I need to weld a stainless steel catalytic convertor back onto a cast
iron (forged steel maybe?) flange. The weld was very weak out of the
factory and its a known trouble spot on vw passats.
I have no problem with the welding itself but I am unsure of what filler
rod to use. My weak knowledge of metallurgy leads me to believ if I use
any old stainless rod on its own (the flange is much more massive than
the ss tube of the cat) I will not only end up with a weld that corrodes
quickly as I will lose chromium (not such a big deal really) but that
carbon from the flange will mix with the stainless and cause a brittle
weld that may well fail.
I understand it may be possible to put a layer of nickel down on the
flange first and then use the stainless rod to weld the tube to the
flange over the nickel layer. Is this as obvious as grinding some flange
away and welding a layer of nickel on so as to allow enough room for the
tueb to be inserted?
How thick should this layer be?

I know it might be the wrong group for this but I know theres some
pretty keen minds in here and I am doing it myself!

BTW probably will gas weld it as the preheating inherent should help, I
doubt there anything that can be damaged by the heat, I've had the turbo
almost incandescent in the past!
--
Mark Roberts