The main points of high-frequency welding quality control
In the process of high-frequency welding of steel pipes, the control of the welding process and process parameters, the placement of the induction coil and the impedance device, etc. have a great influence on the welding quality of the steel pipe weld.
1. Control of opening angle of steel pipe weld. After the steel strip enters the welded pipe unit, is formed by the forming roller and oriented by the guide roller, a steel pipe blank with an open gap is formed. Due to the proximity effect, when the high-frequency current passes through the edge of the steel plate, the edge of the steel plate will form a preheating section and a melting section. When the melting section is heated violently, the molten steel inside is quickly vaporized and exploded and splashed out, forming a flash.
The size of the opening angle has a direct effect on the melting section. When the opening angle is small, the proximity effect is significant, which is beneficial to increase the welding speed. However, if the opening angle is too small, the preheating section and the melting section become longer, and the result of the longer melting section makes the flashing process unstable, and it is easy to form deep pits and pinholes. Lamination. Due to excessive heat, it will also cause welding seam burnout, molten metal splash, and affect the welding quality of the weld seam. When the opening angle is too large, the melting section becomes shorter and the flash is stable, but the proximity effect is weakened, the welding efficiency is significantly reduced, and the power consumption increases, which will cause the weld to be poorly welded and cause confusion or cracking. At the same time, when forming thin-walled steel pipes, if the opening angle is too large, the edges of the pipe will be elongated, causing wavy wrinkles. Generally, it is advisable to adjust the opening angle within 2°～6°. The speed is faster when producing thin plates, and the smaller opening angle should be used in extrusion molding; the speed is slow when producing thick plates, and the larger one should be used in extrusion molding. Opening angle.
2. Adjustment of the position of the high-frequency induction coil. The induction coil should be placed on the same centerline as the steel pipe. A small distance between the induction coil and the surface of the steel pipe is more efficient, but it is easy to cause discharge between the induction coil and the pipe. Generally, the induction coil should be kept 5-8 mm away from the surface of the steel pipe.
The distance between the front end of the induction ring and the centerline of the squeeze roller should be as close as possible depending on the specifications of the steel pipe without burning the squeeze roller. If the induction coil is far away from the squeeze roller, the effective heating time is longer and the heat-affected zone is wide so that the strength of the steel pipe weld is reduced or the weld is not penetrated; otherwise, the induction coil is easy to burn the squeeze roller.
3. Adjustment of the impedance position. Impedance is one or a set of special magnetic rods for welded pipes. Its function is to make the induction coil, the edge of the welded seam of the tube blank, and the magnetic rod form an electromagnetic induction loop to produce a proximity effect. The eddy current heat is concentrated near the edge of the welded tube The edge of the billet is heated to the welding temperature.
The cross-sectional area of the resistor should generally be no less than 70 of the cross-sectional area of the inner diameter of the steel pipe. The resistor should be placed concentrically with the pipe. The gap between the resistor and the inner wall of the pipe is generally 6-15 mm, and the upper limit is taken when the pipe diameter is large.
The distance between the impedance device and the welding point also affects the welding efficiency. The distance between the head and the welding point is 10-20 mm. Similarly, the larger the pipe diameter is. If the impedance device is not placed properly, it will affect the welding speed and welding quality of the welded pipe, and cause the steel pipe to crack.
4. High-frequency welding process parameters-control of input heat. When the high-frequency input heat is insufficient and the welding speed is too fast, the edge of the heated pipe body cannot reach the welding temperature, and the steel still maintains its solid structure and cannot be welded, forming cracks that are not fused or penetrated. Will cause false welding, desoldering, pinch welding, and other unwelded defects; when the high-frequency input heat is too large and the welding speed is too slow, the edge of the heated pipe body will exceed the welding temperature, which is likely to cause overheating or even overburning. Weld breakdown, causing metal splashes to form shrinkage holes, causing serious splashes, pinholes, slag inclusions, and other defects. It can be seen from formulas (1) and (2) that the amount of high-frequency input heat can be controlled by adjusting the high-frequency welding current (voltage) or adjusting the welding speed so that the weld of the steel pipe must be penetrated and not Weld through to obtain steel pipes with excellent welding quality.
The input heat must be adjusted and determined according to the tube wall thickness and the forming speed. Different forming methods, different unit equipment, and different material steel grades all require us to summarize from the first line of production and prepare a high-frequency process suitable for our unit equipment.
5. Extrusion force.
Extrusion force is also the main parameter of high-frequency welding. Theoretical calculations believe that the extrusion force should be 100-300MPa, but the actual pressure in this area is difficult to measure in actual production. It is generally estimated based on experience and converted into the amount of extrusion of the pipe edge. Different wall thickness takes different extrusion amounts, usually, the extrusion amount below 2mm is t; 0.5t～t for 3～6mm; 0.5t for 6～10 mm; 0.3t～0.5t for more than 10 mm.
Post time: Nov-01-2023