Lets look at it from an actual welding aspect. We know that a purge gas normally affects the root pass to the greatest extent. Duplex, already considered a high nitrogen alloyed steel, definitely is altered by a high percentage of nitrogen in the purge gas. High percentages of nitrogen in the purge gas can certainly render the Duplex out of specification requirements. Being a strong austenite former, it can lower the ferrite to levels not acceptable to the applicable specification. Below approximately 25% ferrite, the duplex is at a loss of strength and corrosion resistance. This is a definite mechanical and corrosion detriment. Up to approximately 2-3% nitrogen in the purge gas is sometime recommended to compensate for the loss of nitrogen in the weld arc. On the same hand, that is why filler metals are over alloyed to the base metal...to compensate for the loss of chemistry in the weld arc. Duplex is noted for it's chloride stress corrosion resistance. That is directly tied to it's austenite/ferrite balance. If the balance is out of kilter, so is it's mechanical and corrosion resistance altered. We know that in Duplex specifications, or all that I've been associated with over the last 15 years, ferrite and hardness values have been taken at the root, mid-weld, and cap of the weld, and most of the time in the HAZ. Excessive nitrogen can alter these values to unacceptable ranges. I guess you could say that it could be rendered unsuitable to service, depending what that service is. Sec. IX, as we all know, does not set the requirements for acceptability of Duplex steel for any application. That logic is decided by the metallurgical values that are achieved by proper welding techniques (heat input, interpass temperatures, gas mixture, cooling rate, etc). Those are all things that needed to be recognized or the Duplex can certainly be rendered as unsuitable for service. But, speaking strictly of nitrogen, I think it needs to be closely monitored, even if it is a purge gas, or a percentage in the purge gas.