I weld stainless sheet and tubes all the time.
The 2 main things that effect warping is heat (overheating really) and the amount of filler wire used.
Something like that is going to be hard to weld with 0 warping if it is not self supporting.
The amount of heat needs to be kept to a minimum. Now that doesnt mean low amps, what it does mean is work at a good steady pace with the right heat. For example, if you run 60 amps for 10 seconds, or 30 amps for 30 seconds, the 30 amps will put more heat in to the plate then the 60 amp setting.
The amount of filler also plays a huge role in warping. As filler heats to melt and then cools to solidify, it shrinks x amount. X is small, but when it all gets added together for multiple joints, it adds up to a lot of shrinkage, with out even having a cold shower.
If you dont need full penetration, I would use a good pulse setting, but for a full pen weld on all of that, expect at best, some warpage.
Best thing you could do for a full penn weld, is to build custom chill blocks and run cold water through them. Copper works the best but would be very expensive.
If you clamp it, do not remove the clamps until it is fully cooled down.
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