Effect of milling edge defects on the quality of spiral seam submerged arc welded pipes and countermeasures

Causes of milling defects of spiral seam submerged arc welded steel pipes and their impact on steel pipe quality

1. Milling slip or working width change

During the milling process, the main reasons for the milling machine to mill or the change of the working width of the steel plate are as follows.

(1) The steel plate itself has defects. For example, when the "crescent bend" of the steel plate is large, the plate edge will deviate from the delivery line, resulting in single-sided milling, causing the working width to decrease; when the original width of the steel plate is smaller than the processing width of the milling machine, milling slip will also occur.

(2) There is a problem with the milling machine. The headstock of the milling machine is not fixed firmly on one or both sides, and it deviates in the direction of the steel plate processing width, which will cause the processing width to change. When the headstock moves outward from the edge of the steel plate, it will cause a milling slip.

(3) The position of the steel plate is not well controlled. The position of the steel plate is generally controlled by the vertical roller. If the control is not good, a "crescent bend" will be artificially created, causing the edge of the steel plate to deviate from the delivery line, resulting in milling slip or defects such as "crescent bend" being too large. Milling slip. The change in the working width of the steel plate will lead to unstable forming, poor meshing of the forming seam, and even misalignment, which will cause unstable welding during welding and even defects such as burn-through. In addition, the rest of the plate edge that was not milled during milling will also affect the welding quality. The misalignment will cause the welding molten pool metal to flow to one side, deviating from the center position of the forming seam, resulting in welding deviation.

2. Poor processing accuracy, burrs, and other defects

When the blade of the milling machine is worn out, the quality of the processed plate edge is significantly reduced, and defects such as poor processing accuracy and burrs will occur. When the blade is not worn too seriously or partially worn, because there is a small notch in the blade, the notch blade leaves a "line" on the steel plate when cutting. If the blade behind is intact, it will "scrape" this line out to form a metal line. One end of the thin metal wire is adhered to the edge of the steel plate, and the other end is attached to the upper and lower surface edges of the steel plate within 10mm. Its width is 0.5-1.0mm and its length ranges from 1mm to 30mm. If the blade at the back is severely worn, several "lines" will be left unprocessed on the edge of the steel plate, and the roughness of the steel plate cut is poor, that is, the processing accuracy is poor. The thin metal wires adhering to the surface of the steel plate will be "rolled" into the steel plate when passing through the delivery machine. Shorter metal wires will be melted by the high-temperature molten pool during welding and will not affect the quality of the steel pipe, but longer metal wires will cause indentations on the edge of the weld. When the blade is severely worn, the roughness of the plate edge is poor, mainly manifested as "broom pattern" scratches on the plate edge; in more serious cases, thick and short "fish scale pattern" plate edges appear, indicating that the blade has no cutting edge and has changed from milling to "grinding". The plate edge with poor processing accuracy entering the former will inevitably lead to instability in the forming seam and instability in the welding process. Even the phenomenon of pulling the welding head occurs, that is, the unmelted burrs stick to the welding wire and pull it away.

3. The milling groove does not meet the requirements

I-type groove has no shape requirements and will not have such defects, but Y-type groove and X-type groove often have such defects. The fundamental reason for such defects is that the relative position between the fine milling cutter and the steel plate has changed.

When the cutter head and the steel plate change in the vertical direction, the depth of the upper and lower grooves of the steel plate will change and are related to each other. If the cutter head moves up relative to the steel plate, the depth of the upper groove will decrease, and the depth of the lower groove will increase accordingly.

After excluding the reason for poor adjustment of the cutter head, the main cause of this problem is the poor flatness of the steel plate. Although the milling machine has a follow-up device that allows the cutter head to float with the "wave bend" of the steel plate, when the "wave bend" of the steel plate is too large, the flatness of the local plate edge deteriorates, the follow-up device fails, and the steel plate is partially offset relative to the cutter head, causing milling defects.

The single milling process is not affected by the change in the relative position of the cutter head and the steel plate in the horizontal direction. It will only cause changes in the milling width or de-milling. However, in the double milling process, the changes are more complicated:

① Rough milling moves inward, making the working width of the steel plate smaller, and the fine milling blunt edge knife is equivalent to de-milling, and the depth of the upper and lower grooves becomes smaller;

② Rough milling moves outward, increasing the amount of fine milling processing, and does not affect the groove

③ Fine milling moves inward, making the working width of the steel plate smaller, but the groove does not change

④ Fine milling moves outward, making the upper and lower groove depths of the steel plate smaller, but the working width becomes larger.

Grooves that do not meet the requirements will lead to the following steel pipe quality defects:

(1) Internal weld deviation. During the welding process of spiral welded pipes, fine-tuning is generally based on looking at the "red line". If the "red line" deviates from the forming seam, it is generally considered that the weld point is deviated, and fine-tuning is required to bring the weld point closer to the forming seam. However, in the X-shaped groove or Y-shaped internal welding groove, due to the inconsistency of the grooves on both sides of the steel plate, the wall thickness on the left side is thinner and the wall thickness on the right side is thicker, which makes the left side easier to be heated by the molten pool, making it easier to show the "red line", causing the operator to misunderstand that the "red line is offset to the left", so he adjusts to the right side based on experience until the "red line" coincides with the forming seam, resulting in welding deviation. In addition, the asymmetric groove causes the molten iron in the welding pool to flow to one side under the action of gravity, deviating from the center position of the forming seam, resulting in internal welding deviation.

(2) Incomplete penetration and burn-through. The purpose of milling the groove is to "reduce" the thickness of the steel plate and increase the penetration of the welding wire. When the shape of the milling groove is unstable, it is equivalent to the thickness of the steel plate constantly changing. During the welding process, the welding parameters cannot be adjusted in time and accurately, which will lead to defects such as burn-through and incomplete penetration.

(3) Irregular welds and excessive weld height. Similar to the principle of incomplete penetration and burn-through, if the welding groove is too large or too small, it will cause excess or lack of welding wire filler metal, resulting in irregular welds and too high or too low height.

Measures to prevent milling defects of spiral seam submerged arc welded steel pipes

1. Reasonable selection of milling process and parameters:

Reasonable selection of milling process according to actual production conditions. Under the condition of ensuring production, try to use a single milling process and select an I-type groove, to reduce milling defects and make it easy to process. Selecting a suitable working width can not only reduce the probability of de-milling but also maximize the life of the blade, thereby improving the processing accuracy. Theoretically, the milling amount of 8~10mm is generally selected, that is, the milling working width = the narrowest actual original plate width of 8mm. However, one thing needs to be pointed out if the narrowest width deviation of the steel plate is large during production, it is not appropriate to adopt a fixed milling width. For example, it is often encountered that a batch of steel pipes is produced using steel coils from two steel mills. Generally speaking, the width deviation of steel coils from different steel mills is large. At this time, to improve the yield rate and reduce the milling amount of the blade, a flexible adjustment method should be adopted. A reasonable selection of groove depth according to the thickness of the steel plate gives it a large room for maneuvering.

2. Strengthen the control of steel plates to ensure the quality of steel plates:

Strengthen the leveling control of the leveling machine to ensure that the flatness of the steel plates is consistent and prevent the steel plates from entering the milling machine with "wave bends"; strengthen the control of material discharge and vertical rollers to try to eliminate the influence of the "crescent bend" of the original plate and prevent the generation of artificial "crescent bends".

3. Standardize the use of the milling machine and the replacement of milling blades:

The milling machine must be lubricated and maintained regularly to ensure the integrity and stability of the equipment. The milling blade is the only consumable in the milling machine. Almost every work shift needs to be replaced. The quality of the blade is crucial to the milling quality. Choosing better blades can reduce the number of stops, not only reduce arc breaking, and improve the one-time pass rate, but also improve the yield rate. Therefore, it is necessary to select coated blades with good wear resistance and heat dissipation performance and replace them in time and regularly. It should be noted that the milling cutter disc pursues an overall effect. If only some blades are replaced, it will cause "metal wire" defects and increase the working intensity of other blades, thereby reducing the service life of the overall blade. Therefore, all blades must be replaced when changing the blade.

4. Flexible adjustment of the milling machine:

Under certain circumstances, the milling machine can be flexibly adjusted to suit different situations. For example, when the working width deviation of the steel plate is 1~3mm, it generally does not affect the forming quality; when the edge of the steel plate deviates from the delivery line by 1~3mm, it can also be controlled. Therefore, when the steel plate cannot be controlled and is about to be de-milled, it can be controlled by fine-tuning the headstock. Another situation is that if the width of some steel coils does not exceed the standard, the working width of the milling machine can be fine-tuned to reduce the amount of blade milling and extend its service life.

5. Other methods:

Some relatively simple equipment or methods can be used to eliminate some milling defects. For example, for the slender "metal wire" defect of the steel plate, a simple wire brush can be used to "sweep it away".