A shop foreman observes a blue discoloration (sugaring) on the root side of an ASTM A36 carbon steel butt joint welded with GTAW, despite a decent top bead appearance. The welder states they were using pure argon as backing gas. What is the most probable reason for the sugaring?

Question: A shop foreman observes a blue discoloration (sugaring) on the root side of an ASTM A36 carbon steel butt joint welded with GTAW, despite a decent top bead appearance. The welder states they were using pure argon as backing gas. What is the most probable reason for the sugaring?

Answer options:

• Too much heat input during welding. ✅ Insufficient purging gas flow or inadequate sealing of the purge chamber.
• Using the wrong filler metal for carbon steel.
• The use of argon as a backing gas is inappropriate for carbon steel.

Correct answer: Insufficient purging gas flow or inadequate sealing of the purge chamber.

Explanation: Blue discoloration or 'sugaring' on the root of a weld indicates oxidation of the molten metal due to insufficient protection from atmospheric oxygen. Even with argon backing, if the flow rate is too low or the purge chamber isn't adequately sealed, oxygen can ingress and cause this issue. While excessive heat can affect the HAZ, sugaring is primarily an oxygen issue. Argon is an appropriate inert backing gas for carbon steel, and wrong filler metal typically affects mechanical properties, not directly sugaring.