The last two Tuesdays and the last two Saturdays were spent in what I call the very bowels of this project - welding out the primary weldments of the boiler. The two tube sheets are now in, as is the furnace. The horizontal stays are the last weldment.
It is one thing to summarize; another to carry out. First, the 0.375" thick tube sheets are machined or ground for the relief bevel with @ 0.90" land. The sheet is then tack welded in place with short tacks. Then a 7018 (a low hydrogen rod) root pass is welded in. (Our shop utilizes a 7018 root pass procedure in order to avoid root cracking, a common problem with 6010 wire on circular or pipe weldments.) The root pass is almost ground out with a 4.5" grinder to ensure complete penetration and to ensure that there are no inclusions in the metal. Following passes are laid in and ground out as needed. I skipped around to minimize bowing and distortion, but it is hard to avoid.
Here is an image off the web. It is of a MIG weld but it is still illustrative. It shows a root pass, the pass ground down to eliminate inclusions and the "keyhole" at the arc, a mark of good penetration.