Among the various forms of the weld, the metal streamline is an important display feature. It is a special shape of crystalline structure, which is formed by partially molten or semi-melted metal under high-temperature conditions and pressure. It is also a welding extrusion. The comprehensive manifestation of force size and direction, line energy and welding speed, has fixed metallurgical characteristics.
Metal streamline shape of high frequency welded pipe(ERW)
Under normal circumstances, the greater the rising angle of the metal streamline, the greater the extrusion force during welding. The greater extrusion force will squeeze out more molten metal, resulting in poor welding of the edge metal and easy to produce "cold welding". It is a common welding defect, and the following two situations generally occur in production:
There is no welding on the edge of the plate, which may occur on the inside or outside of the weld. When the inside is not welded, the fracture is generally smooth and tidy, mostly gray, accounting for a large proportion of the weld area, and the streamline rising angle can sometimes reach 80°~90°. When there is no welding on the outside, due to the low current density, low-temperature welding is easy, and the welding seam is not fused well. In most cases, the streamline rises at a small angle and the display is not clear. In severe cases, the streamline is even unclear.
When the online energy and welding speed remain relatively stable, if the extrusion force is too small, it is easy to cause the metal streamline rising angle to be too small, and the streamline display is often unclear or even invisible. In this case, the fusion in the middle of the weld Wire tends to produce more "oxide inclusions", which becomes the source of cracks in weld cracking, which is a common defect that affects the quality of welded pipes.
This weld defect is mainly caused by the following three reasons:
The squeezing force is unbalanced. When adjusting the squeezing rollers, the squeezing force distribution of the top roller or the rollers on both sides is asymmetric, which is easy to cause different changes in the shape of the streamline, and some of the rise angle is too small, and the streamline is fine or unclear. Some have large rising angles, thick streamlines, display abnormalities, and various shapes. If the edges of the plates are not parallel, it is easy to produce wrong edges on the weld, resulting in one-way loss of weld metal and stress concentration, and the probability of welding defects will also be Increase, so pay special attention when adjusting the roll.
The parallelism of the board edges is not good, and the edges of the board are not parallel, and “∨” and “∧” shapes are prone to appear, because high-frequency welding is the result of the edge proximity effect and skin effect. If the edges are not parallel, the high-frequency current distribution It will be uneven, and the local temperature difference will be significant, and the edges of the board cannot be contacted simultaneously to achieve tight welding.
(3) Unreasonable parallelism of the edges of the plate, which may cause the edge of the coil to bend, easily cause the edge to be "wavy", and increase the tendency to form "grey spots". At the same time, during the forming process, the weld may appear "staggered" "And continue to the welding point, which may cause "offset welding" or cracks in the solidified weld metal.
The center distance is the distance between the centerline of the metal streamline and the centerline of the coil wall thickness. It is not only related to the stability of the extrusion force, but also affected by the parallelism of the plate edges.
(1) When the squeezing force is adjusted improperly, the center line of the metal streamline may be lower than the upper side, and it is difficult to match the center line of the coil wall thickness. The larger the center distance, the more uneven the gap between the seams, and the welding seam Prone to "wrong edges", "oxide inclusions" are difficult to squeeze out, and the welding quality is poor. When the center distance is close to zero, the welding point can be close to the center of the squeeze roll, the squeezing force distribution is more uniform, and the welding quality can reach very high Good condition.
(2) When the sides of the coil are not parallel, it is easy to cause the center distance to be too large, and it is difficult to achieve parallel joints. The thermal effect of the local solder joints may be reduced, resulting in a low-temperature welding state, and it is difficult to remove the oxides in the solder joints and increase the formation of " When the center distance reaches 1/2 of the thickness of the coil wall, the weld metal is easily discharged in one direction, and the opposite side is likely to form "grey spots" or poor local welding.
