Analysis of process factors affecting high-frequency straight seam welded pipes

The main process parameters of high-frequency straight seam welded pipes include welding heat input, welding pressure, welding speed, opening angle, position and size of the induction coil, the position of impedance, etc. These parameters have a great impact on improving the quality, production efficiency, and unit capacity of high-frequency welded pipes. Matching various parameters well 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 input heat. When the external conditions are certain and the input heat is insufficient, the edge of the heated strip steel cannot reach the welding temperature and still maintains a solid structure to form a cold weld or even fail to fuse. Unfused due to too little welding heat input. During detection, this unfused state is usually manifested as an unqualified flattening test, steel pipe bursting during the water pressure test, or weld cracking during steel pipe straightening, which is a more serious defect. In addition, the welding heat input will also be affected by the quality of the strip edge. For example, when there are burrs on the strip edge, the burrs will cause sparks before entering the squeeze roller welding point, resulting in welding power loss and reduced heat input, thus forming unfused or cold welding. When the input heat is too high, the heated strip edge exceeds the welding temperature, resulting in overheating or even overburning. The weld will also 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 are mainly manifested as unqualified 90° flattening test, unqualified impact test, and steel pipe bursting or leakage during water pressure test.

2. Welding pressure (reduction)
Welding pressure is one of the main parameters of the welding process. After the strip edge is heated to the welding temperature, the metal atoms are combined under the extrusion force of the squeeze roller to form a weld. The size of the welding pressure affects the strength and toughness of the weld. If the welding pressure applied is too small, the welding edge cannot be fully fused, and the residual metal oxides in the weld cannot be discharged and form inclusions, resulting in a significant reduction in the tensile strength of the weld and the weld is prone to cracking after being stressed; if the welding pressure applied is too large, most of the metal that reaches the welding temperature will be squeezed out, 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. The influence of welding extrusion force on the shape of burrs. If the welding extrusion is too large, the spatter is large and the extruded molten metal is large, the burrs are large and overturned on both sides of the weld; if the extrusion is too small, there is almost no spatter, and the burrs are small and piled up; when the extrusion is moderate, the extruded burrs are upright, and the height is generally controlled at 2.5~3mm. If the welding extrusion is properly controlled, the metal streamline angle of the weld is symmetrical up and down, left and right, and the angle is 55°~65°. The metal streamlines the shape of the weld when the extrusion is properly controlled.

3. Welding speed
Welding speed is also one of the main parameters of the welding process. It is related to the heating system, weld deformation speed, and metal atom crystallization speed. For high-frequency welding, welding quality improves with the increase of welding speed. This is because the shortening of heating time makes the width of the edge heating zone narrower and shortens the time of forming metal oxide. If the welding speed is reduced, not only the heating zone becomes wider, that is, the heat-affected zone of the weld becomes wider, but also the width of the melting zone changes with the change of input heat, and the internal burr formed is larger. Fusion line width at different welding speeds. When welding at low speed, welding will be difficult due to the corresponding reduction of input heat. At the same time, it is easy to cause a series of defects due to the quality of the plate edge and other external factors, such as the magnetism of impedance and the size of the opening angle. Therefore, when welding at high frequency, the fastest welding speed should be selected as much as possible 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 of the edge of the strip before the extrusion roller, as shown in Figure 6. Usually, the opening angle varies between 3° and 6°. 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 welding stability and welding quality. When the V angle is reduced, the distance between the edges of the strip will be reduced, thereby strengthening the proximity effect of the high-frequency current, which can reduce the welding power increase the welding speed, and improve productivity. Too small an opening angle will lead to premature welding, that is, the welding point is squeezed and fused before reaching the temperature, which makes it easy to form defects such as inclusions and cold welding in the weld, reducing the quality of the weld. Although increasing the V angle increases power consumption, it can ensure the stability of the strip edge heating under certain conditions, reduce the edge heat loss, 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. Size and position of the induction coil
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 sharply reduced. If the gap is too small, it is easy to spark with the surface of the steel pipe or be damaged by the steel pipe head. Usually, the gap between the inner surface of the induction coil and the tube body is selected to be 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 sensor 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 increase. Generally, the width of the induction coil is more suitable at 1 to 1.5D (D is the outer diameter of the steel pipe). The distance from the front end of the induction coil to the center of the extrusion roller is equal to or slightly greater than the tube diameter, that is, 1 to 1.2D is more suitable. If the distance is too large, the proximity effect of the opening angle will be reduced, resulting in a long edge heating distance, so that the welding point cannot obtain a higher welding temperature; if the distance is too small, the extrusion roller will generate higher induction heat, reducing its service life.

6. The role and position of the impedance
The impedance magnetic bar is used to reduce the high-frequency current flowing to the back of the steel pipe while concentrating the current and heating 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 bar will exceed its Curie temperature (about 300 ℃) and lose magnetism. If there is no impedance, the current and the induced heat will be dispersed around the entire pipe body, increasing the welding power and causing the pipe body to overheat. The thermal effect of the presence or absence of an impedance in the pipe blank. The placement of the impedance has a great influence on the welding speed and also on the welding quality. The practice has proved that when the front end of the impedance is exactly at the center line of the extrusion roller, the flattening results. When it exceeds the center line of the extrusion roller and extends to the side of the sizing machine, the flattening result will be significantly reduced. When it does not reach the center line but on the side of the guide roller, the welding strength will be reduced. The position is that the impedance is placed in the pipe blank under the inductor, and its head coincides with the center line of the extrusion roller or is adjusted 20 to 40 mm in the forming direction, which can increase the back impedance in the pipe, 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) It is more appropriate to control the extrusion volume at 2.5~3 mm. The extruded burrs are upright and the weld can obtain higher toughness and tensile strength.
(3) Controlling the welding V angle at 4°~5° and producing at a higher welding speed as much as possible under the conditions allowed by the unit capacity and welding equipment 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 front end of the impedance is exactly at the center line of the extrusion roller, which can obtain higher weld tensile strength and a good flattening effect.