Common Defects and Prevention of Welded Pipes

Sometimes, the defects in the actual production of high frequency welded pipe are not caused by one reason, but usually the result of a variety of factors. The welding defects may also be caused by other reasons outside the welding area. Therefore, many factors should be comprehensively considered for the defects, and the causes should be carefully analyzed to solve the problem.

inclusion

The formation mechanism of inclusion defect is that the metal oxide is not extruded with the molten metal, but is sandwiched on the welding fusion surface.

These metal oxides are usually formed on the metal surface of V-angle melting. When the approaching speed of strip edge is lower than the melting speed and the melting speed is higher than the discharging speed of molten metal, the inclusion band containing molten metal and metal oxides will be formed at the apex of V-shaped opening, and the metal oxides can not be completely discharged after normal extrusion, The surface of clean metal solution is doped with these metal oxides, thus forming defects during forging and welding.

This kind of defect will lead to crack after flattening the weld, and inclusions will be seen in the fracture of the weld. This kind of defect exists in different forms, sometimes single, sometimes chain.

Preventive measures for inclusion defects:

1.The V-angle should be strictly controlled between 4 and 6

2.Adjust the unit to ensure a stable length of opening angle

3.The ratio of Mn / Si in the chemical composition of strip steel is greater than 8:1

4.Reduce oxidation in welding area

Pre arc

This kind of defect is actually the lack of fusion caused by pre arc. Usually, the burr or oxide scale and rust on the edge of strip steel fall in front of the vertex of V-angle to form a bridge, which causes the current jump and pre arc phenomenon. The short-circuit current changes the current direction and reduces the heat at V-angle.

For the defects caused by instantaneous shunt, a bright and flat plane fracture can be seen from the weld fracture. Sometimes there is no burr or oxide scale, rust, etc. on the edge of the strip. However, too small V angle or too high voltage will also produce pre arc phenomenon, which is caused by the high voltage discharge on the edge of the strip.

Preventive measures for pre arc defects:

1.The V-angle should be strictly controlled between 4 and 6

2.The strip edge is clean, smooth and free of burr

3.Keep the cooling water clean, control the flow direction of cooling water, and try to avoid V-angle

Insufficient fusion

This defect is due to the two strip edges are heated but not completely fused, and a good weld is not formed. The direct cause of insufficient fusion is insufficient heat during welding. There are many related factors leading to insufficient welding heat, such as high frequency power output, V angle and heating length, position of magnetic rod, working conditions and cooling of magnetic rod, size of induction coil, welding speed, etc, These factors interact with each other, resulting in such defects.

Preventive measures for insufficient fusion defects:

1.The matching of welding heat input and welding speed, and the characteristics of raw materials

2.Magnetic rod working condition

3.v angle and heating length

4.Induction coil specification

The stability and good condition of the equipment are the basic conditions to avoid defects. Improving the recording and analysis of process parameters can increase the quality of pipes.