Tack weld parts of similar thickness, then do the whole weld in a pass. For the .060 material I prefer about 50 amps and 10-15psi on the argon gauge. Large flanges force me to go to at least 75 amps.
Placing prethreaded conflats directly to the sphere with no tube is OK provided you undercut with a lathe and form a weld lip extending into the vessel. I tend to like this as it avoids a second weld which can leak, as when a tube is used.
It is really hard to screw up a TIG weld on SS. I have welded with most forms of welding (even Thermite). Nothing I have ever down approaches the ease of TIG welding SS.
The longer the bead and the larger the surface area, the more likelihood there is of subsequent warpage and micro cracks at thin weldment spots. This cracking may take place during cool down following welding or when torquing the thing down and go un-noticed until you try and pull and maintain a vacuum.
Richard Hull
Created on Monday, May 07, 2001 12:09 AM EDT by Richard Hull