The hemisphere should be welded on the inside surface. Always weld on the inside unless its not possible to do because of some strange geometry. Stay out of that V-groove!
The 8" hemispheres you are working with are the exact same size I welded into 10" Conflats. The difference in material thickness between hemisphere and flange shouldn't be an issue. Nothing is going to warp. The flange will soak up a lot of heat, and nothing should get too hot. In fact, when I welded my hemispheres in place, I just wore a pair of gloves and slowly spun the hemisphere as I welded it. After a complete pass the flange was still cool enough to hold with gloves. Welding shouldn't damage the sealing surfaces either, if there is any molten metal flying around when you are TIG welding, something is really wrong.
Joe Zambelli
Created on Monday, May 07, 2001 9:55 PM EDT by Joe Zambelli