456A TIG (GTAW - Gas Tungsten Arc Welding) 2

Question 1

What is 'back purging' (backing gas) used for in GTAW root pass welding of stainless steel pipe?

Accepted Answer

Prevents oxidation (sugaring) on the inside of the pipe by flooding the back side with inert gas (argon)

Answer

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?

Accepted Answer

ER308L

Answer

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?

Accepted Answer

Carbide precipitation at grain boundaries in the heat-affected zone, depleting chromium and reducing corrosion resistance

Answer

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?

Accepted Answer

Start at full current to achieve fusion, then gradually reduce current near the end to prevent burn-through and cratering

Answer

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?

Accepted Answer

A porous mesh insert that produces laminar shielding gas flow, improving coverage and allowing longer tungsten extension

Answer

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.

Red Seal Welder 456A Practice Test

Red Seal Welder 456A Practice Test

456A TIG (GTAW - Gas Tungsten Arc Welding) 2