The uneven wall thickness in steel tubes is a common issue that manifests as irregularities along the length of the pipe, with variations in thickness at both the head and tail sections. This problem is often influenced by the continuous rolling process, which plays a key role in determining the final quality of the steel tube. Below are the main causes and corresponding solutions for different types of wall thickness inconsistencies:
1. **Causes of Spiral Wall Thickness Variation**
- One primary cause is the non-uniform wall thickness from the piercing mill, where the angle of the two rolls may not be equal or the initial rolling amount before the head is too small. These adjustments can lead to a spiral distribution of thickness along the entire length of the pipe.
- Another reason is the premature opening of the centering roll, misalignment of the centering roll, or jerking of the ejector pin during the rolling process, which also results in a helical pattern of thickness variation.
**Solutions:**
- Adjust the alignment of the piercing mill so that both rolls have equal angles, and ensure the rolling mill settings match the given parameters on the rolling table.
- For the second case, adjust the timing of the centering roll opening based on the capillary exit speed. Avoid opening it too early to prevent instability caused by the top rod shaking. The opening degree should be adjusted according to changes in the capillary tube diameter and the size of the capillary bounce.
2. **Causes of Linear Wall Thickness Variation**
- Improper adjustment of the mandrel pre-worn saddle height can lead to uneven contact between the mantle and the capillary tube, causing rapid temperature drops and resulting in uneven wall thickness or even pulling and concave defects.
- An incorrect gap between tandem rolls, deviation in the roller centerline, or unbalanced rolling in single-pipe direction can create linear symmetric deviations.
- A broken safety cup or differences in inner and outer roll gaps can result in asymmetric wall thickness.
- Poor adjustment during continuous rolling, steel rolling, or pull steel rolling can also cause linear thickness inconsistencies.
**Solutions:**
- Ensure the mandrel pre-worn saddle is properly adjusted and aligned with the capillary tube.
- When changing the hole type or rolling specifications, measure the roll gap to ensure consistency with the rolling table.
- Use an optical centering device to correct the rolling centerline, especially during annual overhauls.
- Replace damaged or broken rolls promptly and regularly measure the internal and external roll gaps.
- Avoid steel stacking during continuous rolling to maintain uniform wall thickness.
3. **Causes of Uneven Wall Thickness at the Head and Tail**
- A large slope at the front end of the tube blank or an improperly sized centering hole can lead to uneven thickness at the head.
- Excessive extension coefficient during piercing, high roller rotational speed, or unstable rolling can result in inconsistent wall thickness at the tail.
- Unstable steel throwing by the thrower can also cause uneven thickness at the end of the capillary tube.
**Solutions:**
- Check the quality of the tube blank to avoid excessive tapering and ensure proper centering hole adjustments when changing hole shapes or during overhauls.
- Reduce the perforation speed to stabilize the rolling process and maintain consistent wall thickness. Adjust the guide plate accordingly when changing roller speeds.
- Monitor the condition of the guide plates and inspect their bolts regularly to reduce turbulence and ensure stable operation of the thrower.
By addressing these issues through precise adjustments and regular maintenance, manufacturers can significantly improve the uniformity of wall thickness in steel tubes, ensuring higher product quality and performance.
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