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Hi Sean. With 4 and 6 inch plate, there really is no alternative but to
weld from both sides for a full penetration weld. With material that thick, the
bevel needed to reach the root would indeed be expensive to fill, owing to it's
width. On our application, dealing with 3/8 to maybe 3/4 inch thick, the bevel
doesn't get that wide, so it's easier to do the welding from one side.
Now for the distortion, that is again dependant on the thickness, along
with several other factors including the shape of the weldment, alloys used,
set-up, pre-heat temp, and others.
With any weld, metal always cools smaller than before it was heated. (
shrinkage )
This can be used to advantage if understood. The shrinkage puts the metal
in tension as it cools. Too much tension in any one direction and you get
warpage.
Again, with the relatively thin material we work with, the stresses and
tensions built up in the piece can be controlled fairly well and actually add
strength to the finished piece.
I can't explain every means/method for every shape but the basic principles
are the same, just applied differently to specific circumstances.
First, use lots of tacks to get the pieces aligned and held in place
strongly. Often times this may require additional tabs, strips, jigs,
backstraps, or other hold-downs.
Next, if it's possible, pre-heat the entire piece and hold the temperature
during the entire process.
Weld the piece using a cross rotational pattern as you lay the beads down.
This is much like cranking down on head bolts with a torque wrench. There's a
specific pattern that must be followed, depending on the shape of the weldment.
Keeping the piece HOT reduces the tension build-up and as you lay down opposing
beads, they need to counteract with the beads already made so the tension builds
evenly. Of course, proper cleaning of the stop/start area is critical in getting
a sound weld.
If the piece requires more passes, the same methods apply to all subsequent
welds. Again, maintaining the heat as uniformly throughout the piece as
possible.
Once all the welding is done, maintain the heat for at least an hour or so
( for the small welds we use ) and then cool the piece slowly and uniformly so
the tensions built up during this process remain more or less equal. On large
and/or thick pieces this " hold then cool " process can take several days.
With our little subs, at 3/8 to 3/4 inch thick, most of the tension will
spread out into the surrounding metal fairly evenly, and can be relieved by just
applying heat via a torch. We don't need to heat the entire hull, just enough of
it to prevent the shrinkage from being too localized. Even just heat lamps
will add stability to the weldment, without getting the metal that hot. With the
current subject of a through-hull plate running 6 to 8 inches in diameter, this
is easily controlled because of its relatively small size. Whether it's done
from both sides or mainly from one side, the same principals of heat management
apply. This may sound complicated, but in practice it really isn't, once the
principals are understood. It's also much easier to make a good weld with
pre-heated metal, especially for the less experienced welder. The metal joining
process works best when EVERYTHING"S hot, except the novice will probably get a
few burns ! Practice with a few small scrap pieces on the bench, using what
tools you can find laying around. See what makes a piece change shape, warp,
shrink, twist, bend, and then use that experience to plan how you will be
welding the project piece.
After just a few of these, it will become instinctive when you approach a
weldment, and the weld bead pattern will present itself. Frank D.
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