A fabrication shop is using FCAW-G to weld mild steel pressure vessels. The shop foreman notices that despite consistent welding parameters, some welds show intermittent worm tracking near the start and stop points. The welding current and voltage are within specification for the wire, and the shielding gas flow is checked regularly. What is the most probable cause of this defect?

Question: A fabrication shop is using FCAW-G to weld mild steel pressure vessels. The shop foreman notices that despite consistent welding parameters, some welds show intermittent worm tracking near the start and stop points. The welding current and voltage are within specification for the wire, and the shielding gas flow is checked regularly. What is the most probable cause of this defect?

Answer options:

• Excessive travel speed in the passes
• Inadequate cleaning of the joint edges prior to welding
• Incorrect torch angle leading to poor gas coverage ✅ A contaminated or incorrect shielding gas mixture

Correct answer: A contaminated or incorrect shielding gas mixture

Explanation: Worm tracking, characterized by internal elongated gas pockets, is often caused by gas-generating contaminants within the weld pool. A contaminated or incorrect shielding gas mixture would introduce such contaminants. Travel speed and torch angle primarily affect bead shape and surface defects respectively, and while joint cleanliness is important, intermittent worm tracking often points to gas-related issues.