General information on welding practices for vacuum systems can be found in any decent book on vacuun technology, but I can give a few welding tips.
It doesn't take a lot of current to weld flanges to a chamber. Currents are normally in the 30-60 amp range. Since the voltage drop across an Argon arc is about 9V, this is only 300-500W. You just need enough current to get good fusion of the material. A weld depth 0.025-0.050" is sufficient. This is not like welding bulldozers or battleships.
It's always a good idea to tightly clamp pieces together before welding or they tend to jump out place. I always tack my welds in a few spots, and then check to make sure everything looks right. I then continue the last tack on around to finish the weld.
The two most important things to achieving a high quality weld are a good fit at the weld joint and cleanliness. I make sure all of my parts fit to within a few thousandths of an inch before I weld. Since the leak rate through the weld needs to be approximately nothing, it is important to not weld in random bits of stuff, as this tends to introduce inclusions. Don't use filler metal, if the pieces fit properly it shouldn't be necessary. Also, if you run the tungsten into the weld, stop and resharpen it. Contaminated tungsten doesn't work very well.
Given the low thermal conductivity of stainless steel, and its resistance to oxidation, it is the easiest thing to weld that I have ever come across. Welding isn't that difficult a skill to learn, it just takes a bit of practice on scrap material. If you ever plan to weld yourself, here are some general rules: Once you strike an arc, if your current is correct, the molten pool should form in about 2 seconds. Hold the tungsten very close above the weld, about 1/32". This helps to concentrate the heat into a smaller area, and gives a nicer weld, it is also easier said than done.
I do all welding myself because I really don't trust anyone else to do it. I feel better doing the work myself, rather than allowing someone else to do it who is not familiar with welding processes for vacuum systems.
Joe Zambelli
Created on Sunday, May 06, 2001 8:15 PM EDT by Joe Zambelli