What are the common defects of high-frequency welded steel pipes and the preventive measures

First, what is the production process of high-frequency welded steel pipes?

In the production process of high-frequency welded steel pipes, a steel strip is fed into a forming machine to form a cylindrical tube blank, which then passes through an induction coil. The magnetic field near the induction coil generates an induced current that passes through the edge of the steel strip. The edge of the steel strip is heated due to its own resistance, and the heated edge is then pressed by extrusion rollers to form a weld. High-frequency welding does not involve additives; it is essentially a forging weld. If the production process is well controlled, there will be no residual molten metal or oxides on the fusion surface. If a weld sample is cut, polished, and etched, and observed under a low-power metallographic microscope, the morphology of a normal high-frequency welded area, the heat-affected zone, resembles a drum shape. This is due to the heat generated by the high-frequency current entering from the ends and edges of the steel strip. The heat-affected zone is slightly darker in color than the base metal because carbon diffuses towards the heated edge of the steel strip during welding, and is absorbed at the edge of the steel strip as the weld cools. Especially near the edge, the carbon oxidizes to CO or CO2, leaving the iron without carbon, resulting in a lighter color. The metal flow lines observed on metallographic specimens are actually flat, discontinuous planes formed when the high-carbon region of the steel billet is rolled into a steel strip. The angle of inclination of the metal flow lines is often used to evaluate the degree of upsetting during welding.

Second, what are the common defects in high-frequency welded steel pipes?

High-frequency welded steel pipes can exhibit a variety of defects, and the terminology for these defects is not entirely consistent. Considering the characteristics of welded steel pipe production, the following defects are frequently observed: ① Inclusions; ② Insufficient fusion; ③ Adhesive welding; ④ Cast welding; ⑤ Porosity; ⑥ Skip welding. Not all of these defects are present, but they frequently occur in high-frequency welded pipe manufacturing.

1. Black overheated inclusions in high-frequency welded steel pipes.

This type of defect occurs when metal oxides are not extruded with the molten metal and are trapped on the molten surface; they form on the surface of the molten metal at the V-notch. In a V-cut weld, if the approach speed of the steel strip edge 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 forms after the apex of the V-cut. These molten metals and metal oxides cannot be completely discharged through normal extrusion, thus forming an inclusion band. After the weld is flattened, black overheated inclusions are easily visible on the weld fracture surface. Compared to the fibrous fracture surface of the weld, the black overheated inclusions have a flatter fracture surface and lack metallic luster. These defects may occur singly or in chains. The probability of inclusions increases when the V-cut angle becomes narrower, for example, less than 4° or when the silicon-to-manganese ratio in the steel is less than 8:1. However, the silicon-to-manganese ratio in the steel is more difficult to control than other influencing factors and mainly depends on the chemical composition of the base metal.

Preventive measures to avoid black overheating inclusions in high-frequency welded steel pipes:

① Control the V-angle to 4°～6°;

② Ensure stable V-length using reliable tooling equipment;

③ Achieve better weld quality with a relatively low welding temperature;

④ Avoid a silicon-to-manganese ratio of less than 8:1 in the base metal of the steel strip.

2. White peroxide inclusions in high-frequency welded steel pipes.

Calling this type of defect "white peroxide inclusions" is not entirely accurate. It is actually insufficient fusion caused by pre-arc welding, and there are no foreign objects trapped on the fusion surface. Typically, burrs or rust form a bridge before reaching the apex of the V-shaped weld, causing a short circuit and resulting in current fluctuations that produce a pre-arc phenomenon. The short-circuit current instantaneously changes the current direction, reducing the heat of the V-shaped weld. The instantaneous current produces a very small defect, generally not exceeding the wall thickness. From the weld fracture surface, a small, bright, flat plane surrounded by fibrous fracture surfaces can be seen.

Preventive measures to avoid white peroxide inclusions in high-frequency welded steel pipes:

① Control the V-not angle to 4°–6°;

② Reduce edge burrs;

③ Provide appropriate edge treatment or reduce edge damage to the steel strip;

④ Keep the cooling water clean and prevent it from flowing into the V-notch.

3. Incomplete fusion at the edges of high-frequency welded steel pipes.

When the edges of the two steel strips are not completely fused to form a good weld, the cracked edges appear blue, indicating that the steel strip has been heated. However, if the steel strip edges are flat and smooth, it indicates that the weld is not completely fused. The most direct cause of this type of defect is insufficient welding heating. However, considering other related factors, such as the weld heat input, the V-not angle, and V-not heating length, the installation and cooling conditions of the magnetic rod, and the size of the induction coil, these factors can individually or in combination cause defects.

Measures to prevent inadequate fusion in high-frequency welded steel pipes:

① Match the welding input to the material characteristics and welding speed;

② Position the magnetic rod 3.2–3.5 mm beyond the center of the extrusion roller;

③ The V-shaped opening angle should not exceed the pipe diameter;

④ The V-shaped opening angle should not exceed 6°;

⑤ The difference between the inner diameter of the induction coil and the outer diameter of the steel pipe should not exceed 6.5 mm;

⑥ The steel strip width should be suitable to meet the requirements of the production pipe diameter.

4. Inadequate edge fusion in high-frequency welded steel pipes.

Inadequate edge fusion is caused by the absence of metal on the fusion surface. This type of defect often appears on the outer or inner edge of the steel strip and is similar to peroxide defects. This type of defect is caused by flattening and cracking of the weld at the 3 o'clock position. The fracture surface is flat and dull.

Measures to prevent inadequate edge fusion in high-frequency welded steel pipes:

① Ensure the steel strip edges are straight and parallel;

② Use a better extrusion amount;

③ If the defect is caused by bulging, the fracture surface is silver-gray; use a larger welding heat input.

5. Insufficient Fusion in the Middle of High-Frequency Welded Steel Pipes.

After a weld with insufficient fusion fails, the middle section of the wall appears as a flat, silvery-gray strip with fibrous edges. This welding defect occurs when the welding speed requires power exceeding the rated power amplifier of the welding machine, and the entire edge of the steel strip does not have sufficient time to heat to the optimal temperature and depth required for the weld. Insufficient fusion in the middle can also be caused by incomplete removal of molten metal from the joint surface.

Measures to prevent insufficient fusion in the middle of high-frequency welded steel pipes:

① Increase welding machine power;

② Increase welding extrusion amount;

③ Increase V-not length or reduce welding speed.

6. Cast Welding of High-Frequency Welded Steel Pipes.

Cast welding occurs when molten metal on the joint surface is not completely removed. The cast metal on the fusion surface contains metal oxides, similar to overburned oxides. The fracture morphology varies depending on the content of residual cast metal. Currently, most exhibit a flat, brittle morphology. Metallographic examination reveals cast metal on the joint surface. Cast-welded steel pipes crack when flattened.

Measures to prevent porosity in high-frequency welded steel pipes:

① Increase weld venting;

② Increase steel strip width.

7. Porosity in high-frequency welded steel pipes.

Porosity on the weld joint surface is caused by insufficient venting during high-temperature welding. The fracture morphology is fibrous, with spherical bright white spots randomly distributed across the entire fracture surface. When white spots appear on the outer wall, their surface appearance turns black due to oxidation. Small pores are visible before external burr removal, and pores are also visible on the fusion line after burr removal.

Measures to prevent porosity in high-frequency welded steel pipes:

① Reduce welding input;

② Increase extrusion amount.

8. Skip welding in high-frequency welded steel pipes.

Skip welding has various forms. Typically, these defects are regularly and continuously distributed, with defects on the outer side of the wall thickness resembling wavy defects, generally appearing at equal intervals. Measures to prevent skip welding in high-frequency welded steel pipes:

① Add welding current filtering equipment;

② Check input voltage;

③ Check rollers and shafts.

Third, Recommendations for Preventing Defects in the Production of High-Frequency Welded Steel Pipes.

In actual production, defects often arise from the combined effect of several factors. A narrow V-shaped opening does not necessarily lead to overheating oxides unless the extrusion amount is slightly less than a normal value. A small extrusion amount may be caused by a slightly narrow slitting width of the steel strip, wear of tooling, or improper equipment installation. Welding defects can also be caused by factors outside the weld zone. For example, cold welding may occur due to cavitation in the cooling pump, preventing sufficient cooling of the magnetic rod. The magnetic rod heats up instantly, reducing the heat concentrated in the V-shaped opening and causing cold welding. Increasing the welding input can prevent cold welding defects before all cooling pumps fail and the magnetic rod completely fails. The best way to prevent defects is to identify the root cause and collect as many operating parameters as possible that may contribute to them. Determine relevant parameters such as working width, welding speed, screen current, screen pressure, grid current, and extrusion amount. Observe actual operation and record data, focusing on identifying abnormal fluctuations to analyze the causes of defects. During production, set values may slightly exceed normal values, but if several related variables simultaneously exceed slightly, the cumulative results are sufficient to cause defects.