How to prevent high frequency welded pipe weld seam cracking

In the high-frequency straight seam welded pipe, the cracks appear in the form of long cracks, local periodic cracks, and irregular intermittent cracks. Some steel pipes have no cracks on the surface after welding, but after flattening, straightening or hydraulic test after the crack

Factors Affecting Cracks
1. Poor quality of raw materials
In the production of welded pipes, there are often problems of large edge burrs and excessive width of raw materials. If the burrs are outward during welding, it is easy to produce intermittent cracks. , the outer welding scar is large, the inner welding rib is small or even no, and it cracks after straightening.
2. Edge joint state
Fillet joint at the edge of tube billet is a common phenomenon in the production of welded pipe. The smaller the pipe diameter, the more serious the fillet joint is. Improper forming adjustment is the prerequisite for producing fillet joint. The elevation angle of the pressure roller adjustment is the key factor affecting the fillet joint.
The pass pattern of the single-radius extrusion roll cannot eliminate the corner joint problem caused by poor molding. Increasing the extrusion force, or the pass pattern of the extrusion roll wears into a vertical ellipse in the later stage of production, will aggravate the pointed peach-shaped welding state, resulting in serious damage. Fillet joints, fillet joints will cause most of the metal to flow out from the top, forming an unstable melting process. At this time, there will be large metal splashes, and an overheated weld joint structure will appear. It is not easy to scrape off, and the internal burrs are very small. If the welding speed is not controlled properly, there will be "false welding" of the weld.
The outer fillet on the extrusion roller is large, so that the tube billet is not filled enough in the extrusion roller, and the edge contact state changes from parallel to V-shaped, resulting in the phenomenon that the inner weld is not penetrated, and the extrusion roller shaft is subjected to long-term force wear. Coupled with the wear of the base bearing, the two axes form an elevation angle, resulting in insufficient extrusion force, vertical ellipse and serious corner joints.
3. Unreasonable selection of process parameters
The process parameters of high-frequency welded pipe production include welding speed, welding temperature, welding current, extrusion force, opening angle, size and placement of inductors and resistors, etc., high-frequency power, welding speed, welding extrusion force and opening angle It is a relatively important process parameter and needs to be matched reasonably, otherwise it will affect the welding quality.
If the speed is too high or too low, it will cause low-temperature weld impermeability and high-temperature overheating, and the weld will crack after being flattened; when the extrusion force is insufficient, the metal at the edge of the weld cannot be fully pressed, and the remaining impurities in the weld are not easy to discharge. The strength of the weld seam decreases; 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, but if the pressure is too high, it will cause large sparks to splash and melt The oxide and part of the metal in the plastic layer are extruded together, and the weld seam is thinned after scratching, thereby reducing the strength of the weld seam and cracking. Appropriate extrusion force is an important prerequisite for weld seam quality.
If the opening angle is too large, the high-frequency proximity effect will decrease, the eddy current loss will increase, and the welding temperature will decrease. If the welding speed is kept at the original speed, cracks will appear; if the opening angle is too small, the welding current will be unstable, and small The rupture (intuitively, it is a discharge phenomenon) forms cracks.
The inductor is the main part of the welding part of the high-frequency welded pipe. The gap between the inductor and the tube blank and the opening width have a great influence on the welding quality. If the gap between the inductor and the tube blank is too large, the efficiency of the inductor will drop sharply; If the gap between the sensor and the tube blank is too small, it is easy to cause discharge between the inductor and the tube blank, resulting in welding cracks, and it is also easy to be damaged by the tube blank; the opening width of the sensor is too large, which makes the welding temperature of the butt edge of the tube blank Reduced, in the case of fast welding speed, it is easy to produce false welding, and it will crack 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 defect in any link will affect the quality of the weld. The above are some common factors .