piperdolan,
Nirosta is a German - ThyssenKruppSteel - brand for high alloyed base materials.
Please find attached the ThyssenKrupp Nirosta + (heat resistant) Thermax material overview and comparison to ASTM and others.
Following from that you can easy conclude what kind of filler can be used in comparison to the US american Stainless Steel Standards.
Regards from Germany,
Stephan