Several Important Factors Affecting the Quality of ERW Welded Pipes

The main process parameters of high-frequency ERW welded pipes include welding heat input, welding pressure, welding speed, opening angle size, position and size of induction coil, position of impedance device, etc. These parameters have a significant impact on improving the quality, production efficiency, and unit capacity of high-frequency welded pipe products. Matching these parameters well can

 

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

 

Welding Heat Input
In high-frequency ERW welded pipe welding, the welding power determines the amount of welding input heat. When the external conditions are constant and the input heat is insufficient, the edge of the heated strip steel cannot reach the welding temperature and still maintains a solid structure, forming cold welding and even unable to fuse. This lack of fusion during testing is usually manifested as failure in the flattening test, bursting of the steel pipe during the Call or email us, or cracking of the weld seam during the straightening of the steel pipe, which is a more serious defect.

When the external conditions are constant and the input heat is too high, the edge of the heated strip steel exceeds the welding temperature, resulting in overheating or even overburning. The weld seam will also crack under stress, and sometimes holes may be formed due to molten metal splashing caused by weld seam breakdown. These defects during testing mainly manifest as failure in the 90 ° flattening test, and bursting or leakage of the steel pipe during the hydrostatic test.

Welding Pressure
Welding pressure is one of the main parameters of welding process. After heating the edge of the strip to the welding temperature, the metal atoms combine with each other under the extrusion force of the squeezing rollers to form a weld seam. The magnitude of welding pressure affects the strength and toughness of the weld seam.
If the applied welding pressure is too small, the welding edge cannot be fully fused, and residual metal oxides in the weld seam cannot be discharged, forming inclusions, which greatly reduces the tensile strength of the weld seam and makes it easy for the weld to crack under stress; If too much welding pressure is applied, most of the metal that reaches the welding temperature will be extruded, which not only reduces the strength and toughness of the weld seam, but also produces defects such as excessive internal and external burrs or overlap welding. Welding pressure is generally measured and judged by the diameter change of the steel pipe before and after the squeezing roller, as well as the size and shape of the burrs.

Excessive welding extrusion, large spatter, and more molten metal being extruded, with large burrs and flipping on both sides of the weld seam; The extrusion amount is too small, with almost no splashing, and the burrs are small and piled up; When the extrusion amount is moderate, the burrs extruded are upright and the height is generally controlled between 2.5-3mm. If the welding extrusion amount is controlled appropriately, the metal flow line angle of the weld seam is basically symmetrical, with an angle of 55 °~65 °.

 

Welding Speed
Welding speed is also one of the main parameters of welding process, which is related to heating system, weld deformation speed, and metal atom crystallization speed. For high-frequency welding, the welding quality improves with the acceleration of welding speed, because the shortened heating time narrows the width of the edge heating zone, shortening the time for the formation of metal oxides; If the welding speed decreases, not only will the heating zone become wider, that is, the heat affected zone of the weld become wider, but the width of the melting zone also changes with the input heat, resulting in larger internal burrs.

When low-speed welding, reducing the corresponding input heat can lead to welding difficulties, and it is also affected by the quality of the plate edge and other external factors, such as the magnetic properties of the impedor and the size of the opening angle, which can easily cause a series of defects. Therefore, during high-frequency welding, the production should be carried out at a faster welding speed as much as possible according to the product specifications, under the conditions allowed by the unit's capacity and welding equipment.

 

Opening Angle
The opening angle, also known as the welding V-angle, refers to the angle between the edges of the strip steel in front of the extrusion roller. The opening angle usually 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 plate. The size of the V-angle has a significant impact on welding stability and welding quality.
When reducing the V-angle, the distance between the edge of the strip steel will decrease, thereby strengthening the proximity effect of high-frequency current, which can reduce welding power or increase welding speed, and improve productivity. A small opening angle can lead to early welding, where the welding joint is compressed and fused before reaching the maximum temperature, which can easily form inclusions and defects such as 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 edge heating of the strip under certain conditions, reduce the loss of edge heat, and reduce the heat affected zone. In actual production, in order to ensure the quality of the weld seam, the V-angle is generally controlled between 4 ° and 5 °.

 

Size and Position of Induction Coil
Induction coils are important tools in high-frequency induction welding, and their size and position directly affect production efficiency. 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 power loss of the back of the tube and the induction coil will also increase. The width of the general induction coil is between 1-1.5OD, which is more suitable. The distance between the front end of the induction ring and the center of the squeezing roller is equal to or slightly greater than the pipe diameter, which is 1-1.2OD is more suitable. If the distance is too large, it will reduce the proximity effect of the opening angle, causing the heating distance at the edges to be too long, making it impossible to obtain a higher welding temperature at the welding point; If the distance is too small, it will cause the squeezing roller to generate higher induction heat, reducing its service life.

 

Function and Position of Impedor
The impedor magnetic bar 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 the cooling is not sufficient, the magnetic bar will exceed its Curie temperature (about 300 ℃) and lose magnetism. If there is no impedor, the current and induced heat will be dispersed around the entire tube, increasing the welding power and causing the tube to overheat.
The placement position of the impedor has a significant impact on the welding speed and also on the welding quality. Practice has shown that the best flattening result is achieved when the front end of the impedor is located exactly at the centerline of the squeezing roller.

 

Conclusion
a. Reasonable control of welding heat input can achieve higher weld quality.
b. The extrusion amount is generally controlled between 2.5-3mm, which is more suitable. The burrs extruded are upright, and the weld seam can achieve high toughness and tensile strength.
c. Controlling the welding V-angle between 4 ° and 5 °, and producing at a higher welding speed as much as possible under the allowable conditions of the unit capacity and welding equipment, can reduce the occurrence of some defects and achieve good welding quality.
d. The width of the induction coil is 1-1.5OD of the outer diameter of the steel pipe, and it is more suitable to be 1-1.2OD away from the center of the extrusion roller, which can effectively improve production efficiency.
e. Ensure that the front end of the impedor is located exactly at the centerline of the extrusion roller, which can achieve high weld tensile strength and good flattening effect.