NITRONIC 50
Welding this grade can be performed with AWS A5.9 ER209. If it is a thick cross section, it is recommended to use AWS A5.9 ER209. ER209 has a small addition of ferrite to resist cracking in the weld for thick sections.
Filler metal ER309 provides good strength, but not similar corrosion resistance. Welding the low carbon austenitic grades, it would be recommended to look at 316L (no weld strength) vs ER209 (which provides strength and corrosion, but not fully austenitic in the weld).
NITRONIC 60
Welding this grade can be performed with AWS A5.9 ER218. It has similar composition to the base, which means similar corrosion resistance, strength and wear resistance. The weld would remain basically austenitic, with little change in the magnetic permeability in the region.
Filler metal ER308 provides good strength, but not similar corrosion resistance and no wear or galling resistance in the deposit.
Information from "Welding Nitrogen-Strengthened Stainless Steels" (R.H. ESPY, AWS Welding Research Supplement, May 1982) follows:
Welding with EB (Electron Beam) can be achieved using slower welding passes or utilizing a two pass technique.This helps to provide more time for outgassing of the Nitrogen, which if trapped causes porosity. The outgassing of Nitrogen is proportionate to the amount of Nitrogen contained.
Matching fillers should contain a lower amount of Nitrogen than the base. This helps to control weld porosity as well.
More on Welding Nitrogen Strengthened Stainless.