I should say hi-nickle content stainless rod to be precise. Used for dissimilar materials and supposed to be more crack resistant as a consequence.
Relentless dont backpurge their thin wall material welds.. and the most common area of failure is around the collector, as it gets the hottest. (like yellow hot!)
The "hope" is the additional welding around their welds (I back purge when going over them also) is to get thicker weld on the joins and to try and get full penetration weld in that area.
I say "hope" as its welding on top of their welding.. and quite what *actual* material it is is unknown.. I know what they claim it to be but I am a sceptical ******, hence using the stainless/nickle rods for dissimilars on them. I use the same rods for welding stainless/ferrous to cast components also... 100% reliable so far.
The V3 relentless is supposed to be better, but I have'nt seen one, and heard they still have'nt backpurged their welds despite changing material to a titanium induced 321 stainless.. (which sounds great as a sales pitch, but full properties will never be achieved unless full solution annealed to normalise welds........ which they dont do, not likely ever to)