Hi Deb,
We used to do a huge amount of welding stainless steels (tubing) in Dairy Factories ( I think you call them milk processing plants) in New Zealand. We used nitrogen purge/ argon shielding all the time and it was a huge cost saving. Chuck will probably be able to help you with the metallurgical side and Jon can probably give you advice on the requalification (if required) of your WPS if you are changing from argon to nitrogen. I know there is a post regarding this because I put a query to the forum about purging chromolly.
The main difference is argon is heavier than air so you have the purge inlet lower than the outlet so it pushes the oxygen out. With nitrogen it somehow mixes with the oxygen to create an oxygen free environment. (Someone can probably explain that a little bit better.)
Hope that helps,
Regards,
Shane
Shane and Jon are absolutely correct. Studies show that argon/argon purge/shield is the most common, but nitrogen/argon is right in there, too. For the common grades of SS, Nitrogen has no adverse affects. Pure nitrogen is not always recommended for welding Duplex SS because the nitrogen, being a strong austenite former, can lower ferrites to an unacceptable range.
Chuck