Common welding defects of high-frequency welded pipes Various welding defects may occur in high-frequency pipe making. Each defect has many different names. There is no recognized professional terminology at present. The names of the defects are given below, and the other common names of the defects are in brackets: a inclusion (black overburned oxide); b pre-arc (white overburned oxide); c insufficient fusion (open seam); d insufficient fusion of the edge (edge wave); e insufficient fusion of the middle (middle cold welding); f sticking welding (cold welding); g cast welding (brittle welding); h porosity (pinhole); i jump welding. Although these are partial defects, they are the most common high-frequency welding defects in production.
1. Inclusion (black overburned oxide) This type of defect is that the metal oxide is not squeezed out with the molten metal and is clamped on the fusion surface. These metal oxides are formed on the surface of the molten metal at the V-mouth. At the V-mouth, if the approach speed of the edge of the steel strip is less than the melting speed, and the melting speed is higher than the molten metal discharge speed, a narrow fan-shaped area containing molten metal and metal oxides is formed after the apex of the V-mouth. These molten metals and metal oxides cannot be completely discharged after normal extrusion, thus forming an inclusion belt.
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 of the inclusion is flat and has no metallic luster. This type of defect may appear individually or in a chain. When the V-shaped mouth angle becomes narrower, for example, the angle is less than 4 degrees or the Mn/Si ratio in the steel strip is less than 8:1, the probability of inclusions increases. However, the Mn/Si ratio is more difficult to control than other influencing factors.
Measures to prevent inclusions: a. The V-shaped mouth angle is controlled at 4°~6°; b. Reliable tooling and equipment installation ensures a stable V-shaped mouth length; c. Relatively low welding temperature to obtain better weld quality; d. Avoid the Mn/Si ratio in the chemical composition of the steel strip to be less than 8:1 (avoid less than 4:1 when the impact toughness requirement is low).
2. Pre-arc (white overburned oxide) It is not accurate to call this type of defect "oxide" because there is no foreign matter sandwiched on the fusion surface. In fact, it is insufficient fusion caused by pre-arc. Normally, burrs or rust fall to the top of the V-shaped mouth to form a bridge, causing a short circuit and causing current jump to produce pre-arcing. The short-circuit current instantly changes the direction of the current, reduces the heat of the V-shaped mouth, and instantly shunts to produce very small defects. Generally, the length of the defect will not exceed the wall thickness. From the weld fracture, you can see a bright, flat plane surrounded by a fibrous fracture. There is no rust or burr to cause a short circuit in the production of vacuum welded pipes, but narrow V-shaped angles and high voltages can also produce pre-arcing. It is the high voltage at the edges of the two steel strips that causes arc discharge and produces the same defect. Pre-arcing defect prevention measures: a. The angle of the V-shaped mouth is controlled at 4°~6; b. Reduce shearing burrs; c. Appropriate edge treatment reduces damage to the edge of the steel strip; d. Keep the cooling water clean and do not flow to the V-shaped mouth.
3. Insufficient fusion (open seam) As the name suggests, the edges of the two steel strips are not completely fused to form a good weld. The edge of the open seam is blue, indicating that the steel strip has been heated, but the edge of the steel strip is flat and smooth, and the surface weld is not completely fused. The most direct cause of this type of defect is insufficient welding heating, but other related factors should also be considered, such as weld input heat, V-shaped mouth angle and V-shaped heating length, magnetic rod installation position and cooling conditions, induction coil size, etc. These factors can cause defects alone or in combination. But sometimes appropriate heat input can also cause seams. This is because the extrusion volume is insufficient, the oxides still remain on the surface of the molten metal, the edges are melted but not fused together, and the weld passes through the extrusion roller, and the steel strip rebounds to form a seam. Measures to prevent insufficient fusion: a. The welding input heat matches the material properties and welding speed; b. The position of the magnetic rod exceeds the center of the extrusion roller by 3.18mm; c. The length of the V-shaped opening does not exceed the length of the pipe diameter; d. The angle of the V-shaped mouth does not exceed 7°; e. The difference between the inner diameter of the induction coil and the outer diameter of the steel pipe is not greater than 6.35mm; f. The width of the steel strip is suitable and meets the production diameter requirements.
4. Insufficient edge fusion (edge wave) The reason for insufficient edge fusion of the weld is that there is no metal on the fusion surface. This type of defect often occurs on the outside or inside of the edge of the steel strip, or is similar to the defect formed by overburned oxides. This type of defect is caused by the weld being flattened and cracked at 3 o'clock. The fracture morphology is flat and has no metallic luster.
Another form is that the bulging causes the outer temperature of the steel strip edge to be lower than the inner temperature, and its fracture morphology is silver-gray. The bulging defect is a form of overburned oxide and insufficient fusion.Preventive measures for overburned and insufficient fusion; a. The edges of the steel strip are straight and parallel to each other; b. Use a larger extrusion volume; c. If the fracture caused by the defect of bulging is silver-gray, use a larger welding heat input. 5. Insufficient fusion in the middle (cold welding in the middle) After the weld with insufficient fusion is destroyed, the middle section of the wall thickness is flat, silver-gray, and the edge is fibrous. This welding defect is caused by the power required by the welding speed exceeding the rated power of the welding machine, and the entire end face of the steel strip edge does not have sufficient time to heat to the optimal temperature and heating depth required for the weld. Insufficient fusion in the middle may also be caused by insufficient discharge, and there is incomplete discharge of molten metal on the joint surface.
Prevention of insufficient fusion in the middle: a. Increase the power of the welding machine; b. Increase the amount of welding extrusion; c. Increase the length of the V-shaped mouth or reduce the welding speed.
6. Sticking welding (cold welding) Sticking welding defects cannot be detected by current detection methods, so it is the most dangerous welding defect in high-frequency welding. The joint surface formed by sticking welding has no gaps, can transmit ultrasonic signals, and cannot be detected by electromagnetic detection, but it cracks when flattened, and the fracture is flat and brittle. Compared with the fracture of a completely fused weld, it is slightly fibrous. Some gaps can be detected. If you observe the transverse metallographic section, you can see that the HAZ (heat affected zone) is very narrow, there is no white fusion line, and the metal flow line rise angle is very small.
Prevention measures for sticking welding: a. Use sufficient welding power for different specifications and welding speeds; b. Sufficient extrusion and increase the width of the steel strip.
7. Cast welding (brittle welding) Cast welding is that the molten metal on the joint surface is not completely discharged, and the cast metal on the fusion surface contains oxides like overburned oxides. Its fracture morphology varies according to the content of residual cast metal. But most of them are flat and brittle. Metallographic inspection shows that there is cast metal on the joint surface. The cast weld cracks when flattened. This type of defect has enough heat to melt the edge of the steel strip, but it is only a simple melting.
Prevention measures for cast welding: a. Increase welding discharge; b. Increase the width of the steel strip.
8. Porosity (pinhole) The pores on the welding joint surface are caused by high-temperature welding but insufficient discharge, and the fracture morphology is fibrous. Spherical bright white spots are randomly distributed on the entire fracture. When white spots appear on the outer wall, the surface of the white spots is black due to oxidation. Small pores can be seen before the external burrs are removed, and pores can be seen on the fusion line after the external burrs are removed.
Prevention measures for pores: a. Reduce welding input heat; b. Increase extrusion. 9. Jump welding Jump welding has various manifestations. This type of defect is slightly regularly distributed continuously. The outer wall thickness defect is similar to a wavy defect, and the spacing is an integer multiple of the power frequency (60HZ). For example: the welding speed of the unit is 120 ft/min (36 576 mm/min), and the defect spacing is 4in (101.6mm), then 36576÷1016=360 is an integer multiple of 60. (1 in=25.4mm, 1 ft=304.8mm)
Preventive measures for skip welding: a. Add filtering equipment for welding current; b. Check input phase voltage; c. Check rollers and shafts.
