One side note here is that ABS
and the ASME code frowns on profile work on pressure vessel welds. The issue as
I understand it was all about maintaining thickness on full penetration welds.
Professor Lance and Brother Frank will know more about that than I do, or
they're in more trouble than I think.
Vance
-----Original Message-----
From: Sean T. Stevenson <cast55@telus.net>
To: personal_submersibles@psubs.org
Sent: Thu, 16 Apr 2009 4:27 pm
Subject: Re: [PSUBS-MAILIST] Re: Pressure vessel welding
Backgouging, or backgrinding, is
necessary to ensure that the weld is full-penetration. The root pass is the one
most susceptible to inclusions, voids or incomplete fusion as a result of
insufficient shielding, poor temperature control (rapid cooling), contamination
(inclusions from dirt or moisture, etc.) or pool flow into the gap you are
bridging and rapid cooling of the bead without actual fusion to the parent
material. Ideally, a full-penetration weld between two cylinders would be a
double-V butt joint, in order to assure complete fusion with a minimum amount
of rod fill. In your example, a 1/2" wall would entail a 0.20" - 0.25
chamfer" on both sides, such that you either have a perfect V or a small
parallel land in the center of the wall. In either case, the root pass fills
the gap and joins! the two cylinders at the wall centerplane
(center-cylinder?), and then you begin to lay passes on one side. Given that
the root pass could have poor fusion due to the cooling / bead flow problem,
you grind it away from the side you haven't welded yet, to the extent that you
are 100% assured of having ground back into material which is high quality,
strong weld that is fully fused to both sides of the assembly - essentially
removing the weld toes of the root pass on both sides and exposing the fusion
interface to assure its quality. Then, you start laying passes into the
ground-out root to guarantee 100% fusion.
Granted, the double V is a lot of work to prepare, and if you can't or won't do
that, you can effect a full-penetration weld on a strict square-ended butt
joint, but you have to ensure that if you lay a root pass on one side, your
electrode can reach fully down into it from the opposite side to achieve 100%
fusion - here, you have conflicting goals, because the larger the gap between
the two cylinders, the easier access you have for the electrode (essentially
achieving the effect that the V preparation is intended for), but in increasing
this gap you make it much more difficult to lay a quality root pass to begin
with, necessitating deeper grinding when you backgouge it to ensure that you
expose perfect weld before covering it. You may end up having to grind out this
pass entirely, exposing your first covering passes from the other side, and
then covering those. I would recommend considering the weld-preparation, as it
may very well save you the same amount of effort in back! gouging and
recovering.
As for your covering passes, you should be able to maintain high quality 100%
fusion welds as you proceed, but if you notice a mistake, or are at all suspect
about the extent of fusion of any given bead (or any other quality issue), just
backgouge it until you have it back to a perfect weld, and proceed from
there.
Once you have a high-strength, high-quality weld, you can further improve its
fatigue life by profile grinding (where you remove geometric discontinuities /
stress concentrations at the exposed weld surface) and stress-relieving with
heat.
-Sean
Jon Wallace wrote:
>
>
> Thanks for that Frank. What about 1/4inch rod? Is it logical to > think
that 1/4 rod would produce results twice as fast as 1/8 rod > (assuming all
other things equal), or does it just not work like > that? I'm sure 1/4 rod
requires more amperage to cause an arc which > is one drawback, but if you
have the power available, does 1/4 rod > make for faster work?
>
> Also, I hate to sound dumb, but somebody has to take that role so it >
might as well be me. Can someone explain "backgrinding" and why it is
> necessary? Let's say I've got the rolled cylinder for my main hull. >
There's an open seam running longitudinally which essentially is a >
"butt" joint, correct? So I put down a root pass which I hope is >
going to fill the void between the two sides of the seam at least on > the
inner side of the cylinder. I likely won't fill the entire seam > on the
outside of the cylinder depending on the thickness of the > material (if
it's 1/6 thickness I will, but if its 3/4 thickness I > won't). So let's
assume I'm working with 1/2 inch material and need > multiple passes because
I'm using 1/8 rod. What am I backgrinding on > the root pass? The inner
side, or the outer side? And assuming my > root pass is perfect, is
"backgrinding" necessary on the build-up > passes that go over the
root pass? >
> I'm going to take a welding course, but these questions have bugged me
> for some time and I can't wait for the class. :)
>
> Jon
>
>
> ShellyDalg@aol.com wrote:
>> Miller makes good machines but definitely you want a DC machine. AC
>> "buzz boxes" are OK for some small farm quality welding, but
not much >> use beyond that. ( like fences or gates.)
>> A good DC machine ( and they also have an AC switch on them) can be
>> used for TIG as well. A cheap TIG torch runs about $80 and a flow
>> meter about $75. Those two items with a bottle of argon mix and
>> you're all set to TIG just about anything. As for magnetizing the
>> metal, that's not a problem. A 250 amp machine is enough for what
>> we're doing. The "duty cycle" relates to how long you can
burn rod at >> the maximum setting, based on a 10 minute cycle. 50% duty
cycle means >> you can burn rod on maximum for 5 minutes, then let the
machine cool >> down. It's pretty hard to "smoke" a stick weld
machine. The older or >> better machines have copper windings on the
transformer, the newer or >> cheaper machines have aluminum wire on the
transformer. Get a copper >> wound one if you can. You won't be welding
at max power anyway. 3/32 >> and 1/8 size rod is the most common.
Typically running a! t 90 to 125 >> amps.
>> Make sure you keep the rod DRY or use new rod. I buy my welding rod
>> in small batches ( 5 pound box ) so it's always new. Frank D..
>>
>> ------------------------------------------------------------------------
>> Access 350+ FREE radio stations anytime from anywhere on the web. Get
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>>
>
>
>
>
>
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