Analysis of Influencing Factors of High Frequency Longitudinal Welded Pipe Technology

The main process parameters of high-frequency straight seam welded pipe include welding heat input, welding pressure, welding speed, opening angle, the position and size of the induction coil, the position of the impedance, and so on. 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.

1 Welding heat input

In the welding of high-frequency straight-seam welded pipes, the welding power determines the amount of heat input to the welding. When the external conditions are constant and the heat input is insufficient, the edge of the heated strip cannot reach the welding temperature, and still maintains a solid structure to form a cold. Welding can't even fuse. The welding heat input is too small resulting in a lack of fusion

This lack of fusion during inspection usually manifests as an unqualified flattening test, bursting of steel pipe during the hydraulic test, or cracking of weld seam during straightening of steel pipe, which is a serious defect. In addition, the welding heat input is also affected by the quality of the edge of the strip. For example, when there is a burr on the edge of the strip, the burr will cause ignition before entering the welding point of the squeeze roll, resulting in a loss of welding power and a reduction in the heat input. small, resulting in unfused or cold welds. When the input heat is too high, the edge of the heated strip exceeds the welding temperature, resulting in overheating or even overburning, and the weld will crack after being stressed. The blisters and holes formed by the excessive heat input are mainly manifested in the failure of the 90° flattening test, the failure of the impact test, and the burst or leakage of the steel pipe during the hydraulic test.

2 Welding pressure (reduction amount)

The welding pressure is the main parameter of the welding process. After the edge of the strip is heated to the welding temperature, the metal atoms are combined under the action of the squeezing roller to form the welding seam. The size of the welding pressure affects the strength and toughness of the weld. If the applied welding pressure is too small, the welding edges cannot be fully fused, and the residual metal oxides in the welding seam cannot be discharged to form inclusions, which will greatly reduce the tensile strength of the welding seam and cause the welding seam to crack easily after being stressed; if the applied welding pressure If it is too large, most of the metal reaching the welding temperature will be extruded, which not only reduces the strength and toughness of the weld but also produces defects such as excessive internal and external burrs or lap welding.

The welding pressure is generally measured and judged by the diameter change of the steel pipe before and after the extrusion roller and the size and shape of the burr. The effect of welding extrusion force on burr shape. The welding extrusion amount is too large, the spatter is large and the molten metal is extruded, the burr is large and overturns on both sides of the weld; the extrusion amount is too small, there is almost no spatter, the burr is small and it is piled up; When it is moderate, the extruded burr is upright, and the height is generally controlled at 2.5~3mm. If the welding extrusion amount is properly controlled, the metal streamlines angle of the weld is symmetrical up and down, left and right, and the angle is 55°~65°. The shape of the metal flow line of the weld when the amount of extrusion is properly controlled.

3 Welding speed

The welding speed is also the main parameter of the welding process, which is related to the heating system, the deformation speed of the weld, and the crystallization speed of the metal atoms. For high-frequency welding, the welding quality increases with the welding speed, because the shortening of the heating time narrows the width of the edge heating zone and shortens the time for metal oxide formation; if the welding speed is reduced, not only does the heating zone become wider, but also That is, the heat-affected zone of the weld becomes wider, and the width of the melting zone changes with the change of the input heat, and the internal burr formed is also larger. Weld line width at different welding speeds. During low-speed welding, it is difficult to weld due to the reduction of the corresponding input heat, and at the same time, affected by the quality of the edge of the board and other external factors, such as the magnetism of the resistor, the size of the opening angle, etc., it is easy to cause a series of defects. Therefore, during high-frequency welding, the fastest welding speed should be selected for production according to the specifications of the product under the conditions allowed by the unit capacity and welding equipment.

4 open corners

The opening angle, also known as the welding V angle, refers to the angle between the edge of the strip before the squeeze roll, as shown in Figure 6. Usually, the opening angle varies between 3° and 6°, and the size of the opening angle is mainly determined by the position of the guide roller and the thickness of the guide sheet. The size of the V angle has a great influence on the welding stability and welding quality. When the V angle is reduced, the strip edge distance will be reduced, so that the proximity effect of the high-frequency current can be strengthened, which can reduce the welding power or increase the welding speed and improve productivity. If the opening angle is too small, it will lead to premature welding, that is, the welding point will be squeezed and fused before reaching the temperature, which will easily form defects such as inclusions and cold welding in the weld, reducing the quality of the weld. Although the power consumption is increased when the V angle is increased, it can ensure the stability of the strip edge heating under certain conditions, reduce the loss of edge heat and reduce the heat affected zone. In actual production, to ensure the quality of the weld, the V angle is generally controlled at 4° to 5°.

5 Induction coil size and location

An 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 clearance 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 cooling is not in place, 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. There is no thermal effect of the resistor in the tube blank. The placement of the resistor has a great influence on the welding speed, but also on the welding quality. The 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 result is flattened. 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 the side of the guide roller. The position is that the impedance device is placed in the tube blank below the inductor, and its head coincides with the center line of the extrusion roller or is adjusted to 20-40mm in the forming direction, 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, the extruded burr is upright, and the weld can obtain high toughness and tensile strength.

(3) Control the welding V angle at 4° to 5°, and carry out production at a higher welding speed as much as possible under the conditions allowed by the unit capacity and welding equipment, which can reduce the occurrence of some defects and obtain 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 production efficiency.

(5) Ensure 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.