Nitrogen in the shielding gas is used to replace nitrogen that is lost in the arc process.
2% has been used. Anymore than that and you will notice tungsten deterioration, since Nitrogen is an active gas.
Nitrogen is added predominantly to acheive phase balance with rapid weld regime cooling. If your getting your balance with Ar there is little point,. Unless for some other specific purpose. Do a Search on the forum you will find voluminous discussion on these matters.

Duplex stainless steels (DSS) are finding increasing use in
the refining industry, primarily because they often offer an
economical combination of strength and corrosion resistance.
These stainless steels typically have an annealed structure
that is generally half austenite and half ferrite, although the
ratios can vary from approximately 35/65 to 55/45. The benefits
expected from the use of DSS are maintained even to a
ratio of 25:75 austenite/ferrite volume fraction. Most refinery
applications where DSS are used are corrosive, and DSS or
other higher alloys are required for adequate corrosion resistance.
However, some plants are also starting to consider DSS
as a "baseline" material.

They are using it in applications
where carbon steel may be acceptable, but DSS have been
shown to be more economical considering their higher
strength and better long-term reliability.
DSS are often used in lieu of austenitic stainless steels, in
services where the common austenitics would have problems
with chloride pitting or chloride stress corrosion cracking.

For more information see API 938C (use of Duplex stainless steels in the oil refining industry)