456A TIG (GTAW - Gas Tungsten Arc Welding) 2 — Questions and Answers
Question 1: What is 'back purging' (backing gas) used for in GTAW root pass welding of stainless steel pipe?
- Prevents oxidation (sugaring) on the inside of the pipe by flooding the back side with inert gas (argon) (Correct answer)
- Provides additional shielding gas from the front side of the weld
- Cools the weld faster to reduce distortion
- Prevents root concavity by pressurizing the pipe
Correct answer: Prevents oxidation (sugaring) on the inside of the pipe by flooding the back side with inert gas (argon)
When welding stainless steel pipe root passes, the back side (inside) of the pipe is exposed to air unless back purged. Without back purging, the back of the weld oxidizes severely ('sugaring'), which depletes chromium from the heat-affected zone, reduces corrosion resistance, and creates a rejectable rough surface. Argon at 2–5 L/min is typically used.
Question 2: What filler metal would be used for GTAW welding of 304 stainless steel?
- ER308L (Correct answer)
- ER70S-6
- ER4043
- ER309L
Correct answer: ER308L
ER308L is the standard filler for GTAW of 304 and 304L stainless steel. The '308' composition matches 304 stainless (18% Cr, 8% Ni), and the 'L' designation indicates extra-low carbon content, which reduces sensitization (carbide precipitation) during welding. ER309L is used for dissimilar metal or high-temperature applications.
Question 3: What is 'sensitization' in austenitic stainless steel and how does it affect weld quality?
- Carbide precipitation at grain boundaries in the heat-affected zone, depleting chromium and reducing corrosion resistance (Correct answer)
- Hardening of the HAZ due to martensite formation during rapid cooling
- Distortion of the base metal from thermal gradients during welding
- Porosity from carbon dioxide released during welding
Correct answer: Carbide precipitation at grain boundaries in the heat-affected zone, depleting chromium and reducing corrosion resistance
Sensitization occurs in austenitic stainless steels when the material is heated to 425–870°C and chromium combines with carbon to form chromium carbides at grain boundaries. This locally depletes the chromium below 12%, destroying the passive oxide layer and making those zones susceptible to intergranular corrosion. Using L-grade (low-carbon) fillers and base metals minimizes this risk.
Question 4: What is the correct foot pedal technique for controlling amperage during GTAW root pass welding on thin stainless?
- Start at full current to achieve fusion, then gradually reduce current near the end to prevent burn-through and cratering (Correct answer)
- Maintain constant full current throughout the weld
- Use the foot pedal to increase current whenever adding filler metal
- Keep the foot pedal at approximately 50% throughout for consistent results
Correct answer: Start at full current to achieve fusion, then gradually reduce current near the end to prevent burn-through and cratering
Thin stainless steel heats up rapidly during GTAW. The correct technique is to start with higher current to establish the arc and pool, then progressively reduce current using the foot pedal as the workpiece heats up, and further reduce it at the end to fill the crater gradually without burn-through. This provides consistent fusion throughout.
Question 5: What is 'gas lens' and why is it used in GTAW torches?
- A porous mesh insert that produces laminar shielding gas flow, improving coverage and allowing longer tungsten extension (Correct answer)
- A glass lens in the torch that focuses the arc on the workpiece
- A gas regulator fitting that maintains constant flow pressure
- A nozzle attachment that reduces gas consumption
Correct answer: A porous mesh insert that produces laminar shielding gas flow, improving coverage and allowing longer tungsten extension
A gas lens is a porous mesh screen assembly that replaces the standard collet body in a GTAW torch. It produces laminar (smooth, non-turbulent) shielding gas flow, which provides better coverage at the same flow rate. It also allows the tungsten to be extended further from the nozzle without compromising shielding, improving visibility and access in tight joints.
Question 6: When GTAW welding titanium, what additional shielding is required beyond the standard torch shielding?
- Trailing shielding and back purging are required to protect the weld and HAZ until temperature drops below 315°C (Correct answer)
- No additional shielding is needed if standard argon flow rates are used
- A welding curtain to prevent ambient air circulation is sufficient
- Only back purging is required; trailing shielding is optional
Correct answer: Trailing shielding and back purging are required to protect the weld and HAZ until temperature drops below 315°C
Titanium is extremely reactive with oxygen and nitrogen above 315°C (600°F). In addition to torch shielding, a trailing shield (a separate gas cup that follows the torch) protects the hot weld and HAZ as it cools below the contamination threshold. For pipe work, back purging is also mandatory. Contaminated titanium welds turn from silver/light gold to blue, purple, or grey, indicating oxygen/nitrogen pickup.
456A TIG (GTAW - Gas Tungsten Arc Welding) 2
What is 'back purging' (backing gas) used for in GTAW root pass welding of stainless steel pipe?