Nitrogen Purge with Argon Shield?

Date 11-03-2005 00:38

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

By Shane Feder (****)

Date 11-03-2005 00:47

Deb,
Go into Search and enter purging, there is quite a lot of info in there,
Regards,
Shane

By jon20013 (*****)

Date 11-03-2005 10:52

Deb; Shane and your boss are correct, it is common to use nitrogen as a purge medium. Given the delays you had on your last set of PQR coupons, is your boss willing to stop welding until the results are in? How much cost difference is the boss expecting and how much would the mechanical tests cost? Depending on where you are on the project it may or may not make sense to requalify.

By welderette (**)

Date 11-03-2005 15:35

Gentlemen, Thanks for your input. I learn something new every day. I think we are on too tight of a schedule to regroup in this way. But now I am curious about this. Does the mix of the two gasses in the weld puddle affect the metalurgy of the product? This is deffinately a procedure that could be valuable in the future.

By chuck meadows (***)

Date 11-07-2005 16:42

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