Last night I saw some interesting things I thought I'd share with the group. Sorry if this is old news to the experienced folks.
I can "stick metal together", but I'm no professional welder. I'm doing most of my welding, but decided not to chance it when it comes to the pressure boundary seams. Those I'm having done by a pro. So I called a local welding supply store and they put me in touch with a gentleman who is a certified pressure vessel welder and pressure vessel inspector, with 30 years experience. Last night he came by and did the longitudinal seam on the pressure hull. This is the single largest weld on the boat.
1) The starting point was a cylinder 94" in length, of 516 gr 70 plate 3/8" thick. Per welder's instructions, I had ordered this with a double beveled edge. The first surprise was that he prefers the edges not to touch. The seam was just how he liked it, with a small gap about 1/16th between the beveled edges. The reason for the gap is he wants to know he has filled it, and without a gap he would be hoping (but not certain) that the metal had melted to the center of the material.
2) He started by doing a continuous weld on the inside with 7018 rods, 1/8". Its interesting seeing the technique. When the rods were still long, he actually held them with both hands -- two fingers were near the hot end, to keep it perfectly steady. Sometimes he'd just be going straight down the seam, other times going side to side "depending on what the puddle is doing". The end result looked like it had been made by a machine. An interesting detail was what he did at the ends of the seams. He'd get to the end and then back up again and redo the last 2-3 inches, all in one movement without removing the slag. He said this "avoids an end of the weld on the end of the seam", which would be a potential weakness.
3) He then went at the outside with a blowtorch and a 90 degree nozzle. He "torch-gouged" the beveled channel turning the V into a wider sort of U, about 1/2" wide. He took it down so the material at the bottom of this U was the inside weld. This also removed the tack welds, but was not limited to the tack welds (he did the whole length). This step could have been done with a grinder, but the torch was a noise reduction tactic -- I live in a quiet residential neighborhood and try to minimize grinding on the driveway.
4) He finished the channel with a light grinding. The result was a shiny and very solid looking channel without discontinuities of any sort. This did get one neighbor out onto the street, but fortunately she was one of the friendly ones. Did I mention his welding equipment was gas powered and 1963 vintage? About as subtle as a locomotive.
5) He welded the whole length from the outside. This weld filled the channel to half its depth but covered its entire width.
6) Removed slag, wire brushed, and laid another bead along the whole length to bring it flush with the surface.
One interesting thing is that there was almost no spatter, and the slag just fell off almost on its own. He had a whole truck full of welding equipment, but did not posess a chipping hammer... no need apparently. The slag required at most a brush of the glove or a scrape with the end of a rod to remove. For some reason I have to pound my welds.
The entire process took 4 hours, working efficiently and without a break. His rate was $65/hour which included all equipment and consumables. He turned up with this self-contained welding-shop-on-a-truck and did everything right there on the driveway. Boy do I have no regrets about this one, the peace of mind is well worth it!