AWS Certified Welding Inspector — FundamentalsWelding ProcessesHard
A CWI is overseeing the fabrication of a critical pressure vessel using Gas Tungsten Arc Welding (GTAW) for the root passes of stainless steel pipe. The WPS specifies a pure argon shielding gas at 20 cubic feet per hour (CFH) for the torch and a pure argon backing gas at 10 CFH. During inspection, the CWI observes a dark, sugared appearance on the root pass interior, indicating severe oxidation. What is the most probable cause for this condition?
- AInsufficient torch shielding gas flow.
- BImproper tungsten electrode type.
- CExcessive backing gas flow rate.
- DInadequate backing gas coverage.
Show answer & explanationAnswer & explanation
Correct answer: D. Inadequate backing gas coverage.
Sugaring or severe oxidation on the root pass interior of stainless steel welded with GTAW is a direct indication that the molten weld pool on the backside was exposed to oxygen from the atmosphere. This is almost always due to inadequate or lost backing gas coverage, allowing air to contaminate the hot metal.
Why the other options are wrong
- A. Insufficient torch shielding gas would cause oxidation on the *face* of the weld, but the question specifies the *interior* (root pass).
- B. Improper tungsten electrode type (e.g., pure tungsten with DCEN) would affect arc stability, penetration, or contamination from the tungsten itself, but not cause widespread oxidation (sugaring) on the root interior.
- C. Excessive backing gas flow could cause turbulence and potentially draw air in, but more commonly leads to a concave root or issues with maintaining a stable gas column, not direct severe oxidation unless it somehow creates a venturi effect to pull in air.
GTAW Stainless Steel Backing Gas
Backing gas, typically argon, is essential in GTAW of stainless steel to protect the root side of the weld from atmospheric contamination and oxidation.
- Prevents 'sugaring' or chromium oxidation.
- Maintains corrosion resistance.
- Requires proper sealing and flow rates.
Memory trick: No back gas, sweet root, sour weld.