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Up Topic American Welding Society Services / Technical Standards & Publications / Backing bar tolerance
- - By Marvinrex Date 11-18-2013 18:45
I am having a hard time finding anything that specifically defines the allowable fit-up variables concerning the allowable "gap" between the base material and the backing bar.  The pipe is out of round and variations in pipe I.D. create a situation where the backing ring is not in total contact with the back side of the root.  All I've really found is the requirement for the backing to be "thoroughly fused".
Parent - - By welderbrent (*****) Date 11-18-2013 19:08
Marvin,

WELCOME TO THE AWS WELDING FORUM!!

Since you posted your question under the D1 heading I assume it is not API nor ASME.  It might help if we did not the precise code application though. 

But, if this is indeed a structural welding code, then you will mainly be interested in the subclause in Clause 5 that deals with Tolerance of Joint Dimensions (5.22.1.1 deals with backing).  Now, I am referencing my 2006 because that is what is in front of me on my computer, 2010 may be slightly different in either the clause number or the wording.  There is also info through 5.22.3 and 5.22.3.1 that may apply. 

Hope this is helpful.

Have a Great Day,  Brent
Parent - - By 46.00 (****) Date 11-18-2013 20:30
Still the same in both 2006 and 2010 editions.
Parent - - By SCOTTN (***) Date 11-18-2013 21:01
If you're not getting a tight fit between the backing bar and the base metal, you can end up with slag inclusions in the gap.
Parent - By Marvinrex Date 11-18-2013 22:31
Thanks for your input.  The first pass is going to be with GMAW.  Please look at my reply to the first response.  I'm also aware that with a loose fit-up here I could end up with some undercut on the back side of the pipe end bevel.  I think we are going to do some mockup sample welds.  However, my main concern at this point is overlooking some "nit-picky" code issue.  (Sorry, I'm old school)
Parent - - By Marvinrex Date 11-18-2013 22:27
Thanks, this joint is being welded to D1.1.  And we are doing a full PQR due to the first pass being GMAW (in the gray area of short-circuit vs globular transfer).  The joint is actually a "spigot" (not bell and spigot) because it is a solid member with a bevel machined on the part then it necks down to the same OD as the ID of the pipe (1.5" sch.80).  This fit-up was designed to allow to aid alignment but primarily acts as backing because a full penetration weld is required. The problem is that the pipe end is out of round and the wall thickness varies slightly from lot to lot along with the ID.  The original tolerance was .040" on the drawing but a more realistic fit is closer to .060-.080 in order to get the two pieces together in the fixture and still be able to rotate the solid piece so back of the part is right with the bends in the pipe.  I personally don't see a problem from a welder's point of view; however, from the technical side; I don't want to overlook some code issue here.

Thanks again, I did review the sections you recommended.
Parent - - By SCOTTN (***) Date 11-19-2013 13:35
In reference to some nit picky code issue, the minimum thickness of backing for the GMAW process is 1/4", but if you're doing a full PQR, the backing thickness, i.e., the wall thickness of the "backing pipe" shouldn't be an issue if the backing pipe wall thickness is less than 1/4.
Parent - - By welderbrent (*****) Date 11-19-2013 15:47
Scott,

If you are referring to D1.1, Clause 5.10.3 then notice that is states "The recommended minimum...".  (emphasis mine)

It is not an absolute even without a PQR.  These are suggestions to keep the welder from burning through the backing. 

Have a Great Day,  Brent
Parent - - By SCOTTN (***) Date 11-19-2013 16:12
I'm aware that it is a recommended minimum, though, I personally wouldn't attempt to use anything thinner due to possible melt through.  I was just implying that the thickness shouldn't be an issue if the pipe wall is thinner than 1/4" and it works.  Though, if someone wanted to nit pick the code, they could come back and say that the recommended minimum for the process being used is 1/4", but they really wouldn't have a leg to stand on if the PQR was successful.  As you know, the recommended minimum thickness is based on the process.  I've always used the recommended minimum thickness for all the listed processes without any problems.  I have also seen 1/4" backing used with the FCAW-G process with no melt through issues.  If the recommended minimum thickness is used in conjunction with a given process and there is melt through, I would think that there's a bigger problem than the backing bar being too thin.  I'd be curious to know if anyone has used the minimum thickness with a prequalified WPS, and have had issues with melt through.
Parent - By Marvinrex Date 11-19-2013 19:25
Just heading off to the lab to look at the results of the mock-up samples.  As soon as we see what they look like, we'll go ahead and do the actual testing, documentation etc.  Thanks for the input to all.  I haven't used this forum much over the years and I need to use it more.  Having input from different perspectives usually leads to a happy ending.  The thickness of the backing really isn't the issue.  If my memory serves me correctly, I think it's about 3/16 +" anyway.  This part is being welding in a rotating positioner so there isn't any vertical-up going on and with a GMAW root, burn through is unlikely at the parameters we're running.  My only real concern has been the "gap" between the backing and the root.  We did some samples with an intentional "gap" to see what it looks like.  After I get the results back from the cross-sections, I'll post the results.

Thanks again,
Parent - - By welderbrent (*****) Date 11-19-2013 19:36
Kinda thought you knew that, just wanted to make sure we didn't give Marvin the wrong idea on that one.

I agree with you, though I have a couple of contractors who regularly use thinner bars and only have problems when they bring in new people who would probably even blow through the heavier backing bars.  It is generally as much about technique as anything else.  But you are absolutely correct that there is very seldom a burn through with the recommended backing thicknesses.

Have a Great Day,  Brent
Parent - - By alstonwelding (*) Date 11-20-2013 14:27
AWS D1.1 (2010)

5.22.1.1 Faying Surface. The separation between
faying surfaces of plug and slot welds, and of butt joints
landing on a backing, shall not exceed 1/16 in [2 mm].
Where irregularities in rolled shapes occur after straightening
do not allow contact within the above limits, the
procedure necessary to bring the material within these
limits shall be subject to the approval of the Engineer.
The use of filler plates shall be prohibited except as specified
on the drawings or as specially approved by the Engineer
and made in conformance with 2.13.

I hope this helps....
Parent - - By Marvinrex Date 11-20-2013 18:28
This has all helped very much.  We all know how critical joint fit-up is especially considering that all important root pass; however, in some cases there are times when conditions exist that make the job a little tougher.  The sample joints that we did yesterday utilizing "worst case" conditions that we expect to encounter all turned out OK.  We did several cross-sections, polish and etch.  The root was fused to the backing and there did not appear to be any negative conditions at the root of the pipe bevel such as undercut or incomplete fusion.  Interestingly, the largest "gap" that we were dealing with was right around 1/16".
Parent - By welderbrent (*****) Date 11-20-2013 20:33
Thanks for the update. 

Hope it all works out for you.

Have a Great Day,  Brent
Up Topic American Welding Society Services / Technical Standards & Publications / Backing bar tolerance

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