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In high-frequency straight seam welded pipes, crack manifestations include long cracks, local periodic cracks and irregular intermittent cracks. There are also some steel pipes with no cracks on the surface after welding, but cracks will appear after flattening, straightening or hydrostatic testing.
Causes of cracks
1. Poor quality of raw materials
In the production of welded pipes, problems with large burrs and excessive raw material width usually occur.
If the burrs are outward during the welding process, it is easy to produce continuous and long intermittent cracks.
The width of the raw material is too wide, the extrusion roller holes are 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 and corner joint 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.
Insufficient forming adjustment is a prerequisite for corner joints.
Improper design of the extrusion roller pass shape, large outer radius and elevation angle of the pressure roller are key factors affecting the corner joint.
A single radius cannot eliminate corner joint problems caused by poor molding. Increase the extrusion force, otherwise the extrusion roller will wear and become oval in the later stage of production, thereby exacerbating the sharp peach-shaped welding state and causing serious corner connections.
A corner joint will cause most of the metal to flow out from the upper side, creating an unstable melting process. At this time, there will be a lot of metal spatter, the weld will be overheated, and the external burrs will become hot, irregular, large, and difficult to scratch. If the welding speed is not properly controlled, "false welding" of the weld seam will inevitably occur.
The outer angle of the extrusion roller is large, so that the tube blank is not fully filled in the extrusion roller, and the edge contact state changes from parallel to "V" shape, resulting in the phenomenon that the internal weld seam is not welded.
3. Unreasonable selection of process parameters
The process parameters of high-frequency welded pipe production include welding speed (unit speed), welding temperature (high-frequency power), welding current (high-frequency frequency), extrusion force (design and material of grinding tool), angle opening (grinding) tool Design and materials, location of the induction coil), size and location of the inductor (coil material, winding direction, location) and the resistor.
(1) High-frequency (stable and continuous) power, welding speed, welding extrusion force and opening angle are the most important process parameters and must be matched reasonably, otherwise the welding quality will be affected.
① If the speed is too high or too low, it will lead to low-temperature welding impermeability and high-temperature overburning, and the weld will crack after being flattened.
② When the extrusion force is insufficient, the edge metals to be welded cannot be completely pressed together, and the remaining impurities in the weld are not easily discharged and the strength is reduced.
When the extrusion force is too large, the metal flow angle increases, the residue is easily discharged, the heat-affected zone becomes narrower, and the welding quality improves. However, if the pressure is too high, it will cause large sparks and splashes, causing the molten oxide and part of the metal plastic layer to be extruded, and the weld will become thinner after being scratched, thereby reducing the strength of the weld.
Appropriate extrusion force is an important prerequisite for ensuring welding quality.
③The opening angle is too large, which reduces the high-frequency proximity effect, increases eddy current loss, and reduces the welding temperature. If welded 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 cracks will form.
(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 impact on the welding quality.
The extrusion roller is worn for a long time and the basic bearing is worn. The two axes form an elevation angle, resulting in insufficient squeezing force, vertical ellipses and severe angular joints.
①The gap between the inductor and the tube blank is too large, causing the inductor efficiency to drop sharply;
If the gap between the inductor and the tube blank is too small, discharge will easily occur between the inductor and the tube blank, causing welding cracks, and it will also be easily damaged by the tube blank.
② If the opening width of the sensor is too large, it will reduce the welding temperature of the butt edge of the tube blank. If the welding speed is fast, false welds and cracks will easily 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 so many variables in the high-frequency welding process that any link defects will ultimately affect the quality of the weld.