In tube and pipe fabrication, it’s essential to inspect for defects in the skelp—the strip of raw material to be welded in the tube or pipe—before it is welded.
But it’s also necessary for adequate weld process control to be able to monitor the skelp in real time as it’s being welded.
Using laser-based and optical systems for this real-time monitoring, fabricators can better assure quality by detecting gradual changes in the skelp (or "slit mult,” as it’s also called).
Common Skelp Variances
The slitting process itself causes some variances in the skelp:
- Because the original coil is thicker in the middle than at the edges, two mults slit from the same coil can have varying thickness.
- Because the material is cold-worked during the slitting process, its hardness changes, which can cause variations in the material’s flatness.
- Because the slitter tooling dulls as it cuts the coil, the edge quality is better at one end of the skelp than at the other—the shear zone is smaller and the break zone is larger.
- Burrs are more likely to develop as the slitter tooling wears.
- Although the ideal shear angle is 90 degrees, in practice it varies.
In addition to these variances, which can be detected by pre-weld inspection, additional variances can occur during the welding process. That’s why real-time monitoring of the skelp is necessary to ensure proper welds.
Using a Laser-Based System to Monitor Skep
Change in the skelp’s shape (e.g., camber, edge wave) will affect the joint fit-up, also known as edge presentation. Varying edge presentation causes the squeeze-out to change accordingly—at times resulting in a significant increase or decrease of the bead on the ID or OD of the tube.
A laser-based monitoring system is ideal for detecting these changes because the user gets instantaneous feedback of the weld bead shape and strip presentation as it’s welded.
If there has not been enough squeeze-out on the bead, the operator will see the bead size reduce.
If the edge breaks were not working properly, or other alignment issues resulted in welding on a peak, the operator will see a deflection on the control screen.
An increase in the amount of edge wave—a defect in which the edges appear scalloped—also indicates that an adjustment is needed, such as increasing stretch.
More edge wave might also signify an upstream issue (e.g., dull slitter blades) that needs to be corrected.
Laser monitoring systems are also very effective in detecting mismatch, which may be caused by edge wave. A histogram of mismatch measurement is a good monitoring tool to look at during live production, as seen on the top histogram line in this figure.
Another useful aspect of skelp to monitor is the variance between the input and output speeds of the skelp. While not a “defect,” this measurement can help in determining the amount of stretch, which is a critical element to control to produce a good yield.
Conclusion
Ensuring proper welding of skelp in a tube or pipe requires careful inspection of the skelp before welding, but this should be supplemented by in-process monitoring of the weld to detect changes in the skelp that indicate problems with the weld. Laser-based monitoring systems are a highly effective way to achieve this real-time monitoring.
