In high-frequency longitudinally welded pipes(ERW steel pipe), the manifestations of cracks include long cracks, partial periodic cracks, and irregular intermittent cracks. Some steel pipes have no cracks on the surface after welding, but they have been flattened, straightened or hydraulically tested. Cracks appeared later.
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 the raw material. If the burrs are outward during welding, it is easy to produce intermittent cracks. The width of the raw material is too wide, and the squeeze roll pass is too full, forming a welded peach shape. , The outer welding scar is large, the inner welding rib is small or even not, and it will crack after straightening.

2. Edge corner joint state
Fillet joints of tube edges are a common phenomenon in the production of welded pipes. The smaller the diameter of the tube, the more serious the corner joints. Improper adjustment of forming is a prerequisite for corner joints. Improper pass design of the squeeze roll, large outer round corners of the pass, and extrusion The elevation angle of the pressure roller adjustment is the key factor that affects the angle joint.

The single-radius squeeze roller's pass cannot eliminate the corner joint problem caused by poor forming. Increase the extrusion force, or the pass of the squeeze roll in the later stage of production will wear out and become a vertical ellipse, which will aggravate the sharp peach-shaped welding state and cause serious problems. 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 large metal splashes and overheated weld joints. The external burrs appear to be hot, irregular, and large. It is not easy to be scraped off, and the internal burrs are small. If the welding speed is slightly improperly controlled, "false welding" of the weld will occur.

The outer round angle on the squeeze roll is large, which makes the tube blank insufficiently filled in the squeeze roll, and the edge contact state changes from parallel to V shape, which causes the phenomenon of incomplete welding of the inner weld, and the squeeze roller shaft is stressed for a long time. In addition, the base bearing is worn, causing the two shafts to 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 position 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 welding impermeability and high-temperature overburning, and the weld will crack after being flattened; when the extrusion force is insufficient, the welding edge metal cannot be fully pressed, and the residual impurities in the weld will not be easily discharged. The welding seam 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 narrow, and the welding quality increases. However, if the pressure is too high, it will cause larger sparks to splash and melt The oxide and part of the plastic layer of metal are squeezed together, and the weld is thinned after being scratched, thereby reducing the strength of the weld and cracking. Proper extrusion force is an important prerequisite for the quality of the weld.

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 decrease. If the welding maintains the original speed, cracks will occur; if the opening angle is too small, the welding current will be unstable and small at the extrusion point. The rupture (intuitively a discharge phenomenon), the formation of cracks.

The inductor is the main component of the welding part of the high-frequency welded pipe. The gap and the width of the opening between the inductor and the tube blank have a great influence on the welding quality. The gap between the inductor and the tube blank is too large, which makes the efficiency of the inductor drop sharply; The gap between the tube and the tube blank is too small, which will easily cause the discharge between the inductor and the tube blank, leading to welding cracks, and it is also easy to be damaged by the tube blank; the width of the sensor opening is too large, which will cause the welding temperature of the tube blank Reduced, in the case of faster welding speed, it is easy to produce false welding, which 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 different. There are too many variables in the high-frequency welding process. The defects in any link will affect the quality of the weld. The above are some common factors.