High-frequency welding process of submerged arc steel pipe

First, the introduction of the high-frequency welding process of submerged arc steel pipe:
1. Control of weld gap: After multiple rollers roll, the strip steel is fed into the welded pipe unit. The strip steel is gradually rolled up to form a round tube blank with a tooth gap. The pressing amount of the extrusion roller is adjusted to control the weld gap to 1~3mm and make the two ends of the weld flush. If the gap is too large, the proximity effect is reduced, the eddy current heat is insufficient, and the weld crystal is not directly connected, resulting in unfused or cracked. If the gap is too small, the proximity effect is increased, the welding heat is too large, and the weld is burned; or the weld forms a deep pit after extrusion and rolling, affecting the weld surface.
2. Control of welding temperature: According to the formula, the welding temperature is affected by the high-frequency eddy current thermal power. The high-frequency eddy current thermal power is affected by the current frequency, and the eddy current thermal power is proportional to the square of the current excitation frequency; the current excitation frequency is affected by the excitation voltage, current, capacitance, and inductance. Inductance = magnetic flux / current Where: f-excitation frequency (HzC-capacitance in the excitation circuit (FCapacitance = charge/voltage; L-inductance in the excitation circuit. The excitation frequency is inversely proportional to the square root of the capacitance and inductance in the excitation circuit, or directly proportional to the square root of the voltage and current, as shown in the above formula. The excitation frequency can be changed by simply changing the capacitance, inductance, or voltage and current in the circuit, and then the purpose of controlling the welding temperature can be achieved. For low-carbon steel, the welding temperature is controlled at 1250~1460℃, which can meet the requirements of 3~5mm penetration of the pipe wall thickness. In addition, the welding temperature can also be achieved by adjusting the welding speed. The edge of the heated weld does not reach the welding temperature. When the input heat is insufficient, the metal structure remains solid, resulting in unfused or incomplete penetration; when the input heat is insufficient, the edge of the heated weld exceeds the welding temperature, overburning or molten droplets occur, and the weld forms a molten hole.
3. Control of extrusion force: Under the extrusion of the extrusion roller, the two edges of the tube blank are heated to the welding temperature. The metal grains formed together penetrate and crystallize each other, and finally form a strong weld. If the extrusion pressure is too small, the number of crystals formed together will be small, the strength of the weld metal will decrease, and cracking will occur after the force; if the extrusion pressure is too large, the molten metal will be squeezed out of the weld, which will not only reduce the strength of the weld, but also produce a lot of internal and external burrs, and even form defects such as weld overlap.
4. Regulation of the position of the high-frequency induction coil: The effective heating time is long, and the high-frequency induction coil should be as close to the position of the extrusion roller as possible. If the induction coil is far away from the extrusion roller. The heat-affected zone is wide and the weld strength is reduced; on the contrary, the weld edge is not heated enough, and the forming is poor after extrusion. The cross-sectional area of the resistor should generally be not less than 70% of the cross-sectional area of the inner diameter of the steel pipe. Its effect is to make the induction coil, the edge of the tube blank weld, and the magnetic bar form an electromagnetic induction loop,
5. The impedance is a magnetic bar or a group of magnetic bars specially used for welding pipes. The proximity effect occurs, and the eddy current heat is concentrated near the edge of the weld of the tube billet so that the edge of the tube billet is heated to the welding temperature. The impedance is dragged in the tube billet with a steel wire, and the center position should be relatively fixed near the middle position of the extrusion roller. When the machine is turned on, due to the rapid movement of the tube billet, the impedance is greatly worn by the friction of the inner wall of the tube billet and needs to be replaced frequently.
6. Weld scars will be generated after welding and extrusion. The weld scars are scraped flat by the rapid movement of the welded pipe. The burrs inside the welded pipe are generally not cleaned.
7. Process example: Process parameters: Take the welding of φ322mm straight seam welded pipe as an example. Strip steel specification: 298mm belt width is expanded according to the middle diameter plus a small amount of forming allowance. Steel material: Q235A. Input excitation voltage: 150V excitation current: 1.5A frequency: 50Hz. Output DC voltage: 11.5kV DC: 4A frequency: 120000Hz. Welding speed: 50 meters/minute. Parameter adjustment: Adjust the output voltage and welding speed in real-time according to the change of welding line energy. After the parameters are fixed, there is no need to adjust them.

Second, technical requirements and inspections of high-frequency welded pipes:
The nominal diameter of the welded pipe is 6~150mm and the nominal wall thickness is 2.0~6.0mm. The length of the welded pipe is generally 4~10 meters, according to the provisions of GB3092 "Welded Steel Pipe for Low-pressure Fluid Transportation". It can be shipped in fixed length or multiple lengths. The surface of the steel pipe should be smooth, and no defects such as folding, cracks, delamination, lap welding, etc. are allowed. The surface of the steel pipe is allowed to have minor defects such as scratches, weld dislocation, burns, and scars that do not exceed the negative deviation of the wall thickness. Thickening of the wall thickness at the weld and the existence of internal weld ribs are allowed and must meet the requirements of the standard regulations. The steel pipe should be able to withstand a certain internal pressure, and the welded steel pipe should be subjected to mechanical function tests, flattening tests, and expansion tests. When necessary, a 2.5Mpa pressure test is carried out, and there is no leakage for one minute. Answer: Use eddy current flaw detection instead of water pressure test. Eddy current flaw detection is carried out by GB7735 "Eddy Current Flaw Detection Method for Steel Pipes". The eddy current flaw detection method is to fix the probe on the frame, keep the flaw detection and weld at a distance of 3~5mm, and conduct a detailed scan of the weld by the rapid movement of the steel pipe. The flaw detection signal is automatically processed and automatically sorted by the eddy current flaw detector to achieve the purpose of flaw detection.