High-frequency welded pipes may have various defects, and the defects are not completely uniform. In combination with the characteristics of our welded pipe production, there are several defects that often occur:
2 Insufficient fusion;
4 Cast welding;
5 Air holes;
6 Skip welding.
These defects are not all present, but are often found in high frequency welded tubes. The first defect — inclusions have been expressed last week. Now we will list the rest five defects and their preventive measures in this article.
Insufficient fusion – incomplete fusion (cracking) at the edge
The edges of the two steel strips were not completely fused to form a good weld seam, and the cracked edges were blue, indicating that the steel strip was heated, as shown in Figure 5.
Fig 5 Fracture profile of incomplete fusion at the edge
Measures to prevent insufficient fusion:
(1) The welding input matches the characteristics of the material factory and the welding speed;
(2) The position of the magnetic rod is 3.2-3.5 mm higher than that of the center of the extrusion roll;
(3) The V-shaped opening angle does not exceed the length of pipe diameter;
(4) The angle of V-shaped mouth should not exceed 6 degrees;
(5) The difference between inner diameter of induction coil and outer diameter of steel tube is not more than 6.5 mm.
(6) The width of steel band is suitable for the requirement of pipe diameter in production.
Insufficient Fusion – Insufficient Fusion at the Edge (Edge wave)
The reason for insufficient fusion at the weld edge is that there is no metal on the fusion surface. Such defects often occur on the outside or inside of the edge of the strip and are similar to peroxide defects. This kind of defect is due to the flattening and cracking of the weld at 3 points. As shown in Fig. 6, the fracture surface is flat and dull.
Fig 6 Schematic diagram of edge fusion insufficient defect
Preventive measures for insufficient fusion at the edge:
(1) The edge of the steel strip is straight and parallel butt;
(2) Using better extrusion quantity;
(3) If the fracture surface of the defect caused by bulging is silver gray, the larger welding heat input is used.
Insufficient fusion – insufficient central fusion (internal cold welding)
The middle section of the wall thickness shows a flat silver-gray strip after the weld failure due to insufficient fusion, as shown in Figure 7.
Fig 7 Schematic diagram of the defect port defect in the middle
Preventive measures for insufficient fusion in the central region:
(1) Increase the power of welding machine;
(2) Increase the welding extrusion quantity;
(3) Increase the length of V-shaped port or decrease the welding speed.
Cast welding(Brittle Welding)
Cast welding is that the molten metal on the joint surface is not exhausted completely. The as-cast metal on the fusion surface contains metal oxides as the over-burnt oxide. The fracture morphology varies according to the content of residual as-cast metal. Most of them present flat and brittle morphologies. Metallographic examination shows that there are as-cast metals on the interface, as shown in Figure 8. Cracking of cast-welded pipe during flattening.
Fig 8 Schematic diagram of metallographic morphology of cast welding defects
Preventive measures for cast welding:
(1) Increase welding discharge;
(2) Increase the width of steel band.
Air holes (pinholes)
The air holes on the welded joint surface are caused by high temperature welding but insufficient discharge. The morphology of the fracture is fibrous. Spherical bright white spots are randomly distributed over the whole fracture. When white spots appear on the outer wall, the surface of white spots is black due to oxidation. As shown in Figure 9.
Fig 9 Schematic diagram of air holes defects
Preventive measures for air holes:
(1) Reducing welding input;
(2) Increase the amount of extrusion.
There are many forms of skip welding, as shown in Figure 10.
Fig 10 Schematic diagram of the distribution of skip welding defects
Preventive measures for skip welding:
(1) Filter equipment for increasing welding current;
(2) Check the input voltage;
(3) Check rollers and shafts.
Suggestions to Prevent Defects
In actual production, defects are often caused by a combination of several factors. A narrow V-shaped port does not necessarily produce overburnt oxide unless the extrusion volume is slightly less than a normal value. The small extrusion may be caused by narrow longitudinal shear width of steel strip or inappropriate installation of tool wear equipment.
Welding defects also occur outside the welding zone, such as cold welding may be due to the phenomenon of evacuation of the cooling pump, which can not make the magnetic rod fully cooled. The instantaneous heating of the magnetic rod reduces the heat concentration at the V-shaped port and leads to cold welding. Increasing welding input can prevent the occurrence of cold welding defects before all cooling pumps fail and magnetic rods fail completely.
The best way to prevent defects is to find out the root causes of defects and collect as many operation parameters as possible that may cause defects. To determine the relevant parameters, such as working width, welding speed, screen pressure gate current, extrusion, etc., and observe the actual operation, records should be based on the detection of abnormal fluctuations, mainly to analyze the causes of defects. The set value may slightly exceed the normal value in production, but several related variables are slightly exceeded at the same time. The cumulative results show that it is sufficient to produce defects.
In the production process, the common defects and their causes are analyzed, classified and summarized, which is very useful for defect experience operators, not only to improve efficiency, but also to reduce costs.
Our company WRD, MANUFACTURER of high quality tube mill line, such as ERW/SSAW tube mill line, hot-dip galvanizing line, H-beams mill line, slitting/cut to length mill line and other tube mill related equipment.
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Post time: Mar-12-2019