Analysis of Factors Affecting High-Frequency Straight-Seam Welded Steel Pipe Processing

The main process parameters for high-frequency straight-seam 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. Optimizing these parameters can enable manufacturers to achieve significant economic benefits.

1. Welding Heat Input as a Process Factor for High-Frequency Straight-Seam Welded Steel Pipes

In high-frequency straight-seam welded steel pipes, 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 pipe straightening, a serious defect. Furthermore, welding heat input 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 can crack under stress, and sometimes weld breakdown causes molten metal to splash and form holes. Excessive heat input can cause pinholes and holes. These defects are primarily detected as failures in the 90° flattening test, impact test, and pipe cracking or leakage during the hydrostatic test.

2. Process Factors of High-Frequency Straight Seam Welded Steel Pipe: Welding Pressure (Reduction)

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 weld's tensile strength 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, reducing the weld's strength and toughness and 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 squeeze is moderate, the burrs are upright, generally controlled at 2.5-3mm in height. When the squeeze is properly controlled, the weld's metal flow line angles are generally symmetrical, ranging from 55° to 65°.

3. Welding Speed: A Key Process Parameter for High-Frequency Longitudinal Welding of Steel Pipes

Welding speed is also a key parameter in the welding process. It is related to the heating system, weld deformation rate, and metal atom crystallization rate. For high-frequency welding, 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. Lowering the welding speed not only widens the heating zone (i.e., the heat-affected zone) but also increases the width of the melting zone with varying heat input, resulting in larger internal burrs. Low-speed welding can lead to welding difficulties due to the correspondingly reduced heat input. Furthermore, it can be affected by plate edge quality and other external factors, such as the magnetism of the impeder and the size of the opening angle, which can easily lead to a series of defects. Therefore, when welding high-frequency steel pipes, 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. Opening Angle: A Key Process Parameter for High-Frequency Longitudinal Welding of Steel Pipes

The opening angle, also known as the welding V-angle, refers to the angle between the strip edges before the squeeze rollers. The opening angle typically ranges from 3° to 6°. Its size is primarily determined by the position of the guide rollers and the thickness of the guide blades. The V-angle significantly impacts both 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 lead to defects such as inclusions and cold welds in the weld, reducing weld quality. While 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 is generally controlled between 4° and 5°.

5. Induction Coil Size and Position: Process Elements of High-Frequency Straight Seam Welding of 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 pipe surfaces. Too large a gap can dramatically reduce production efficiency, while too small a gap can easily spark with the pipe surface or damage the pipe ends. Typically, the gap between the inner surface of the induction coil and the pipe body is around 10mm. The width of the induction coil is selected based on the outer diameter of the 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 losses on 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 pipe). The distance from the front end of the induction coil to the center of the squeeze roller should be equal to or slightly greater than the pipe diameter, meaning a range of 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 weld temperature at the weld point. Too small a distance generates excessive induction heat in the squeeze roller, shortening its service life.

6. The Role and Position of the Impeder in High-Frequency Longitudinal Welding of Steel Pipes

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 preventing heat loss due to the heating of the pipe body. If the cooling is inadequate, the magnet will exceed its Curie temperature (approximately 300°C) and lose its magnetism. Without an impeder, the current and induced heat will be dispersed throughout the 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 flattening effect is best when the impeder 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 effect is significantly reduced. If it is positioned below the centerline and toward the guide roll, the weld strength is 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 20-40 mm in the forming direction. This increases the back impedance inside the pipe, reduces circulating current loss, and lowers the 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 2.5~3 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) 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 good flattening effect.