Josehua Nitrogen as a backing and purging gas is used by some users with special precaution and control of moisture and also purging gas out let checks. For Aus S.S., if the root weld is not accessible and have defects and sharp notches, using nitrogen gas as purging and backing gas may have adverse effect in developing brittlenes in weld / HAZ. Recent years there have been quite success using nitrogen by special care to over come such problems Hope this helps C.V.Srinivasan Nishi Engineers Pvt Ltd India E-mail: nishi@vsnl.com Nov 17, 2008 >There have been some occasions when Argon cannot be >introduced as backing gas inside piping during welding >repair of austenitic stainless steel piping. > >This was done because the system was already filled with >nitrogen i.e. to protect catalyst filled equipment. > >Just want to know the effect of using N2 as backing gas to >the properties (i.e. mechanical or corrosion resistance) >especially the root passes. > >As a precaution during the welding activities, the N2 >pressure in the system is maintained as low as possible and >argon is applied locally at the back of the weldment (where >accessible) by an assistant.