Metal streamline shape of high frequency welded pipe

Metal streamline shape of high frequency welded pipe

Among the various forms of weld, metal streamline is an important display feature. It is a special shape of crystalline structure, formed by local melting or semi melting metal under high temperature and pressure. It is also a comprehensive embodiment of welding extrusion force size and direction, linear energy and welding speed, and has fixed metallurgical characteristics.

1.The rising angle of streamline is too large

In general, the larger the rising angle of the metal streamline, the greater the extrusion force during welding. The larger the extrusion force will extrude more molten metal, resulting in poor welding of the plate edge metal, which is easy to produce "cold welding". This is a common welding defect. In production, there are generally the following two cases:

(1)The plate edge is not welded, which may occur on the inside or outside of the weld. When the inside is not welded, the fracture surface is generally smooth and neat, mostly gray, accounting for a large proportion of the weld area, and the rising angle of the streamline can sometimes reach 80 degrees~ When the outer side of 90 ° is not welded, it is easy to weld at low temperature due to the low current density, and the weld fusion is not good. In most cases, the rising angle of streamline is small, and the display is not clear, and even the flow line can not be seen in serious cases.

(2) In fact, this kind of defect is a kind of "false welding", and the fusion line in the middle of the weld is generally very slight, sometimes even difficult to distinguish. Only the "waist drum" shape of thermal effect can be observed. The microstructure of the non welded part is mostly "oxide inclusion" or "gray spot", and the two sides are fine ferrite + pearlite The weld is easy to crack when it is subjected to 90 ° flattening test.

2.The rising angle of streamline is small

When the online energy and welding speed remain relatively stable, if the extrusion force is too small, it is easy to cause the rising angle of the metal streamline to be too small, and the streamline display is often not clear or even invisible. In this case, the fusion line in the middle of the weld often produces more "oxide inclusions", which become the crack source of the weld cracking, and is a common defect affecting the quality of the welded pipe.

3.asymmetric rising angle of streamline

This kind of weld defect is mainly caused by the following three reasons:

(1) The extrusion force is unbalanced. When adjusting the extrusion roll, the distribution of extrusion force on the top roll or both sides of the roll is asymmetric, which is easy to cause different changes in the streamline shape. Some rising angles are too small, the streamline is fine or not clear, some rising angles are too large, the streamline is thick, the display is abnormal, and the shape is diverse. If the plate edge is not parallel, it is easy to produce the wrong edge on the weld, resulting in the weld metal One way loss and stress concentration, the probability of welding defects will also increase, so special attention should be paid when adjusting the roll.

(2) If the edge is not parallel, the high-frequency current distribution will be uneven, and the local temperature difference is significant, so the plate edge can not be synchronized to achieve close welding.

(3) The plate edge parallelism is unreasonable, which may lead to the bending of the coil edge, which is easy to cause the edge "wave" shape, and increase the tendency of forming "gray spot". At the same time, in the forming process, the weld may appear "dislocation" and continue to the solder joint, which may cause "partial welding" or cracks in the solidified weld metal.

4.The center distance is too large

The center distance is the distance between the center line of the metal streamline and the center line of the thickness of the rolled plate. It is not only related to the stability of the extrusion force, but also affected by the parallelism of the plate edge.

(1) When the extrusion force is not adjusted properly, the center line of the metal streamline is not up or down, which is difficult to match with the center line of the wall thickness of the coiled plate. The larger the center distance is, the more uneven the butt joint spacing is. The weld is prone to "misalignment" and "oxide inclusion" is difficult to extrude. The welding quality is poor. When the center distance is close to zero, the welding point can be close to the center of the extrusion roll, and the extrusion force distribution is poor The welding quality can reach a good state.

(2) When the edge of the plate is not parallel, it is easy to cause the center distance to be too large and it is difficult to realize parallel joint. The thermal effect of local welding points may be reduced, which leads to low temperature welding state, difficult to remove oxides in the weld, and increase the trend of forming "Grey Spot". When the center distance reaches 1 / 2 of the wall thickness of the plate, the weld metal is easy to form single direction discharge, and the reverse side will easily form "Grey Spot" ”, or poor local welding.