Hi asarkar,
Nitrogen is not normally a component of sheilding gases but it is added in amounts of around 3 % to gases used for the welding of duplex or other nitrogen containing stainless steels. Excessive amounts of nitrogen can form insoluble nitrides with titanium or niobium added for carbide stabilisation ( Quoted from ISSF- Belgium)

Nitrogen is common enough as a purge gas. Not a shield gas. And fundimentally purge gas requirements remain somewhat similar in GTAW and GMAW weldng of stainless pipe and tanks (especially 300 series).

Gapless joint fitup becomes more important when using nitrogen purge as nitrogen will break down when exposed to the welding arc as explained below by Ed Craig;

"Nitrogen typically a diatomic, "two atoms" per molecule. Nitrogen in the diatomic form is usually insoluble in molten stainless. However if the nitrogen gets into the weld arc, the plasma arc energy can seperate the diatomic molecules and create monatomic molecules.

"The monatomic molecules are soluble in the weld. The nitrogen, monatomic (seperated molecules) become an alloying element and can reduce the ferrite in a stainless weld. A reduction in ferrite in some alloys can cause the weld to be more austenitic and sensitive to hot cracking. If nitrogen enters a weld or the welding arc, it can have a negative and sometimes a positive influence".

"With closed root, austenitic stainless welds, as found in tanks, corner, edge welds, or thin gage, partial penetration tube welds, nitrogen is the logical, economical, purge gas choice for all austenitic, duplex, martensitic and precipitation hardening stainless steel applications. The only concern would be a few specific, ferritic alloys in which nitrogen could cause severe weld mechanical issues".

Here is the source link to the above data.

http://www.weldreality.com/stainlesswelddata.htm