In high-frequency straight seam welded steel pipes, cracks can manifest as full-length cracks, localized periodic cracks, and irregular intermittent cracks. Some pipes may not show cracks on the surface after welding, but may develop cracks after flattening, straightening, or hydrostatic testing.
Factors Affecting Cracks in High-Frequency Welded Steel Pipes
1. Poor Raw Material Quality:
In welded steel pipe production, large burrs on the raw material edges and excessive width are common problems. Outward-facing burrs during welding can easily cause full-length intermittent cracks. Excessive width and overfilling of the extrusion roll pass can lead to weld peaches, large external weld scars, and small or even no internal weld ribs, leading to cracking after straightening.
2. Fillet Condition of Steel Pipe Edges:
Fillet joints on the edges of steel pipe blanks are a common problem in welded steel pipe production. The smaller the pipe diameter, the more severe the fillet joints. Improper forming and adjustment are a prerequisite for fillet joints. Improper roll pass design, large external fillets, and roll adjustment with an angled roll are key factors affecting fillet welds. A single-radius roll pass cannot eliminate fillet weld problems caused by poor forming. Increasing extrusion pressure or wear of the roll pass into a vertical ellipse in late production can exacerbate the sharp-peared weld and lead to severe fillet welds. Fillet welds cause most of the metal to flow out from the top, creating an unstable melting process. This results in significant metal splashing and an overheated weld structure. External burrs are high-heat, irregular, and large in volume, making them difficult to scrape off. Internal burrs are minimal, and even the slightest miscontrol of the welding speed can lead to “false welds.” Large external fillets on the rolls result in insufficient filling of the steel pipe billet within the rolls, altering the edge contact from parallel to a “V” shape, resulting in a missing weld on the inner pipe. Prolonged wear on the roll shafts, combined with wear on the base bearings, creates an angled roll between the two shafts, resulting in insufficient extrusion pressure, vertical ellipse welds, and severe fillet welds.
3. Unreasonable selection of process parameters for steel pipes
The process parameters for high-frequency welded steel pipe production include welding speed (unit speed), welding temperature (high-frequency power), welding current (high-frequency frequency), extrusion pressure (mold design and material), opening angle (mold design and material, induction coil position), inductor (coil material, winding direction, position) and the size and placement of the impedance.
(1) High-frequency (stable and continuous) power, welding speed, welding extrusion pressure, and opening angle are the most important process parameters and must be reasonably matched; otherwise, the weld quality of the steel pipe will be affected.
① Too high or too low speed will cause low-temperature welding to be incomplete and high-temperature overburning, and the weld of the steel pipe will crack after being flattened.
② When the extrusion pressure is insufficient, the metal of the welded edge cannot be fully pressed together, and the impurities remaining in the weld of the steel pipe are not easy to be discharged, and the weld strength of the steel pipe is reduced; when the extrusion pressure is too large, the metal flow angle increases, the residue is easy to be discharged, the heat-affected zone becomes narrower, and the welding quality is improved. However, if the pressure is too high, it will cause large sparks to splash, causing the molten oxide and part of the plastic layer metal to be squeezed out together. After scraping, the weld of the steel pipe will be thinned, thereby reducing the weld strength of the steel pipe and causing cracks. Appropriate extrusion pressure is an important prerequisite for the quality of the steel pipe weld.
③ If the opening angle is too large, the high-frequency proximity effect will be reduced, the eddy current loss will increase, and the welding temperature will be reduced. If the welding speed is maintained at the original speed, cracks will be generated; if the opening angle is too small, the welding current will be unstable, and small explosions will occur at the extrusion point (visually known as the discharge phenomenon), forming cracks.
(2) The inductor (coil) is the main component of the high-frequency welding steel pipe welding part. The gap between it and the steel pipe billet and the opening width have a great influence on the weld quality of the steel pipe.
① If the gap between the inductor and the steel pipe billet is too large, the efficiency of the inductor will drop sharply; if the gap between the inductor and the steel pipe billet is too small, it is easy to cause discharge between the inductor and the steel pipe billet, causing weld cracks, and it is also easy to be damaged by the steel pipe billet.
② Excessively wide inductor openings reduce the welding temperature at the butt joint edge of the steel pipe. This can easily lead to false welds at high welding speeds, which can cause cracking after straightening.
In high-frequency welded steel pipe production, numerous factors can cause weld cracking, and preventative measures vary. Numerous variables exist in the high-frequency welding process, and defects in any step can ultimately affect the quality of the weld. The above are some common factors.
Post time: Sep-25-2025