Introduction to the high-frequency welding process of submerged arc steel pipe:
First, weld gap control: After rolling with multiple rollers, the strip is fed into the welded pipe unit. The strip steel is gradually rolled up to form a circular tube blank with an opening gap. The reduction amount of the squeeze roller is adjusted to control the weld gap to 1~3mm and make both ends of the weld flush. If the gap is too large, the proximity effect will be reduced, the eddy current heat will be insufficient, and the weld crystal joint will be poor, resulting in non-fusion or cracking. If the gap is too small, the proximity effect will increase and the welding heat will be too high, causing the weld to burn out; or the weld will form a deep pit after being extruded and rolled, affecting the surface of the weld.
Second, welding temperature control: According to the formula, the welding temperature is affected by the high-frequency eddy current thermal power. High-frequency eddy current thermal power is affected by the current frequency. The eddy current thermal power is proportional to the square of the current-encouraging frequency; and the current-encouraging frequency is affected by the encouraging voltage, current, capacitance, and inductance. Inductance = magnetic flux/current in the formula: f - encouragement frequency (HzC - capacitance in the encouragement loop (F capacitance = power/voltage; L - inductance in the encouragement loop. 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. It can be seen from the above formula that the size of the excitation frequency can be changed by changing the capacitance, inductance, or voltage and current in the loop, thereby achieving the purpose of controlling the welding temperature. For low carbon steel, welding temperature control. At 1250~1460℃, the welding penetration requirement of 3~5mm pipe wall thickness can be met. In addition, the welding temperature can also be achieved by adjusting the welding speed when the heated weld edge cannot reach the welding temperature when the input heat is insufficient. The metal structure remains solid, resulting in incomplete fusion or incomplete welding; when the input heat is insufficient, the heated edge of the weld exceeds the welding temperature, causing overburning or molten droplets, causing the weld to form a molten hole.
Third, control of the 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 a joint penetrate and crystallize with each other, eventually forming a strong weld. If the extrusion force 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 stress; if the extrusion force is too large, the molten metal will be squeezed out of the weld, which will not only reduce The strength of the weld is reduced, and a lot of internal and external burrs will be produced, and even defects such as welding laps will be formed.
Fourth, control of the position of the high-frequency induction coil: the effective heating time is longer, 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 squeeze roller. The heat-affected zone is wider and the strength of the weld is reduced; on the contrary, the edge of the weld is not heated enough and the shape is poor after extrusion. The cross-sectional area of the resistor should generally not be less than 70% of the cross-sectional area of the inner diameter of the steel pipe. Its function is to make the induction coil, the edge of the pipe blank weld, and the magnetic rod form an electromagnetic induction loop. The 5.5 resistor is one or a group of special magnetic rods for welded pipes. The proximity effect occurs, and the eddy current heat is concentrated near the edge of the tube blank weld, causing the edge of the tube blank to be heated to the welding temperature. The resistor is dragged inside the tube blank with a steel wire, and its center position should be relatively fixed close to the center of the extrusion roller. When starting up, due to the rapid movement of the tube blank, the resistor suffers a large loss from the friction of the inner wall of the tube blank and needs to be replaced frequently.
Sixth, weld scars will occur after welding and extrusion. The weld scar is smoothed by the rapid movement of the welded pipe. The burrs inside the welded pipe are generally not removed.
Seventh, process examples: Process parameters: Take the welding of φ322mm straight seam welded pipe as an example. Strip specifications: 298mm wide width 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: Real-time adjustment of output voltage and welding speed according to changes in welding line energy. After the parameters are fixed, there is generally no need to adjust them.


Technical requirements and inspections 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 generally 4~10 meters, according to the provisions of the GB3092 Welded Steel Pipe for Low-Pressure Fluid Transport Standard. It can be shipped from the factory 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, scratches, weld dislocations, burns, and scars that do not exceed the negative direction of the wall thickness. The thickening of the wall thickness at the weld and the presence of internal weld bars must meet the requirements of standard regulations. The steel pipe should be able to withstand a certain internal pressure, and the welded steel pipe should undergo mechanical function tests, flattening tests, and surface expansion tests. When necessary, conduct a 2.5Mpa pressure test and there will be no leakage for one minute. It is allowed to use the eddy current flaw detection method to replace the hydraulic test. Eddy current flaw detection is carried out by the standard GB7735 Eddy Current Flaw Detection Inspection Methods for Steel Pipes. The eddy current flaw detection method is to fix the probe on the frame, keep a distance of 3~5mm between the flaw detection and the weld, and rely on the rapid movement of the steel pipe to conduct a detailed scan of the weld. The flaw detection signal is automatically processed and automatically processed by the eddy current flaw detector. Sorting to reach the target of flaw detection.