456A TIG (GTAW - Gas Tungsten Arc Welding) 4 — Questions and Answers
Question 1: What is the minimum purity level of argon shielding gas required for GTAW of stainless steel and titanium?
- 99.995% (Grade 5.0 or higher) (Correct answer)
- 99.0% (commercial grade)
- 99.5% is sufficient for most applications
- Purity is only critical for titanium, not stainless steel
Correct answer: 99.995% (Grade 5.0 or higher)
GTAW of reactive and sensitive metals requires high-purity argon (Grade 5.0, 99.995% pure) to prevent contamination. Even small amounts of oxygen, moisture, or nitrogen in the shielding gas can cause porosity, embrittlement, or loss of corrosion resistance. For titanium, Grade 5.0 or higher is mandatory.
Question 2: What is the amperage range typically used for GTAW of 3 mm (1/8-inch) stainless steel with a 2.4 mm tungsten electrode?
- 70–110 A DCEN (Correct answer)
- 150–200 A DCEN
- 30–50 A AC
- 200–250 A DCEP
Correct answer: 70–110 A DCEN
For 3 mm stainless steel with a 2.4 mm tungsten on DCEN, a typical range of 70–110 A provides adequate fusion without burn-through. A common rule of thumb is approximately 1 A per 0.025 mm (0.001 inch) of base metal thickness, adjusted for material type, joint configuration, and position.
Question 3: What is 'arc wander' in GTAW and what causes it?
- Erratic movement of the arc, often caused by a contaminated tungsten tip, improper grinding, or magnetic fields (Correct answer)
- The natural movement of the arc along the joint during travel
- Arc instability caused by insufficient shielding gas flow
- Variation in arc length due to workpiece surface irregularities
Correct answer: Erratic movement of the arc, often caused by a contaminated tungsten tip, improper grinding, or magnetic fields
Arc wander is uncontrolled deflection or movement of the GTAW arc away from the intended location. Primary causes include a contaminated or improperly shaped tungsten (especially spiral grinding marks), tungsten contaminated by base metal contact, or arc blow from magnetic fields in ferromagnetic workpieces. Replacing or re-grinding the tungsten usually resolves the problem.
Question 4: What is the purpose of pre-flow and post-flow shielding gas in GTAW?
- Pre-flow purges air from the torch before arc start; post-flow shields the weld and tungsten as they cool after arc stop (Correct answer)
- Pre-flow pre-heats the shielding gas for better coverage; post-flow cools the torch body
- Both are only required for titanium and aluminum
- Post-flow prevents the regulator from freezing in cold conditions
Correct answer: Pre-flow purges air from the torch before arc start; post-flow shields the weld and tungsten as they cool after arc stop
Pre-flow establishes the shielding gas blanket before the arc is struck, preventing air from being present at arc initiation. Post-flow continues shielding gas after the arc is extinguished, protecting the hot tungsten and weld metal from oxidation as they cool. Post-flow time should be sufficient to cool the tungsten to below its oxidation temperature (approximately 5–15 seconds depending on current and electrode size).
Question 5: What type of joint preparation is typically used for GTAW butt welding of 6 mm stainless steel pipe (schedule 40)?
- 37.5° bevel with a 1.5 mm root face and 2.5 mm root opening (60° included angle V-groove with open root) (Correct answer)
- Square butt joint with no root opening
- Double V-groove (X-groove) from both sides
- J-groove with backing ring
Correct answer: 37.5° bevel with a 1.5 mm root face and 2.5 mm root opening (60° included angle V-groove with open root)
For GTAW root pass welding of stainless steel pipe, a 37.5° bevel per side (75° included angle) with a 1.5–2.0 mm root face and approximately 2.5 mm root opening is standard. This allows the root pass to be made with the keyhole technique for complete penetration. The exact dimensions are specified in the WPS.
Question 6: What is the 'walking the cup' technique in GTAW pipe welding?
- Resting the ceramic cup on the pipe and rocking it forward in a consistent oscillating pattern for precise arc control without a free hand (Correct answer)
- Moving the entire welding machine slowly along the pipe during welding
- A technique for positioning the torch at the correct travel angle automatically
- Using the cup to guide the filler rod into the weld pool
Correct answer: Resting the ceramic cup on the pipe and rocking it forward in a consistent oscillating pattern for precise arc control without a free hand
Walking the cup involves resting the gas cup (nozzle) on the pipe surface and pivoting/rocking it forward in a smooth oscillating motion. This stabilizes the torch, maintains consistent arc length, and allows the welder to focus on pool control and filler addition. It is widely used for GTAW pipe root and hot pass welding, especially in tight spaces.
456A TIG (GTAW - Gas Tungsten Arc Welding) 4
What is the minimum purity level of argon shielding gas required for GTAW of stainless steel and titanium?