A CWB certified welder is performing root pass welding on 304L stainless steel pipe, 100 mm diameter and 3 mm wall thickness, using the GTAW process. The specifications require a contamination-free root. Which purging gas and flow rate (for an open-end purge) would be most suitable to ensure proper root protection?

Question: A CWB certified welder is performing root pass welding on 304L stainless steel pipe, 100 mm diameter and 3 mm wall thickness, using the GTAW process. The specifications require a contamination-free root. Which purging gas and flow rate (for an open-end purge) would be most suitable to ensure proper root protection?

Answer options:

• Nitrogen at 5 L/min ✅ Argon at 15 L/min, with an exit hole
• Carbon dioxide at 10 L/min
• A 75% Argon / 25% Helium mix at 8 L/min

Correct answer: Argon at 15 L/min, with an exit hole

Explanation: Argon is the preferred purging gas for stainless steel due to its inert nature and density, which helps displace oxygen effectively. A flow rate of 15 L/min for an open-end purge ensures adequate oxygen displacement for the given pipe size, and an exit hole permits expelling of oxygen and spent purge gas, preventing pressure build-up. Nitrogen can nitride stainless steel, CO2 is not an inert gas, and helium mixes are typically not necessary for basic purging.