A) Common welding defects of high frequency welded pipe (ERW)
High-frequency welding pipe (ERW) is the fastest and most efficient method of pipe-making, but high-frequency welding can produce a variety of welding defects. There are many different names for each defect, and there is currently no accepted terminology. The names of defects are given below, with another common name for defects in parentheses: (1) Inclusions (black peroxide); (2) Prearc (white peroxide); (3) Insufficient fusion ( (4) Insufficient fusion at the edge (edge wave); (5) Insufficient fusion in the middle (cold welding in the middle); (6) Adhesion welding (cold welding); (7) Cast welding (brittle welding); (8) ) porosity (pinhole); (9) jump welding. These defects are not all but the most common high frequency welding defects.
1. Inclusions (black peroxides)
A defect of this type is that the metal oxide is not trapped on the fusion surface as the molten metal is extruded. These metal oxides are formed on the molten metal surface of the V-shaped mouth. In the V-shaped mouth, if the approach speed of the edge of the steel strip is less than the melting speed, the melting speed is higher than the molten metal discharge speed. And the narrow sector of metal twisted oxides, these molten metals and metal oxides cannot be completely discharged by normal extrusion, thus forming an inclusion zone.
After the weld is flattened and cracked, inclusions are easily seen at the weld fracture. Compared with the fibrous fracture of the weld, the fracture is flat and has no metallic luster. Such defects may appear singly or in chains. When the angle of the V-shaped mouth is narrowed, for example, the angle is less than 4 degrees or the ratio of Mn/Si in the steel strip is less than 8:1, the occurrence probability of inclusions increases. But the Mn/Si ratio is more difficult to control than other influencing factors.
Prevention measures for inclusions: (1) The V-shaped angle is controlled at 4 degrees to 6 degrees; (2) Reliable tooling and equipment installation ensure a stable V-shaped mouth length; (3) Relatively low welding temperature obtains better performance. (4) Avoid the Mn/Si ratio in the chemical composition of the steel strip being less than 8:1 (avoid less than 4:1 when the impact toughness requirement is low).
2. Pre-arc (white peroxide)
Calling this type of defect "oxide" is inaccurate, since there is no foreign matter trapped on the fusion surface, it is actually a pre-arc lack of fusion. Usually, burrs or rust form a bridge before falling to the apex of the V-shaped mouth, causing the short-circuit surface to cause the current to jump and produce a pre-arc phenomenon. The short-circuit current instantly changes the current direction and reduces the heat of the V-port.
Instantaneous shunting produces very small defects, generally not exceeding the wall thickness dimension. From the weld fracture, a bright, flat plane can be seen surrounded by the fiber fracture.
There is no short circuit caused by rust or burr in the production of vacuum welded pipe, but the narrow V-shaped angle and high pressure also produce pre-arc phenomenon. It is the same defect caused by the arc discharge phenomenon caused by the high voltage at the edges of the two steel strips.
Pre-arc defect prevention measures: L (1) V-shaped mouth angle is controlled at 4 degrees to 6 degrees; (2) Reduce edge shearing burrs; (3) Appropriate edge treatment reduces edge damage of steel strip; (4) Keep cooling water clean And does not flow into the V-shaped mouth.
3. Insufficient fusion (slitting)
As the name implies, the edges of the two steel strips are not completely fused into a good weld, and the edge of the opening is blue, indicating that the steel strip has been heated, but the edge of the steel strip is flat and smooth, indicating that the weld is not fully fused. The most direct cause of such defects is insufficient welding heating, but other related factors should also be considered, such as the heat input to the welding seam, the angle of the V-shaped mouth and the heating length of the V-shaped mouth, the installation position and cooling conditions of the magnetic rod, and the size of the induction coil. These factors can act individually or in combination to produce defects. But sometimes suitable heat input will also produce seams. This is because the amount of extrusion is insufficient, oxides still remain on the surface of the molten metal, and the edges are melted but not fused together. After the weld passes through the squeeze roll, the steel strip rebounds in the form of Slit.
Measures to prevent insufficient fusion: (1) The welding input heat matches the material properties and welding speed; (2) The position of the magnet bar exceeds the center of the squeeze roller by 1/8 in (3.18 mm); (3) The length of the V-shaped opening angle is not equal. Exceeds the length of the pipe diameter; (4) The angle of the V-shaped mouth does not exceed 7 degrees; (5) The difference between the inner diameter of the induction coil and the outer diameter of the steel pipe is not more than 1/4 in (6.35 mm); (6) The width of the steel strip is suitable and meets the production requirements. pipe diameter requirements.
4. Insufficient edge fusion (edge wave)
The reason for insufficient fusion at the edge of the weld is that there is no metal on the fusion surface. Such defects often appear on the outside or inside of the edge of the steel strip, and are similar to those formed by peroxidation. This type of defect is due to the flattening and cracking of the weld at the 3 o'clock position. Fracture-shaped Limulus flat, no metallic luster.
Another form is that the bulge causes the outer temperature of the edge of the steel strip to be lower than the inner temperature, and its fracture shape is silver-gray. The defect formed by the bulge is a form of peroxidation and insufficient fusion.
Preventive measures for insufficient edge fusion: (1) The edges of the steel strip are straight and parallel butt; (2) Use a larger amount of extrusion; (3) If the fracture caused by the bulge is silver-gray, use a larger welding heat enter.
5. Insufficient fusion in the middle (cold welding in the middle)
After the failure of the insufficiently fused weld, the section in the middle of the wall thickness is a flat, silver-gray strip with a fibrous edge. This welding defect is caused by the fact that the power required by the welding speed exceeds the rated power of the welding machine, and the entire end face of the edge of the steel strip cannot be heated to the optimum temperature and heating depth required by the welding seam for sufficient time. Insufficient fusion in the middle may also be caused by insufficient discharge and incomplete discharge of molten metal on the joint surface.
Preventive measures for insufficient fusion in the middle: (1) Increase the power of the welding machine; (2) Increase the welding extrusion amount; (3) Increase the length of the haircut or reduce the welding speed.
6. Bonding (cold welding)
Adhesion welding defects cannot be detected by current inspection methods, so they are the most dangerous weldability defects in high frequency welding. There is no gap on the surface of the joint formed by stick welding, which can transmit ultrasonic signals, which cannot be detected by electromagnetic inspection (EMI), but crack when flattened, and the fracture is flat and brittle. Compared with the fully fused weld fracture, it is slightly fibrous. Some gaps can be detected. If the transverse metallographic section is observed, it can be seen that the HAZ is very narrow, there is no white fusion line, and the rising angle of the metal streamline is very small.
Preventive measures for sticking welding: (1) Use sufficient welding power for different specifications of materials and welding speeds; (2) Sufficiently squeeze or increase the width of the steel strip.
7. Cast welding
Cast welding means that the molten metal on the joint surface is not fully discharged, and the as-cast metal on the fusion surface contains oxides like peroxidation. The fracture morphology varies according to the residual as-cast metal content. But most of them have flat, brittle morphology. Metallographic inspection can see that there is as-cast metal on the joint surface. Cracks when cast welds are flattened. Such defects have enough heat to melt the edge of the strip, but only a simple melt.
8. Stomata
The pores on the welding joint surface are caused by high temperature welding but insufficient discharge, and the fracture morphology is fibrous. The spherical bright white dots are randomly distributed on the entire fracture. When the white dots appear on the outer wall, the surface of the white dots turns black due to oxidation. Small pores can be seen before the outer burr is removed, and it can be seen on the fusion line after the outer burr is removed. to the stomata.
Prevention measures for blowholes: (1) reduce the heat input of welding; (2) increase the amount of extrusion.
9. Jump Soldering
Jump welding has various manifestations. Usually, such defects have a slightly regular continuous distribution. The defects outside the wall thickness are similar to wavy defects, and the spacing is an integral multiple of the power frequency (60HZ).
B) Recommendations to prevent defects
In actual production, defects are often caused by the combined effect of several factors. A narrow V-shaped mouth does not necessarily produce peroxide unless the amount of extrusion is slightly smaller than the normal requirement. The small amount of extrusion may be caused by the slightly narrow slitting width of the steel strip or the wear of the tooling and the improper installation of the equipment.
Welding defects are also caused by reasons other than the welding area. For example, cold welding may be caused by cavitation in the cooling pump, which cannot be fully cooled with the magnetic rod. The back of the steel pipe is conducted, the heat of the V-shaped mouth is reduced, and the phenomenon of cold welding occurs. Before all the cooling pumps fail to work properly and the magnet bar completely fails, increasing the welding input heat can prevent the occurrence of cold welding defects.
The best way to prevent defects is to find out the root cause of defects, and try to collect various operating parameters that may cause defects. It is very beneficial to determine relevant parameters such as: working width, welding speed, screen current, screen pressure, grid current, extrusion amount, etc. Observing the actual operating records can have abnormal fluctuations and is also very useful for analyzing the causes of defects.
Possible settings during production slightly above the normal value, but several correlated variables are slightly above the required value at the same time, and the cumulative result is enough to produce a defect.
Some factories have classified and summarized common defects and their causes, which is very beneficial for inexperienced operators to deal with such problems, which can improve production efficiency and reduce costs.
C) In conclusion
(1) Most of the welding defects are caused by improper installation or adjustment of the unit;
(2) Selecting a reasonable manufacturing plan, monitoring daily operation records, and regularly training high-frequency welders will help reduce defects;
(3) Improve the quality of edge trimming and edge treatment and the coil energy storage process, which is conducive to reducing edge damage defects;
(4) Pre-maintenance can prevent defects caused by tool wear or damage.
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