Introduction of the high-frequency welding process of submerged arc steel pipe:
1. Control of weld gap: After rolling by multiple rollers, the strip is sent to the welded pipe unit. The strip steel is gradually rolled up to form a circular tube blank with an opening gap, and the reduced amount of the extrusion roller is adjusted so that the gap between the welds is controlled at 1~3mm and the two ends of the weld are flush. If the gap is too large, the proximity effect will be reduced, the eddy current heat will be lacking, and the weld crystal will not be directly bonded, resulting in un-fusion or cracking. If the gap is too small, the proximity effect will increase, the welding heat will be too large, and the welding seam will be burned; perhaps the welding seam will form a deep pit after extrusion and rolling, which will affect the appearance of the welding seam.
2. Welding temperature control: 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 encouragement frequency; the current encouragement frequency is also affected by the encouragement voltage, current, capacitance, and inductance. Inductance=magnetic flux/current In the formula: f-encouraging frequency (HzC-capacitance in the encouragement loop (Fcapacitance=electricity/voltage; L-inductance in the encouragement loop. The encouragement frequency is inversely proportional to the square root of the capacitance and inductance in the encouragement loop, It may be proportional to the square root of voltage and current, as can be seen from the above formula. Only by changing the capacitance, inductance, or voltage and current in the loop can change the size of the encouraging frequency and then achieve the goal of controlling the welding temperature. For low-carbon steel, the welding temperature is controlled At 1250~1460℃, it can be seen that the wall thickness of the foot tube is 3~5mm. In addition, the welding temperature can also be adjusted by adjusting the welding speed. The edge of the heated weld cannot reach the welding temperature when the input heat is lacking. The metal structure remains in a solid state, resulting in incomplete fusion or incomplete penetration; when the input heat is insufficient, the edge of the heated weld exceeds the welding temperature, causing overburning or molten droplets, causing the weld to form a 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 that form together penetrate and crystallize each other, and finally form a strong weld. If the extrusion force is too small, the number of crystals forming together will be small, the strength of the weld metal will decrease, and cracking will occur after being stressed; if the extrusion force is too large, the molten metal will be squeezed out of the weld, not only reducing The strength of the weld is increased, and a lot of burrs on the inside and outside will occur, and even defects such as welding laps will be formed.
Fourth, the 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 as possible to the position of the squeeze roller. If the induction coil is far away from the squeeze roller. The heat-affected zone is wider, and the strength of the weld is reduced; on the contrary, the heating of the edge of the weld is lacking, and the forming is poor after extrusion. The cross-sectional area of the impedance device should not be less than 70% of the cross-sectional area of the inner diameter of the steel pipe. . The proximity effect occurs, and the eddy current heat is concentrated near the edge of the tube blank weld, which heats the edge of the tube blank to the welding temperature. The resistor is dragged in the tube blank with a steel wire, and the center position should be relatively fixed close to the middle position of the extrusion roller. When starting up, due to the rapid movement of the tube blank, the resistor is damaged by the friction of the inner wall of the tube blank and needs to be replaced frequently.
6. Weld scars will occur after welding and extrusion of welds. By the rapid movement of the welded pipe, the weld scar is scraped off. The burrs inside the welded pipe are generally not removed.
7. Process example: Process parameters: Now take the welding of φ322mm straight seam welded pipe as an example. Strip specification: 298mm width is opened 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 current: 4A Frequency: 120000Hz. Welding speed: 50 m/min. Parameter adjustment: adjust the output voltage and welding speed in real-time according to the change in welding line energy. After the parameters are fixed, they generally do not need to be adjusted.
Skill requirements and inspection of high-frequency welded pipes:
The nominal diameter of the welded pipe is 6~150mm, the nominal wall thickness is 2.0~6.0mm, and the length of the welded pipe is 4~10 meters, according to the rules of the GB3092 Welded Steel Pipe for Low-Pressure Fluid Conveying Standard. It can be delivered with fixed length or double length. The surface of the steel pipe should be lubricated, and defects such as folding, cracks, delamination, and lap welding are not allowed. The surface of the steel pipe is allowed to have minor defects such as scratches, scratches, weld dislocations, burns, and scarring that do not exceed the negative deviation of the wall thickness. The increased wall thickness at the weld and the internal weld bead exist and meet the requirements of the standard rules. The steel pipe should be able to accept a certain internal pressure, and the welded steel pipe should be subjected to mechanical function tests, flattening tests, and surface expansion tests. When necessary, a 2.5Mpa pressure test is carried out, and there is no leakage for one minute. A. Use eddy current testing instead of hydraulic test. Eddy current inspection is carried out according to the GB7735 steel pipe eddy current inspection method “standard. The eddy current flaw detection method is to fix the probe on the frame, keep the distance between flaw detection and weld seam 3~5mm, and conduct specific scanning on the weld seam by the rapid movement of the steel pipe. Sorting to reach the target of flaw detection.
Post time: Nov-01-2022