A welder is preparing to weld a full penetration butt joint on 3 mm thick stainless steel pipe using GTAW. To prevent oxidation and sugaring on the root side of the weld, what technique should be employed?

Question: A welder is preparing to weld a full penetration butt joint on 3 mm thick stainless steel pipe using GTAW. To prevent oxidation and sugaring on the root side of the weld, what technique should be employed?

Answer options:

• Increasing the shielding gas flow rate on the torch.
• Using a backing plate made of copper. ✅ Purging the inside of the pipe with an inert gas.
• Preheating the pipe joint to 150°C.

Correct answer: Purging the inside of the pipe with an inert gas.

Explanation: To prevent oxidation and 'sugaring' on the back side (root) of a stainless steel weld, it is crucial to protect it from atmospheric contamination. This is best achieved by purging the inside of the pipe with an inert gas like argon to displace oxygen. Increasing torch gas flow only protects the face, and preheating can exacerbate oxidation.