Dark grey carburized joints TIG welding stainless, help please?

I am attempting welding stainless steel of about 35 though wall thickness. The job is welding a flanged disc into a 6 inch diameter tube, to form a sealed end. My problem, apart from the fact I am totally new to TIG welding, is the bead is dark grey and carburized looking, with little rough nodules forming. My instinct is too much heat, but I am down to 70 amps on a Migatronic 220 TIG welder. Any less current and i don't seem to get enough heat to fuse the materials. Gas is pure argon, tungsten is 3/32 ceriated, with a number 7 gas lens on the torch. Welding DC, with a pre and post gas flow. I can post pics to my web site, embarrassing as it may be for me to allow others to see this disastrous attempt. I have a similar assembly, of exactly the same materials, welded by a pro, and it is perfect, a nice golden bead, shiny and neat. Can anyone please give me an idea where I am going wrong? Thanks.