AWS Certified Welding Inspector — FundamentalsWelding ProcessesEasy
A CWI is inspecting a critical pipe weld performed using Gas Tungsten Arc Welding (GTAW) on 316L stainless steel. Upon visual inspection, the CWI observes a distinct blue-to-purple discoloration (heat tint) on the weld bead and adjacent heat-affected zone. What does this discoloration primarily indicate?
- AThe presence of beneficial chromium carbides, enhancing the strength of the weld.
- BExcessive oxidation due to inadequate shielding or high heat input, potentially reducing corrosion resistance.
- COptimal shielding gas coverage and proper heat input.
- DInsufficient post-weld cleaning and surface preparation.
Show answer & explanationAnswer & explanation
Correct answer: B. Excessive oxidation due to inadequate shielding or high heat input, potentially reducing corrosion resistance.
Blue-to-purple discoloration (heat tint) on stainless steel after GTAW indicates excessive oxidation. This occurs when the hot metal is exposed to oxygen due to inadequate shielding or excessive heat, which can deplete chromium at the surface and reduce corrosion resistance.
Why the other options are wrong
- A. Chromium carbides are typically formed in the grain boundaries, not as a surface discoloration. Furthermore, their formation is generally detrimental to corrosion resistance in stainless steel, not beneficial.
- C. Optimal shielding and heat input on stainless steel should result in a bright, shiny, or straw-colored weld, not blue-to-purple.
- D. While proper cleaning is important, heat tint is a result of the welding process itself, specifically related to heat and shielding, not primarily post-weld cleaning.
GTAW Stainless Steel Heat Tint
Heat tint (blue-to-purple discoloration) on GTAW stainless steel indicates excessive oxidation due to inadequate shielding or high heat input, leading to reduced corrosion resistance.
- Caused by exposure of hot stainless steel to oxygen.
- Results in a surface oxide layer (chromium oxides).
- Depletes chromium from the underlying metal, reducing corrosion resistance.
- Color ranges from straw (minimal) to blue, purple, gray (severe).
Memory trick: Stainless's shine depends on shielding; blue means corrosion's beginning.