Analysis of Process Elements Affecting High-Frequency Longitudinal Welded Steel Pipe

The main process parameters of high-frequency straight seam welded pipe include welding heat input, welding pressure, welding speed, opening angle, position and size of the induction coil, position of impedance, etc. These parameters have a greater impact on improving the quality of high-frequency welded pipe products, production efficiency, and unit capacity. Matching various parameters can enable manufacturers to obtain considerable economic benefits.

1. Welding heat input: In high-frequency straight seam welded pipe welding, the welding power determines the amount of welding heat input. When the external conditions are constant and the input heat is insufficient, the edge of the heated strip cannot reach the welding temperature and remains constant. This kind of solid structure forms a cold weld and cannot even be fused. The lack of fusion caused by welding heat input is too small. This lack of fusion usually manifests as the failure of the flattening test, bursting of the steel pipe during the hydraulic test, or cracking of the weld seam when the steel pipe is straightened. This is a serious defect. In addition, the welding heat input will also be affected by the quality of the edge of the strip. For example, when there are burrs on the edge of the strip, the burrs will cause ignition before entering the welding spot of the extrusion roller, resulting in a loss of welding power and a decrease in 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, and sometimes the molten metal will splash and form holes due to weld breakdown. Sand holes and holes formed by excessive heat input, these defects are mainly manifested as unqualified 90° flattening tests, unqualified impact tests, and burst or leakage of steel pipe during the hydraulic test.

2. Welding pressure (diameter reduction): 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 to form a weld under the extrusion force of the extrusion roller. The size of the welding pressure affects the strength and toughness of the weld. If the applied welding pressure is too small, the welding edge cannot be fully fused, and the residual metal oxides in the weld cannot be discharged to form inclusions, which will greatly reduce the tensile strength of the weld, and the weld will easily crack after being stressed; if the applied welding pressure If it is too large, most of the metal that reaches 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 burrs. Effect of welding extrusion force on burr shape. The welding extrusion is too large, the spatter is large, and the molten metal that is extruded is more, the burrs are large and overturned on both sides of the weld; the extrusion amount is too small, there is almost no splash, and the burrs are small and piled up; the extrusion amount When it is moderate, the extruded burrs are 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 seam is symmetrical from top to bottom, left and right, and the angle is 55°~65°. The metal streamlines the shape of the weld seam 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 metal atoms. For high-frequency welding, the welding quality increases with the increase of welding speed, because the shortening of heating time narrows the width of the edge heating zone and shortens the time for forming metal oxides; if the welding speed is reduced, not only the heating zone becomes wider, That is, the heat-affected zone of the weld becomes wider, and the width of the melting zone changes with the input heat, and the inner burrs formed are also larger. Fusion line width at different welding speeds. When welding at low speed, due to the corresponding reduction of heat input, it will cause welding difficulties. At the same time, it is affected by the quality of the board edge and other external factors, such as the magnetism of the impedance, the size of the opening angle, etc., and 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 Opening angle: The opening angle is also called the welding V angle, which refers to the angle between the edge of the strip before the extrusion roller, 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 edge distance of the strip will be reduced, so that the proximity effect of the high-frequency current is 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 early welding, that is, the welding point will be squeezed and fused before reaching the temperature, and it is easy to form inclusions and cold welding defects in the weld, which reduces the quality of the weld. Although the power consumption is increased when the V angle is increased, it can ensure the stability of the edge heating of the strip 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°~5°.

5 Size and position of the induction coil: An induction coil is an important tool in high-frequency induction welding, and its size and position directly affect production efficiency. The power transmitted by the induction coil to the steel pipe is proportional to the square of the surface gap of the steel pipe. If the gap is too large, production efficiency will be drastically reduced. The gap is selected around 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 loss of the tube back and the induction coil will also decrease. Increase. Generally, the width of the induction coil is 1-1.5D (D is the outer diameter of the steel pipe) which is more suitable. The distance between the front end of the induction coil and the center of the extrusion roller is equal to or slightly larger than the pipe diameter, that is, 1-1.2D is more suitable. If the distance is too large, the proximity effect of the opening angle will be reduced, resulting in too long edge heating distance, so that the solder joint cannot get a higher welding temperature; service life.

6 The function and position of the resistor: The magnet rod of the resistor 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 magnetic rod will exceed its Curie temperature (about 300 ℃) and lose its magnetism. Without the resistor, the current and induced heat would be dispersed around the entire body of the pipe, increasing the welding power and causing the body 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 exactly at the centerline of the extrusion roller, the flattening result is the best. When it exceeds the center line of the squeeze roller and extends to the side of the sizing machine, the flattening effect will be significantly reduced. When it is less than the center line and on the side of the guide roller, the welding strength will be reduced. The position is that the impedance is placed in the tube blank under the inductor, and its head coincides with the center line of the extrusion roller or adjusted 20-40mm in the forming direction, which can increase the back impedance of the tube, reduce its circulating current loss, and reduce the welding power.