A welding supervisor observes an orbital GTAW setup experiencing inconsistent root penetration and severe sugaring on the inside of a 50 mm diameter 316L stainless steel tube during production. The welder confirms the front shielding gas is pure argon at 10 L/min. What is the most probable cause of the sugaring?

Question: A welding supervisor observes an orbital GTAW setup experiencing inconsistent root penetration and severe sugaring on the inside of a 50 mm diameter 316L stainless steel tube during production. The welder confirms the front shielding gas is pure argon at 10 L/min. What is the most probable cause of the sugaring?

Answer options:

• The travel speed is too slow, causing excessive heat input. ✅ The backing gas flow rate is insufficient or absent.
• The tungsten electrode is contaminated.
• The AC balance control is set incorrectly.

Correct answer: The backing gas flow rate is insufficient or absent.

Explanation: Sugaring on the back side of stainless steel indicates severe oxidation due to exposure to atmospheric oxygen while the metal is hot. This is a direct consequence of insufficient or absent backing gas, which is critical for protecting the root of the weld from contamination.