torch at the joint back and forth, and fixed-point heating is not allowed. Each workpiece lasts for about (3-5) seconds.

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