The quality of welded pipes (SSAW/LSAW/ERW) is significantly influenced by the welding seam, which is considered the lifeline of these pipes. As such, it is imperative to conduct a comprehensive analysis and address quality defects related to the welded section.
1) Weld Cracking:
a. Natural Cracking of the Weld:
Features: This type of cracking occurs after the welded pipe leaves the squeeze roller, without any additional external force in subsequent processes. The cracks resemble canine teeth and appear gray with a distinctive luster, akin to non-ferrous metals.
Causes: Causes include insufficient extrusion force, inappropriate welding temperature (either too low or too high), narrow and thin tube blanks, excessive burr removal, and specific shapes of butt joints, resulting in excessive forming transverse residual tensile stress.
Preventive Measures:
- Thoroughly inspect raw materials and eliminate hard tube blanks.
- Employ a stable welding process tailored to different tube blanks to avoid issues like cold welding and over-burning.
- Strengthen forming adjustments to reduce residual stress and achieve parallel butt joints.
b. Weld Cracking Under Force:
Features: Flattening weld cracking includes positive and side flattening cracking, which can be further categorized into D/3 flattening and full flattening. Full flattening involves 100% pressing to the inner wall of the pipe with no gap.
Causes: High-pressure flattening may lead to cracks, but full flattening without cracking is often required for practical use. This involves checking welding quality and the plastic state of the tube blank.
2) Weld Cracks:
Definition: Refers to the presence of small hair-like cracks in the weld, mostly in the superficial part. Causes include high welding temperature, thinner and narrower tube blanks, low extrusion force, reflow, slag inclusion, and other factors.
Preventive Measures: Address issues such as increasing the opening angle, extrusion force, welding temperature, and improving the edge butt state to prevent crack formation.
3) Dislocation of Welds:
Definition: Weld misalignment occurs when two pairs of welding surfaces are not on the same plane. This can be inclined, occasional, or periodic misalignment, with a common feature being the persistence of external burrs on one side of the weld after normal removal.
Causes: Various reasons, including asymmetry in flat rolls, squeeze rollers, or guide rollers, damage to roller bearings, tube blank irregularities, unstable tube billet movement, and deviations in rolling center lines.
Complex Causes: Dislocation may result from single or multiple causes, requiring a systematic approach to identification and elimination, starting with visible and tangible factors.
Addressing weld defects involves a meticulous investigation, considering visible and tangible factors first and proceeding systematically to eliminate or mitigate each issue.
