​ How to prevent cracking of high-frequency welded pipe welds

Posted by BAOWI STEEL MANUFACTURING CO.,LTD on January 6th, 2025


In high-frequency straight seam welded pipes, the manifestations of cracks include long cracks, local periodic cracks, and irregular intermittent cracks. There are also some steel pipes that have no cracks on the surface after welding, but cracks will appear after flattening, straightening or water pressure testing.

Causes of cracks

1. Poor quality of raw materials

In the production of welded pipes, large burrs and excessive raw material width problems are often seen.

If the burrs are outward during welding, continuous and long intermittent cracks are easily generated.

The width of the raw material is too wide, the extrusion roller hole is overfilled, forming a welding peach shape, the external welding marks are large, the internal welding is small or even non-existent, and it will crack after straightening.

2. Edge corner connection state

The corner connection state of the edge of the tube blank is a common phenomenon in the production of welded pipes. The smaller the pipe diameter, the more serious the corner joint.

Inadequate forming adjustment is a prerequisite for the generation of corner joints.

Improper design of the extrusion roller hole type, large outer round angle and elevation angle of the pressure roller are the key factors affecting the corner joint.

Single radius cannot eliminate the corner joint problem caused by poor forming. Increase the extrusion force, otherwise the extrusion roller will wear and become oval in the later stage of production, which aggravates the sharp peach-shaped welding state and serious corner joints.

Corner joints will cause most of the metal to flow out from the upper side, forming an unstable melting process. At this time, there will be a large amount of metal splashing, the weld will be overheated, and the external burrs will be hot, irregular, large and not easy to scratch. If the welding speed is not properly controlled, the "false welding" of the weld will inevitably occur.

The outer angle of the extrusion roller is large, which makes the tube billet insufficiently filled in the extrusion roller, and the edge contact state changes from parallel to "V" shape, and the internal weld is not welded.

The extrusion roller shaft is worn for a long time and the base bearing is worn. The two shafts form an elevation angle, resulting in insufficient extrusion force, vertical oval and serious angle joints.

3. Unreasonable selection of process parameters

The process parameters for high-frequency welded pipe production include welding speed (unit speed), welding temperature (high-frequency power), welding current (high-frequency frequency), extrusion force (grinding tool design and material), opening angle (grinding tool design and material, position of induction coil), inductor (coil material, winding direction, position) and resistor size and position.

(1) High-frequency (stable and continuous) power, welding speed, welding extrusion force and opening angle are the most important process parameters and must be reasonably matched, otherwise the welding quality will be affected.

① If the speed is too high or too low, it will cause low-temperature welding impermeability and high-temperature overburning, and the weld will crack after being flattened.

② When the extrusion force is insufficient, the welded edge metal cannot be completely pressed together, the impurities remaining in the weld are not easy to discharge, and the strength is reduced.

When the extrusion force is too large, the metal flow angle increases, the residue is easy to discharge, the heat-affected zone narrows, and the welding quality is improved. However, if the pressure is too high, it will cause large sparks and spatters, resulting in the extrusion of molten oxides and part of the metal plastic layer, and the weld will become thinner after being scratched, thereby reducing the strength of the weld.

Appropriate extrusion pressure is an important prerequisite for ensuring welding quality.

③ The opening angle is too large, which reduces the high-frequency proximity effect, increases eddy current loss, and reduces the welding temperature. If welding is carried out at the original speed, cracks will appear;

If the opening angle is too small, the welding current will be unstable, and a small explosion (intuitively a discharge phenomenon) will occur at the extrusion point and form cracks.

(2) The inductor (coil) is the main part of the high-frequency welded pipe welding part. The gap between it and the tube blank and the width of the opening have a great influence on the welding quality.

① The gap between the inductor and the tube blank is too large, resulting in a sharp drop in the inductor efficiency;

If the gap between the inductor and the tube blank is too small, it is easy to generate discharge between the inductor and the tube blank, causing welding cracks, and it is also easy to be damaged by the tube blank.

② The opening width of the inductor is too large, which will reduce the welding temperature of the butt edge of the tube blank. If the welding speed is fast, false welding and cracks are likely to occur after straightening.

In the production of high-frequency welded pipes, there are many factors that cause weld cracking, and the prevention methods are also different. There are too many variables in the high-frequency welding process, and any link defects will eventually affect the welding quality.

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BAOWI STEEL MANUFACTURING CO.,LTD

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BAOWI STEEL MANUFACTURING CO.,LTD
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