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RE: [PSUBS-MAILIST] Re: Pressure vessel welding
Thanks Sean,
Even I can understand that. Well put and I guess it is fairly logical.
Cant understand why ASME/ABS are taking a dim view especially now that ASME
DivII is reducing safety factors on the basis of emphasis on fatigue life.
Hugh
-----Original Message-----
From: owner-personal_submersibles@psubs.org
[mailto:owner-personal_submersibles@psubs.org] On Behalf Of Sean T.
Stevenson
Sent: 17 April 2009 09:32
To: personal_submersibles@psubs.org
Subject: Re: [PSUBS-MAILIST] Re: Pressure vessel welding
Profile grinding is a means of eliminating stress concentrations which
occur at weld toes or between exposed weld bead profiles, by smoothing
them out and eliminating toe overlaps. Typically encountered in
T-junctions, but also at the protruding edges of butt joint welds, the
toe of the exposed weld can sometimes overlap the parent plate,
producing a pronounced stress concentration at the "sharp" or obscured
interface. Grinding away the toe smooths the interface, drastically
reducing this stress concentration. Imagine a fillet which is perfectly
round between perpendicular plates. The ideal is to have perfect
tangency between the weld fillet and the parent plates, as though it
were a casting with a radius between the two planes. This is not
physically possible in most cases, so instead of a point of tangency you
have a discontinuity at the weld toe where it intersects the plate
surface. Profile grinding acts to create a point of tangency beneath the
surface of the plate, by grinding the weld toe to a smooth transitional
curve. This reduces the stress concentration factor, but increases the
nominal stress since it removes effective plate thickness in the process
- this is why the practice is prohibited in pressure vessel welds, as
strength and not fatigue is the dominant factor governing the ABS and
ASME welding codes. Of course, if you account for the maximum amount of
material you are likely to remove in the process of profile grinding
when specifying the wall thickness in the first place, you get a strong
hull with excellent fatigue endurance, but would technically be in
violation of code. IIRC, profile grinding was employed on the hull welds
in many US Navy subs, but then the military does whatever it wants in
defiance of commercial regulations.
-Sean
Hugh Fulton wrote:
>
> Can you explain "profile work pls.
>
> '
>
> *From:* owner-personal_submersibles@psubs.org
> [mailto:owner-personal_submersibles@psubs.org] *On Behalf Of
> *vbra676539@aol.com
> *Sent:* 17 April 2009 08:39
> *To:* personal_submersibles@psubs.org
> *Subject:* Re: [PSUBS-MAILIST] Re: Pressure vessel welding
>
> 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
>
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