Common welding defects of ERW pipes

Common welding defects of ERW pipes

High-frequency welding is the fastest and most effective method for manufacturing steel pipes, but high-frequency welding is prone to various welding defects. The following analyzes the causes and preventive measures of nine common welding defects including inclusions, pre-arc, insufficient fusion, insufficient edge fusion, insufficient middle fusion, sticking welding, cast welding, blow holes, and skip welding in the production and manufacturing of high-frequency welded pipes. .

Introduction to high frequency welding process

After the steel strip enters the forming machine and becomes a cylindrical tube blank after slitting, uncoiling, leveling, shearing butt welding, loop storage and other processes, it then passes through the induction coil or contact welding angle, and the magnetic field near the induction coil is induced. Electric current passes through the edge of the steel strip, and the edge of the steel strip is heated due to the resistance heat generated by its own resistance. The edge of the heated steel strip is extruded by an extrusion roller to form a weld. High-frequency welding does not add metal or welding wire, and is actually a kind of forge welding. The amount of pressure exerted by extrusion is an important link. If the extrusion amount is controlled appropriately and stably, there will be no molten metal or oxide remaining on the fusion surface.

If a weld sample is cut, polished, corroded and observed under a metallographic microscope, the heat-affected zone is shaped like a waist drum. This is because the high-frequency current entering the edge of the steel strip enters the steel from the ends and edges of the edge of the steel strip. The belt generates heat. The color of the heat-affected zone is slightly darker than the base metal because carbon diffuses toward the edge of the heated steel strip during welding and is absorbed at the edge of the steel strip as the weld cools. The carbon especially close to the edge of the steel strip is oxidized into CO or CO2, and the remaining iron has no carbon and becomes lighter in color. The metal streamlines seen on metallographic samples are actually the high-carbon areas being rolled into flat, discontinuous planes when the billet is rolled into steel strips. The size of the metal streamline rise angle is often used to evaluate the degree of upsetting during welding.

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 currently no recognized professional term. The name of the defect is given below. Another common name of the defect is in parentheses: aInclusion (Black overburned oxide); b Pre-arc (white overburned oxide); c Insufficient fusion (open seams); d Insufficient edge fusion (edge waves); e Insufficient fusion in the middle (cold welding in the middle); f Sticky welding (cold welding); g cast welding (brittle welding); h pores (pinholes); i jump welding. Although these are some defects, they are the most common high-frequency welding defects in production.

1. Inclusions (black burnt oxide)

This type of defect is that the metal oxide is not clamped on the fusion surface as the molten metal is extruded. These metal oxides are formed on the surface of the molten metal at the V-shaped opening. At the V-shaped opening, 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 molten metal oxide is formed after the apex of the V-shaped opening. A narrow fan-shaped area of metal and metal oxides. These molten metals and metal oxides cannot be completely discharged after normal extrusion, thus forming an inclusion zone.

After the weld is flattened and cracked, inclusions can easily be seen at the fracture of the weld. Compared with the fibrous fracture of the weld, the inclusion has a flat fracture and no metallic luster. Such defects may appear singly or in a chain. When the V-shaped opening 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. Control the V-shaped opening angle between 4° and 6°; b. Reliable tooling and equipment installation to ensure a stable V-shaped opening length; c. Relatively low welding temperature to obtain better welds Quality; d. Avoid the Mn/Si ratio in the chemical composition of the steel strip being less than 8:1 (avoid being less than 4:1 when the impact toughness requirements are low).

2. Pre-arc (white perburnt 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. Usually, burrs or rust fall in front of the top of the V-shaped opening to form a bridge, causing a short circuit and causing the current to jump and produce a pre-arc phenomenon. The short-circuit current instantly changes the direction of the current and reduces the heat of the V-shaped port.

Instantaneous shunting produces very small defects, and generally the length of the defect does not exceed the wall thickness. A bright, flat plane can be seen from the weld fracture surrounded by fibrous fractures.

Vacuum welded pipe production does not have rust or burrs to cause short circuits, but the narrow V-shaped angle and high voltage can also cause pre-arcing. It is the arc discharge phenomenon caused by the high voltage at the edges of the two steel strips that produces the same defect.

Measures to prevent pre-arc defects: a. Control the V-shaped opening angle between 4° and 6; b. Reduce edge trimming burrs; c. Appropriate edge treatment to reduce steel strip edge damage; d. Keep the cooling water clean and not flow to the V-shaped opening .

3. Insufficient fusion (open seams)

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 edges of the steel strip are flat and smooth, and the surface weld is not completely fused. The most direct cause of such defects is insufficient welding heating, but other related factors should also be considered, such as weld heat input, V-shaped opening angle and V-shaped heating length, magnetic rod installation position and cooling conditions, induction coil size, etc. These factors can act individually or in combination to produce defects. But sometimes appropriate heat input will also produce seams. This is because the amount of extrusion is insufficient, the 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 extrusion roller, the steel strip rebounds to form Slit.

Measures to prevent insufficient fusion: a. Match the welding heat input to 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 V-shaped opening The angle 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 needs of the production pipe diameter.

4. Insufficient edge fusion (edge waves)

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, or are similar to defects formed by over-burned oxides. This type of defect is caused by the weld being crushed and cracked at the 3 o'clock position. The fracture surface is flat and has no metallic luster.

Another form is that the bulging causes the outer temperature of the edge of the steel strip to be lower than the inside. The fracture surface is silver-gray. The bulging defect is a form of over-burned oxide and insufficient fusion.

Preventive measures for over-burning and insufficient fusion: a. The edges of the steel strips are straight and parallel butt joints; b. Use a larger extrusion amount; c. If the fracture caused by the bulge is silver-gray, use a larger welding heat input.

5. Insufficient fusion in the middle (cold welding in the middle)

After the insufficient fusion weld is damaged, the middle section of the wall thickness appears as a flat, silvery gray band with fibrous edges. This kind of 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 incomplete discharge of molten metal on the joint surface.

To prevent insufficient fusion in the middle part: a. Increase the power of the welding machine; b. Increase the amount of welding extrusion; c. Increase the length of the V-shaped opening or reduce the welding speed.

6. Sticky welding (cold welding)

Sticky welding defects cannot be detected using current inspection methods and are therefore the most dangerous welding defects in high-frequency welding. The joint surface formed by adhesive welding has no gaps, can transmit ultrasonic signals, and cannot be detected by electromagnetic detection. However, it cracks when crushed, and the fracture is flat and brittle. Compared with the completely fused weld fracture, 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 streamline elevation angle is very small.

Preventive measures for sticking welding: a. Use sufficient welding power for different specifications of materials and welding speeds; b. Fully squeeze and increase the width of the steel strip.

7. Cast welding (brittle welding)

Cast welding means that all the molten metal on the joint surface is not discharged, and the cast metal on the fusion surface contains oxides as well as overburned oxides. The fracture morphology changes according to the residual cast metal content. But most of them have a flat, brittle morphology. Metallographic examination shows that there is cast metal on the bonding surface. Cast welds crack when flattened. This type of defect has enough heat to melt the edge of the strip, but only a simple melt.

Preventive measures for cast welding: a. Increase welding discharge; b. Increase steel strip width.

8. Stomata (pinhole)

The pores on the welding joint surface are caused by high-temperature welding but insufficient discharge, and the fracture surface is fibrous. Spherical bright white spots are randomly distributed on the entire fracture. When the white spots appear on the outer wall, the surface of the white spots turns black due to oxidation. Small pores can be seen before the outer burrs are removed, and they can be seen on the fusion line after the outer burrs are removed. to the stomata.

Measures to prevent pores: a. Reduce welding heat input; b. Increase the amount of extrusion.

9. Jump welding

Jump soldering comes in various forms. Usually, such defects are distributed slightly regularly and continuously. The defects on the outer side of the wall thickness are similar to wavy defects, and the spacing is an integral multiple of the power frequency (60HZ). For example: the unit welding speed is 120 ft/m in (36 576 mm /m in), and the defect distance is 4 in (101. 6 mm), then 36576÷1016=360 is an integral multiple of 60.

(1 in=25.4mm, 1 ft=304.8mm)

Measures to prevent jump welding: a. Increase the filtering equipment for welding current; b. Check the input phase voltage; c. Check the rollers and shafts.

Tips for preventing defects

In actual production, defects are often caused by the combined effect of several factors. A narrow V-shaped opening does not necessarily produce burnt oxide unless the extrusion amount is less than the normal requirement. The small amount of extrusion may be caused by a slightly narrow slitting width of the steel strip, wear of the tooling, or improper installation of the equipment.

Welding defects can also be caused by reasons other than the welding area. For example, cold welding may cause cavitation (evacuation) in the cooling pump, which cannot allow the magnet rod to be fully cooled. The magnet rod heats up instantly, causing the current to concentrate on the V-shaped port. Weakened, the current is conducted along the back of the steel pipe, the heat at the V-shaped mouth is reduced, and cold welding occurs. Before the cooling pump fails to work properly and the magnetic rod completely fails, increasing the welding input heat can prevent the occurrence of cold welding defects.

The best way to prevent defects is to identify the root causes 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 flow, screen pressure, grid flow, extrusion amount, etc. Observing actual operating records can reveal abnormal fluctuations and is also very helpful in analyzing the causes of defects. During production, the set value may slightly exceed the normal value, but several related variables slightly exceed the required value at the same time, and the cumulative result is enough to cause defects.

Conclusion

1. Most 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 trimming and edge processing and the coil energy storage process, which will help reduce edge damage defects;

4. Pre-maintenance can prevent defects caused by wear or damage of tooling.