Analysis of Factors Affecting the Process of High-Frequency Longitudinal Welding of Steel Pipes

The main process parameters for high-frequency longitudinal welded steel pipes include welding heat input, welding pressure, welding speed, opening angle, induction coil position and size, and impedance position. These parameters significantly impact product quality, production efficiency, and unit capacity. Properly matching these parameters can enable manufacturers to achieve substantial economic benefits.

1. Steel Pipe Welding Heat Input

In high-frequency longitudinal welded steel pipe welding, the welding power determines the amount of heat input. Under certain external conditions, insufficient heat input prevents the heated strip edge from reaching the welding temperature, resulting in a solid structure and a cold weld, or even incomplete fusion. This lack of fusion during testing typically manifests as a failure in the flattening test, pipe bursting during the hydrostatic test, or weld cracking during straightening, a serious defect. Furthermore, the heat input for steel pipe welding is also affected by the quality of the strip edge. For example, burrs on the strip edge can cause sparking before entering the squeeze roller weld point, resulting in welding power loss and reduced heat input, leading to incomplete fusion or a cold weld. If the heat input is too high, the heated strip edge exceeds the welding temperature, causing overheating or even overburning. The weld seam can crack under stress, and sometimes, molten metal splashes from the weld seam due to weld breakdown, forming holes. Excessive heat input can cause pinholes and holes. These defects are typically detected as failures in the 90° flattening test, impact test, and pipe cracking or leakage during the hydrostatic test.

2. Welding Pressure (Reduction) of Steel Pipes

Welding pressure is a key parameter in the welding process. After the strip edge is heated to the welding temperature, the squeezing force of the squeeze rollers causes metal atoms to bond, forming the weld. The level of welding pressure affects the strength and toughness of the weld. If the welding pressure is too low, the weld edges won't fuse fully, and residual metal oxides in the weld can't be expelled, forming inclusions. This significantly reduces the tensile strength of the steel pipe weld and makes the weld prone to cracking under stress. If the welding pressure is too high, much of the metal that reaches the welding temperature will be squeezed out, not only reducing the strength and toughness of the steel pipe weld but also causing defects such as excessive internal and external burrs or overlapping welds. Welding pressure is generally measured and assessed by the amount of pipe diameter change before and after the squeeze rollers and the size and shape of the burrs. Excessive squeeze results in significant spatter, a large amount of extruded molten metal, and large burrs that overturn on both sides of the weld. Too little squeeze results in virtually no spatter, and the burrs are small and accumulated. When the squeeze is moderate, the extruded burrs are upright, generally within a height of 2.5-3mm. If the squeeze is properly controlled, the metal flow line angles of the steel pipe weld are generally symmetrical, ranging from 55° to 65°.

3. Steel Pipe Welding Speed

Welding speed is also a key parameter in the welding process. It is related to the heating system, the deformation rate of the steel pipe weld, and the crystallization rate of metal atoms. For high-frequency welding, the steel pipe weld quality improves with increasing welding speed. This is because the shortened heating time narrows the width of the edge heating zone, reducing the time for metal oxide formation. If the welding speed is reduced, not only does the heating zone (i.e., the heat-affected zone) widen, but the width of the melting zone also varies with the heat input, resulting in larger internal burrs. Higher welding speeds produce more ideal steel pipe welds at the same extrusion rate. Lower welding speeds can make welding more difficult due to the correspondingly reduced heat input. Furthermore, due to the influence of plate edge quality and other external factors, such as the magnetism of the impeder and the size of the opening angle, a series of defects can easily occur. Therefore, when welding at high frequency, the fastest welding speed should be selected, within the permitted conditions of the unit capacity and welding equipment, and in accordance with the product specifications.

4. Steel Pipe Opening Angle

The opening angle of the steel pipe, also known as the welding V-angle, refers to the angle between the strip edges before the extrusion rollers. The opening angle of a steel pipe typically ranges from 3° to 6°. The angle is primarily determined by the position of the guide rollers and the thickness of the guide blades. The V-angle of a steel pipe significantly impacts welding stability and quality. Reducing the V-angle reduces the distance between the strip edges, enhancing the proximity effect of the high-frequency current. This can reduce welding power or increase welding speed, thereby improving productivity. Excessively small opening angles can lead to premature welding, where the weld point is squeezed and fused before reaching its maximum temperature. This can easily cause defects, such as inclusions and cold welds in the weld, reducing weld quality. Although increasing the V-angle increases power consumption, it can, under certain conditions, ensure stable heating of the strip edges, reduce heat loss at the edges, and minimize the heat-affected zone. In actual production, to ensure weld quality, the V-angle of steel pipes is generally controlled between 4° and 5°.

5. Size and Position of the Induction Coil for Steel Pipes

The induction coil is a critical tool in high-frequency induction welding, and its size and position directly impact production efficiency. The power transmitted by the induction coil to the steel pipe is proportional to the square of the gap between the steel pipe surfaces. Too large a gap can dramatically reduce production efficiency, while too small a gap can easily spark with the steel pipe surface or damage the pipe ends. Typically, the gap between the inner surface of the induction coil and the steel pipe body is around 10mm. The width of the induction coil is selected based on the outer diameter of the steel pipe. If the induction coil is too wide, its inductance decreases, which in turn reduces the voltage across the inductor and the output power. If the induction coil is too narrow, the output power increases, but also increases the active power loss in the pipe back and in the induction coil. A typical induction coil width is 1 to 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 extrusion roller should be equal to or slightly greater than the steel pipe diameter, meaning 1 to 1.2D is ideal. Excessive distance reduces the proximity effect of the opening angle, resulting in excessive edge heating distance and a failure to achieve a high welding temperature at the weld point. Too small a distance generates excessive induction heat in the extrusion roller, shortening its service life.

6. Function and Position of the Impeder

The impeder magnet is used to reduce the flow of high-frequency current to the back of the steel pipe. It also concentrates the current, heating the V-angle of the steel strip and ensuring that heat is not lost due to the heating of the steel pipe body. If the cooling is not adequate, the magnet will exceed its Curie temperature (approximately 300°C) and lose its magnetism. Without the impeder, the current and the induced heat will be dispersed throughout the steel pipe body, increasing the welding power and causing overheating. The placement of the impeder significantly affects the welding speed and quality. Practice has shown that the best flattening results are achieved when the impeller tip is positioned exactly at the centerline of the squeeze roll. If it extends beyond the centerline of the squeeze roll toward the sizing mill, the flattening results will be significantly reduced. If it is positioned below the centerline and toward the guide roll, the weld strength will be reduced. The optimal position is to place the impeder inside the pipe below the inductor, with its tip aligned with the centerline of the squeeze roll or adjusted 20-40mm in the forming direction. This increases the internal impedance of the pipe, reduces circulating current losses, and reduces welding power.

7. Conclusion

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

(2) It is generally more appropriate to control the extrusion volume at 2.5~3mm. 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) Ensuring that the front end of the resistor is exactly at the center line of the extrusion roller can obtain higher weld tensile strength and a good flattening effect.