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Hi guys. Well......yes, the process of building a pressure vessel isn't
something to take lightly. As Dan has said here, there's lots to consider. Not
really an easy project for a novice welder, but with a little common sense, some
basic parameters, and LOTS of practice it can be done. Provided the builder has
learned the necessary skills, and has the right information and equipment.
For me, I didn't use Mig. I TIG welded all the beads.
Partly because my eyes aren't what they used to be after 40 some years as a
welder, and partly because I didn't want to buy another weld machine to run the
seams. I did however burn up a couple of TIG torches in the process. Still, much
cheaper than buying another MIG machine.
I have been using the TIG on almost everything I weld lately because it's
much easier to see what's going on with the puddle, and the welds are easy to
keep clean. It's a lot slower though. ( A LOT )
As for MIG welds getting brittle, they can if you don't follow Dan's
advice. I would still recommend shielding gas on the back side. The back will
need to be ground out anyway, but the gas will reduce that "back grinding" to
almost nothing. Just grazing the surface with the grinder will take it down to
nice, clean metal.
Pre-heating the base and letting it cool slowly will mitigate most causes
of embrittlement if the right wire and good shielding is used. Heat control is
very important too. The weld bead itself is stronger than the base metal, but
the "heat affected zone" right next to the weld is where problems develop.
The best way to make a pressure tank ( you're all gonna hate this ) is to
get a tank shop to make it for you. They use computer controlled automatic
weld machines. A machine weld is the best, but for a couple of reasons, not
available to us. First off, the tank shops won't build submarine hulls, for
sure. The steel shops won't even sell you the stuff if you tell them you're
building a sub. Second, the price. I got a price to make just the two
circumference welds on my hull, and labor alone was $5800. Damn, I never got
paid that kind of money welding for NASA or the other hi-tech outfits. Guess I
was working too cheap!
Anyway, back to the task at hand. It could be possible to rent a MIG
machine capable of welding the seams, or maybe get a local guy with a portable
machine to come out and do it. A couple of phone calls will tell you what's
available and how much. Or you can use TIG like I did, it just takes a long
time. ( good thing I'm retired! )
Most of the other welding tasks for a home built sub aren't life
threatening, so aren't that critical.
Stick welding has been used on pressure vessels for many years too. It's
kinda dirty and requires a lot of grinding but it's a good way to get X-ray
quality welds if the welder has the skills and patience.
Well, gotta get back up to the shop. Frank D.
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