To repair a stainless steel U-tube fracture, it is necessary to first determine the fracture location (straight section, curved section), degree of fracture (complete fracture, local crack), and operating conditions (such as whether it is under pressure and whether the conveying medium is corroded), and then follow the logic of “pretreatment → repair operation → quality verification” for continuous processing. The core is to ensure that the seal is leak free and the strength meets the standard after repair. The specific steps are as follows:
Firstly, pre-treatment should be carried out before repair. The U-shaped tube should be removed from the system (if it cannot be removed, the medium should be isolated, and the valves at both ends should be closed to drain the residual medium in the empty tube to avoid medium leakage or affecting welding during repair). Then, the surface of the fracture and the surrounding 5-10cm range should be polished with an angle grinder or sandpaper to remove oxide scale, rust, and oil stains, exposing the stainless steel color. At the same time, check whether there are cracks extending or deforming on the fracture surface. If the bending section is fractured and accompanied by severe thinning of the pipe wall, it is recommended to replace the new tube directly (the bending section is under concentrated force and is prone to repeated fracture after repair); For straight section fracture, it is necessary to calibrate the flatness of the fracture surface with a ruler. If the fracture surface is inclined or has burrs, use a file to smooth it out to ensure a tight fit during subsequent docking.
Next, choose the repair method based on the fracture situation. If the straight section is completely disconnected and the pipe diameter is small (such as outer diameter ≤ 50mm), argon arc welding should be used for docking repair first. Align the two broken sections of the pipe to ensure coaxiality (fix them with fixtures to avoid misalignment during welding). During welding, use low current (adjust according to the pipe diameter, such as controlling the current of the Φ 32mm pipe at 80-120A) and short arc welding, and weld in 2-3 layers. The first layer of bottom welding should ensure penetration and no weld beads on the inner wall (to avoid affecting the flow of the medium). After each layer of welding is completed, clean the welding slag with a wire brush. The last layer of weld should be slightly higher than the pipe body to enhance strength; If the fracture is a local crack (not completely disconnected), first drill a 3-5mm crack stop hole at both ends of the crack (to prevent the crack from continuing to extend), then use an angle grinder to make a V-shaped groove along the crack direction (to a depth of 2/3 of the pipe wall thickness), and then fill the groove with argon arc welding. During welding, pay attention to controlling the melting depth to avoid incomplete penetration or porosity.
After welding repair, stress relief and surface treatment are required. Due to the residual stress generated by welding, especially under pressure conditions, local heat treatment should be carried out on the welding area. The welding area should be heated to 600-650 ℃ (austenitic stainless steel) with a flame heating gun, kept for 15-20 minutes, and slowly cooled to room temperature to eliminate internal stress; Afterwards, use sandpaper to polish the weld scars at the welding site to make the surface smooth. If corrosive media are being transported, the welding area needs to be acid washed and passivated. Soak it in citric acid solution for 10-15 minutes, then rinse it with clean water and air dry to form a passivation film to prevent subsequent corrosion.
Finally, quality verification should be carried out by first conducting a visual inspection. The welding area should be visually inspected for defects such as cracks, pores, and undercutting, and there should be no obvious protrusions or depressions when touching the weld bead with hands; Perform another sealing test, seal both ends of the U-shaped tube, introduce compressed air into the tube (at a pressure of 1.2 times the working pressure), apply soapy water to the welding area, and observe for 5-10 minutes without any bubbles, indicating that the sealing is qualified; If used in a pressure bearing system, a water pressure test is also required. Fill the pipe with water and pressurize it to 1.5 times the working pressure, hold the pressure for 30 minutes, check for no pressure drop and no leakage at the welding point. After confirming that the repair meets the standard, reinstall the U-shaped pipe into the system. Before putting it into use, check again whether the interface connection is firm to avoid secondary damage caused by improper installation.
It should be noted that if the U-shaped pipe breaks in a curved section or when transporting high-temperature, high-pressure/highly corrosive media (such as chlorine containing solutions, high-temperature steam), the reliability after repair is low. It is recommended to directly replace it with a new pipe of the same specification; During the repair process, impurities should be avoided from entering the pipe throughout the entire process, especially when transporting food and pharmaceutical media. After repair, purified water should be used to rinse the pipe to ensure that there are no welding residues or contaminants.