Why do common burrs form inside welded steel pipes

Internal burr removal is a crucial process in the production of high-frequency welded steel pipes. Improper adjustment or operation can lead to burrs forming during this process, affecting the quality of the internal burr removal. Internal burr removal has always been a challenging problem in high-frequency welded steel pipe production, especially the burrs (also called weld beads) that appear on the cutting tool during production. These burrs not only severely impact the quality of internal burr removal but also frequently cause production line shutdowns, reducing the yield of welded steel pipes. Furthermore, a large number of welded steel pipes with excessive internal burrs create significant difficulties for subsequent processing, severely reducing production efficiency. Therefore, it is necessary to analyze the burr problem during internal burr removal, identify the root cause, and develop appropriate countermeasures. There are several methods for removing internal burrs from high-frequency welded steel pipes, including rolling, grinding, and tool scraping.

First, what are burrs inside welded steel pipes?

Burs: refer to the lumpy deposits formed by the aggregation of molten metal droplets during high-frequency welding.

Common types of burrs inside welded steel pipes are classified into small-particle burrs and large-particle burrs.

(1) Small-particle burrs: Small-particle burrs are very common in high-frequency welded steel pipes. They appear to be composed of many small particles. During high-frequency welding of steel pipes, sparks generate particles the size of sesame seeds. If these particles are not removed in time, they will continuously accumulate and form lumps, remaining above the inner burr cutter rod. When these lumps grow to a certain size, they will prevent the inner burr cutter rod from lifting properly or block the inner hole of the inner burr ring cutter, preventing normal scraping. Small-particle burrs are generally resolved by adjusting the cooling air pressure and the position of the air vents on the inner burr cutter rod.

(2) Large-particle burrs: Large-particle burrs consist of larger particles that are firmly bonded together. Compared to small-particle burrs, large-particle burrs form faster and are more likely to cause poor scraping of the inner burr, even leading to welding failure and machine shutdown.

Large-particle burrs can be further classified into shank burrs and inner burr cutter burrs according to their location.

(A) Tool Rod Nodules: Primarily formed on the tool rod or protective sleeve below the welding V-angle, caused by excessive welding power. Higher welding power results in greater fluidity of the molten metal at the plate edge. During high-frequency welding, a large amount of squeezed-out molten metal drips onto the tool rod or protective sleeve, forming large nodules. This severely affects the normal rise of the tool rod, leading to poor internal burr scraping.

(B) Internal Burr Nodules: Mainly found at the internal burr knife or upper roller position, also caused by excessive welding power. Higher welding power produces softer internal burrs. During scraping, the internal burrs continuously adhere to the internal burr knife, accumulating in large quantities at the internal burr knife or upper roller position within a short time, forming nodules and severely degrading the quality of internal burr scraping.

Second, what are the causes of nodule formation?

1. Excessively High-Frequency Welding Power: During production, high-frequency welding operators often judge the welding power by the color and shape of the external burrs. The shape and color of external burrs are related to the shape of the strip edge during welding. Different strip edge shapes result in significant differences in the shape and color of the external burrs. Inexperienced operators often misjudge the weld, leading to excessive welding power and the formation of burrs.

A. Misalignment: During high-frequency extrusion welding, the strip edges are misaligned.

B. Excessive curvature on one side of the edge: During roughing or finishing processes, factors such as deformation of the roll stand after prolonged use or malfunction of the roll bearings may cause one side of the edge to experience greater rolling force than the other, resulting in different curvatures on both sides.

C. Chamfer or rounded corner on one side of the edge: Before high-frequency extrusion welding, the strip edge undergoes processes such as overhead crane hoisting, uncoiling, shearing and welding, looping, and forming after longitudinal shearing. These processes may cause one side of the edge to develop a chamfer or rounded corner.

All three of these situations can cause uneven size of external burrs on both sides during welding, leading to operator misjudgment of the welding status and excessive welding power.

2. Unsatisfactory Butt Joint Form

There are three types of butt joint formations for plate edges during welding.

(1) The ideal butt joint is an I-shape between the two plate edges. This results in good weld quality.

(2) If the plate edges are V-shaped, the bottom V-shaped area will converge first during welding, causing a large amount of current to pass through the inner surface, resulting in excessive heating and melting of the inner weld area metal. Simultaneously, to ensure weld penetration, i.e., to ensure the outer weld area metal also reaches a molten state, the welding power must be increased, leading to excessively high welding temperatures, producing a large number of weld beads, and thus forming large burrs. Furthermore, in this welding state, the outer burrs are smaller, while the inner burrs are larger, requiring a large amount of scraping for the inner burrs.

(3) If the plate edges are inverted V-shaped, the outer burrs are generally larger, while the inner burrs are smaller. Therefore, an inverted V-shaped plate edge should be avoided as much as possible during welding.

3. Improper Cooling Air Adjustment: The cooling air on the internal deburring tool shank plays a crucial role in the deburring process. Besides removing slag to prevent it from accumulating on the tool shank or damaging the protective sleeve, it also cools the internal burrs to prevent them from sticking to the tool during deburring. However, in actual production, insufficient cooling air pressure and improper air outlet positioning often lead to slag buildup. If the cooling air pressure is too low, there won't be enough force to blow away the slag, causing small slag particles to accumulate on the tool shank or blade. If the air outlet is improperly positioned, it may prevent slag from being blown away from certain areas of the internal deburring tool or tool shank, resulting in slag buildup.

4. Improper Cooling Water Adjustment: The cooling water on the internal deburring tool shank, besides cooling the magnetic rod, also helps remove weld beads and slag that fall onto the front end of the protective sleeve during welding, preventing them from accumulating on the tool shank and forming hard lumps. In actual production, leaks or blockages in the water supply pipeline can cause excessively low water pressure at the outlet, failing to flush away the slag buildup formed at the front end of the impedance device.

5. Improper Adjustment of the Internal Burr Cutter: In actual production, improper selection of the internal burr cutter's cutting angle, improper adjustment of the burr cutter shank, or a notch in the internal burr cutter can lead to poor internal burr scraping, easily causing burr buildup.

6. Malfunction of the Upper and Lower Idler Rollers: In actual production, operators often overlook the operation of the upper and lower idler rollers on the internal burr cutter shank. Inadequate inspection or improper adjustment can cause the rollers to malfunction. If either the upper or lower idler rollers malfunction, a large amount of weld slag formed during welding will accumulate at the rollers and cannot be discharged, forming weld beads, affecting the normal raising and lowering of the cutter shank, or causing a change in the cutter shank's position.

7. Weld Torsion: For small-diameter steel pipes, weld torsion is prone to occur. Severe torsion can cause internal burr scraping and even weld beads.

Third, observation and judgment of weld beads in welded steel pipes.

During production line operation, it is impossible to inspect for weld beads directly at the burr cutter or cutter position using a "window" method. However, the scraping condition of the internal burr cutter can be judged by observing the internal burr pulled out after the flying saw, thus determining whether weld beads are present. If weld beads are frequently present on the internal burrs, it indicates improper high-frequency extrusion welding parameter settings. If the production line continues to operate under these conditions, blade sticking is highly likely to occur, resulting in large particle build-up. If the build-up is small and can pass smoothly through the circular hole of the internal deburr cutter, it will be clearly visible on the internal burrs produced after the flying saw cuts. If the build-up is large and cannot pass through the circular hole of the internal deburr cutter, it will accumulate on and around the internal deburr cutter, severely affecting the scraping of the internal burrs and even causing the production line to shut down involuntarily. Therefore, once a build-up appears on the internal burrs after flying saw cutting, attention should be paid to checking whether the high-frequency welding power and the cooling air pressure on the internal deburr cutter rod are abnormal.

Fourth, Methods to Prevent the Formation of Build-up in Welded Steel Pipes.

1. Methods to Prevent the Formation of Small Particle Build-up in Welded Steel Pipes: The formation of small particle build-up is mainly caused by the inability to properly remove particles after the sparks from the welding process have cooled. Therefore, preventing the formation of small particle lumps mainly involves checking the air and water channels of the internal burr cutter before starting the machine to ensure they are normal, verifying the air and water pressure gauges are functioning correctly, and inspecting the pipes and pipe ends for blockages. Simultaneously, check the upper and lower support rollers of the internal burr cutter for proper operation.

2. Methods to prevent the formation of large particle lumps in welded steel pipes: The direct cause of large particle lumps is excessively high welding temperature. Excessive welding temperature is caused by unreasonable process parameters, but in most cases, it is due to abnormal plate edge butt joints, resulting in increased welding power to reach the required welding temperature for the entire weld. Therefore, the forming should be adjusted to ensure an I-shape at the butt joint of the two plate edges to reduce the likelihood of large particle lumps.

Many factors contribute to the formation of lumps during welding, such as welding parameters, cooling air, cooling water, plate edge quality, upper and lower support rollers, the straightness of the cutter shank, and the quality of the cutter. Only by having a comprehensive understanding of all influencing factors and paying close attention to their external manifestations can we adjust equipment and process parameters in a timely manner, thereby reducing the generation of weld beads, ensuring the quality of internal burr removal, and ensuring the efficient operation of the production line.