Analysis of Factors Affecting the Process of High-Frequency Straight Seam Welded Steel Pipes

The main process parameters for high-frequency straight seam welded steel pipes include welding heat input, welding pressure, welding speed, opening angle, the position and size of the induction coil, and the position of the impedance device. These parameters have a significant impact on improving the quality, production efficiency, and unit capacity of high-frequency welded pipe products. Properly matching these parameters can bring considerable economic benefits to manufacturers.

1. Welding Heat Input in Straight Seam Welded Steel Pipe Process

In high-frequency straight seam welded steel pipe welding, the welding power determines the amount of heat input. When external conditions are constant, insufficient heat input will prevent the heated strip edge from reaching the welding temperature, resulting in a cold weld or even failure to fuse. Insufficient welding heat input leads to incomplete fusion.

During inspection, this incomplete fusion typically manifests as a failed flattening test, pipe bursting during hydrostatic testing, or weld cracking during pipe straightening. This is a serious defect. Furthermore, the 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 weld point on the extrusion rollers, resulting in a loss of welding power and a reduction in heat input, leading to incomplete fusion or cold welds. When the input heat is too high, the heated strip edge exceeds the welding temperature, causing overheating or even burning. The weld will crack under stress, and sometimes, the weld penetration can cause molten metal spatter, forming holes. Sand holes and pores caused by excessive heat input manifest as failure in the 90° flattening test, failure in the impact test, and pipe bursting or leakage during the hydrostatic test.

2. Welding Pressure in Straight Seam Welded Steel Pipe Process

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 combine under the pressure of the extrusion rollers to form the weld. The magnitude of the welding pressure affects the strength and toughness of the weld. If the applied welding pressure is too low, the weld edges cannot fully fuse, and residual metal oxides in the weld cannot be expelled, forming inclusions. This significantly reduces the tensile strength of the weld, making it prone to cracking under stress. If the applied welding pressure is too high, most of the metal reaching the welding temperature will be extruded, reducing not only the weld's strength and toughness but also causing defects such as excessive internal and external burrs or overlapping welds.

Welding pressure is generally measured and judged by the change in diameter of the steel pipe before and after the extrusion roller, and the size and shape of the burrs. The influence of welding extrusion pressure on burr shape: Excessive welding extrusion results in large spatter and more extruded molten metal, larger burrs that are overturned on both sides of the weld; insufficient extrusion results in almost no spatter, and smaller, accumulated burrs; with moderate extrusion, the extruded burrs are upright, with a height generally controlled between 2.5 and 3 mm. If the welding extrusion is properly controlled, the metal flow line angle of the weld is basically symmetrical vertically and horizontally, with an angle of 55° to 65°. The shape of the metal flow line of the weld is determined by the proper control of the extrusion.

3. Welding Speed in Straight Seam Welded Steel Pipe Process

Welding speed is one of the main parameters in the welding process, related to the heating regime, weld deformation rate, and metal atom crystallization rate. For high-frequency welding, welding quality improves with increasing welding speed because the shortened heating time narrows the edge heating zone, reducing the time for metal oxide formation. Conversely, decreasing the welding speed not only widens the heating zone (i.e., the heat-affected zone of the weld) but also causes the molten zone width to vary with the input heat, resulting in larger internal burrs. (See Figure 6 for fusion line width at different welding speeds.) Low-speed welding, due to the reduced input heat, leads to welding difficulties. Furthermore, the quality of the plate edge and other external factors, such as the magnetism of the impedance device and the size of the opening angle, easily cause a series of defects. Therefore, for high-frequency welding, the fastest possible welding speed should be selected based on the product specifications, within the limits of the unit's capacity and welding equipment.

4. Opening Angle in Straight Seam Welded Steel Pipe Process

The opening angle, also known as the welding V-angle, refers to the angle between the edges of the strip in front of the extrusion roller, as shown in Figure 6. The opening angle typically varies between 3° and 6°, primarily determined by the position of the guide rollers and the thickness of the guide plates. The V-angle significantly impacts welding stability and quality. Decreasing the V-angle reduces the distance between the strip edges, strengthening the proximity effect of the high-frequency current, which can reduce welding power or increase welding speed, thus improving productivity. An excessively small opening angle leads to premature welding, where the weld point is compressed and fused before reaching the required temperature, easily resulting in inclusions and cold welds, thus reducing weld quality. Increasing the V-angle, while increasing power consumption, can, under certain conditions, ensure the stability of strip edge heating, reduce edge heat loss, and minimize the heat-affected zone. In actual production, to ensure weld quality, the V-angle is generally controlled between 4° and 5°.

5. Size and Position of the Induction Coil in Straight Seam Welded Steel Pipe Process

The induction coil is a crucial tool in high-frequency induction welding, and its size and position directly affect production efficiency. The power transmitted from the induction coil to the steel pipe is proportional to the square of the gap between the coil and the pipe surface. An excessively large gap drastically reduces production efficiency, while a gap that is too small can easily cause short circuits or damage from head-on collisions with the pipe. 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 steel pipe. If the induction coil is too wide, its inductance decreases, the voltage of the inductor decreases, and the output power decreases. If the induction coil is too narrow, the output power increases, but the active power losses on the pipe back and the induction coil also increase. Generally, a coil width of 1–1.5D (where D is the outer diameter of the steel pipe) is suitable. The distance from the front end of the induction coil to the center of the extrusion roller should be equal to or slightly greater than the pipe diameter, i.e., 1–1.2D is suitable. A distance that is too large will reduce the proximity effect of the opening angle, resulting in an excessively long heating distance at the edges, preventing the weld joint from reaching a high welding temperature. A distance that is too small will cause the extrusion roller to generate excessive induced heat, reducing its service life.

6. The Role and Position of the Impedance Controller in Straight Seam Welded Steel Pipe Process

The impedance controller's magnetic rod is used to reduce the flow of high-frequency current to the back of the steel pipe, while concentrating the current to heat the V-angle of the steel strip, ensuring that heat is not lost due to the heating of the pipe body. If cooling is inadequate, the magnetic rod will exceed its Curie temperature (approximately 300 ℃) and lose magnetism. Without the impedance controller, the current and induced heat will disperse around the entire pipe body, increasing welding power and causing overheating of the pipe body. The presence or absence of an impedance controller within the pipe blank creates a thermal effect. The placement of the impedance controller significantly affects the welding speed and weld quality. Practice has shown that when the front end of the impedance controller is exactly at the center line of the extrusion roller, the flattening result is optimal. When it extends beyond the center line of the extrusion roller towards the sizing mill side, the flattening result decreases significantly. When it is not at the center line but on the guide roller side, the weld strength is reduced. The position, i.e., the impedance device is placed inside the tube blank below the inductor, with its head coinciding with the center line of the extrusion roller or adjusted 20-40mm in the forming direction, can increase the back impedance inside the tube, reduce its circulating current loss, and lower the welding power.