A welding inspector is observing a Gas Tungsten Arc Welding (GTAW) operation on a thin-walled stainless steel tube. The welder is experiencing frequent 'arc wandering' or instability, making it difficult to control the weld bead. The WPS specifies pure tungsten electrodes. Which of the following is the MOST likely cause of this arc instability?
- AExcessive welding current for the electrode size
- BIncorrect tungsten electrode diameter
- CUse of pure tungsten with DCEN polarity
- DContaminated shielding gas
Show answer & explanationAnswer & explanation
Correct answer: C. Use of pure tungsten with DCEN polarity
Pure tungsten electrodes (EWP) are primarily designed for AC welding due to their ability to form a balled end, which provides a stable origin for the alternating arc. When used with DCEN (Direct Current Electrode Negative), pure tungsten tends to melt and contaminate the weld pool, or the arc becomes unstable and wanders due to poor emission characteristics from a pointed pure tungsten tip. Thoriated or lanthanated tungsten is preferred for DCEN.
Why the other options are wrong
- A. Excessive welding current for the electrode size would cause rapid melting and contamination, but the core issue is the unsuitable electrode type for the polarity.
- B. Incorrect tungsten electrode diameter can affect current density and overheating, but the fundamental issue with pure tungsten on DCEN is its emission characteristics.
- D. Contaminated shielding gas typically leads to porosity and oxidation, not primarily arc wandering with pure tungsten.
GTAW Pure Tungsten with DCEN
Using pure tungsten electrodes (EWP) with DCEN polarity in GTAW is generally not recommended, as it leads to arc instability, electrode melting, and potential weld contamination.
- Pure tungsten (EWP) is best for AC welding (aluminum, magnesium).
- Forms a stable balled end in AC.
- Poor electron emission characteristics on DCEN.
- Melts and contaminates weld pool easily on DCEN.
- Thoriated, lanthanated, or ceriated tungsten preferred for DCEN.
Memory trick: Pure tungsten on DCEN, an arc that won't stay keen.