Usually, I run an 1/8" gap with 1/8" wire as the filler, but on stainless, you should only need 60-70 amps and ofcourse you will need a purge for the inside of the pipe you are welding. Your machine should be set for DCSP where the electrode should be negative.
It may be wise to work on carbon pipe to practice laying in a root as you won't need to purge the I.D. of carbon pipe. While there are differences as to how the dissimilar metals run......the principle to join the two are the same. If you were to weld carbon pipe, you could run 100-120 amps with the same set-up (DCRP) and same gap.
Refering back to stainless steel pipe......you need to set up an inner purge of the pipe by using masking tape and another hose (separate from your tig torch). Tape the hose in one end of the pipe and try to seal it up as tight as you can get it so your argon gas won't leak out where you tape in the hose. On the other end of the pipe (down stream from the actual joint, you can seal it up tight. Set the ID purge (of the pipes you are wanting to weld to 35-45 CFH of argon while you tack in 4 places. Once the pipe is tacked (and assuming the fit-up is acceptable, then you can turn the gas down to around 20 CFH and put a bleeder hole at the end of the pipe which does not have the hose in it.
I really do not know what your skill level is so forgive me if I am giving you too many details which you may already know. I leave my e-mail open so if you want to drop me a line about what you are doing, go ahead. Just put "Tig root of stainless pipe" or something like that so I know I am not getting spammed. Alright?
Be well.
Hawk
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