A welder is performing a full penetration weld on a 50 mm diameter 316L stainless steel pipe. Due to site constraints, the only available purging gas is industrial-grade nitrogen. What is the primary concern when considering the use of nitrogen as a back purge gas for stainless steel, especially for full penetration welds?

Question: A welder is performing a full penetration weld on a 50 mm diameter 316L stainless steel pipe. Due to site constraints, the only available purging gas is industrial-grade nitrogen. What is the primary concern when considering the use of nitrogen as a back purge gas for stainless steel, especially for full penetration welds?

Answer options:

• Nitrogen will reduce the melting point of the stainless steel.
• Nitrogen is more expensive than argon, increasing project costs. ✅ Nitrogen can cause porosity and nitriding, embrittling the weld, especially in the root pass.
• Nitrogen will alter the colour of the weld bead, making inspection difficult.

Correct answer: Nitrogen can cause porosity and nitriding, embrittling the weld, especially in the root pass.

Explanation: While nitrogen is an inert gas, it can react with certain alloying elements in stainless steel (particularly the chromium in 316L), leading to the formation of nitrides. This results in significant porosity and embrittlement, especially in the root pass where the molten puddle is most exposed to the purge gas. Therefore, argon is generally preferred for purging stainless steel.