The main process parameters of high-frequency straight seam welded pipes manufactured by used high frequency welded pipe equipment include welding heat input, welding pressure, welding speed, size of opening angle, position and size of induction coil, position of impedance and other. These parameters have a great impact on improving the product quality, production efficiency and unit capacity of high-frequency welded pipes. Matching various parameters can make manufacturers obtain considerable economic benefits.

5. Induction coil size and position

The induction coil is an important tool in high-frequency induction welding, and its size and position directly affect the efficiency of production.

The power transmitted by the induction coil to the steel pipe is proportional to the square of the gap on the surface of the steel pipe. If the gap is too large, the production efficiency will be drastically reduced. The gap is selected at about 10mm. The width of the induction coil is selected according to the outer diameter of the steel pipe. If the induction coil is too wide, its inductance will decrease, the voltage of the inductor will also decrease, and the output power will decrease; if the induction coil is too narrow, the output power will increase, but the active power loss of the tube back and the induction coil will also be reduced. Increase. Generally, the width of the induction coil is 1-1.5D (D is the outer diameter of the steel pipe).

The distance between the front end of the induction coil and the center of the squeeze roller is equal to or slightly larger than the pipe diameter, that is, 1 to 1.2D is suitable. If the distance is too large, the proximity effect of the opening angle will be reduced, resulting in too long a heating distance at the edge, so that a higher welding temperature cannot be obtained at the solder joint; service life.

6. The role and location of the resistor

The impedance magnet bar is used to reduce the high-frequency current flowing to the back of the steel pipe, and at the same time concentrate the current to heat the V angle of the steel strip to ensure that the heat will not be lost due to the heating of the pipe body. If the used high frequency welded pipe equipment is not cooled properly, the magnet bar will exceed its Curie temperature (about 300 ℃) and lose its magnetism. Without the resistor, the current and induced heat would spread around the entire tube, increasing the welding power and causing the tube to overheat. The thermal effect of the presence or absence of the impedance in the tube blank is shown in Figure  7.

Figure 7 Thermal effect diagram of the presence or absence of impedance in the tube blank

The placement of the resistor has a great influence on the welding speed, but also on the welding quality. Practice has proved that when the position of the front end of the resistor is just at the center line of the squeeze roller, the flattening result is the best. When extending beyond the center line of the squeeze roll to the side of the sizing machine, the flattening result will drop significantly. The welding strength will be reduced when it is not on the center line but on the side of the guide roller. The best position is that the impedance device is placed in the tube blank below the inductor, and its head is coincident with the center line of the extrusion roller or adjusted to the forming direction by 20-40mm, which can increase the impedance of the inner back of the tube, reduce its circulating current loss, and reduce the welding power.

7. Conclusion

(1) Reasonable control of welding heat input can obtain higher weld quality.

(2) The extrusion amount is generally controlled at 2.5~3 mm, and the extruded burr is upright, and the weld can obtain high toughness and tensile strength.

(3) When the used high frequency welded pipe equipment is operated, the welding V angle should be controlled at 4° to 5°, and the production should be carried out at a higher welding speed as much as possible under the conditions allowed by the unit capacity and the welding equipment, which can reduce some defects. produced, resulting in good welding quality.

(4) The width of the induction coil is 1-1.5D of the outer diameter of the steel pipe, and the distance from the center of the extrusion roller is 1-1.2D, which can effectively improve the production efficiency.

(5) Make sure that the position of the front end of the resistor is just at the center line of the squeeze roller, so that a higher tensile strength of the weld and a good flattening effect can be obtained.

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