AWS Certified Welding Inspector — FundamentalsWelding ProcessesEasy
A CWI is inspecting a critical pipe weld performed using Gas Tungsten Arc Welding (GTAW) on a stainless steel alloy. The inspector observes a bluish-purple discoloration on the weld bead and adjacent heat-affected zone (HAZ). This discoloration is MOST likely an indication of:
- AInadequate preheat temperature
- BExcessive interpass temperature
- CContaminated tungsten electrode
- DInsufficient shielding gas coverage
Show answer & explanationAnswer & explanation
Correct answer: D. Insufficient shielding gas coverage
Bluish-purple discoloration, often referred to as 'sugaring' or 'heat tint,' on stainless steel GTAW welds is a classic sign of insufficient shielding gas coverage, leading to oxidation of the chromium in the steel at elevated temperatures. Proper shielding, especially with a trailing shield or back purge, is crucial for stainless steel.
Why the other options are wrong
- A. Inadequate preheat temperature is less critical for stainless steel and would more likely lead to cracking in high-restraint joints, not discoloration.
- B. Excessive interpass temperature can lead to sensitization in stainless steel but typically doesn't directly cause bluish-purple discoloration as the primary visual indicator.
- C. Contaminated tungsten primarily causes arc instability and inclusions, not typically widespread bluish-purple discoloration of the weld and HAZ.
GTAW Stainless Steel Discoloration
Discoloration (heat tint) on stainless steel welds, particularly bluish-purple, indicates oxidation due to inadequate shielding gas protection during welding or cooling.
- Caused by atmospheric oxygen reacting with hot stainless steel.
- Indicates chromium oxidation, reducing corrosion resistance.
- Ranges from straw to blue to black depending on severity.
- Prevented by proper shielding gas coverage (front, back, trailing).
Memory trick: Colors mean oxygen got in, your shield was thin.