Ideas to Determine Weld Root Opening Size

[tc7]

On weld joints that are highly restrained, such as a partial penetration all-around fillet weld on a T-joint, a root opening is often recommended, presumably to minimize shrinkage stress in the cooled filler material and/or to avoid lamellar tearing in the base metal. What size root opening should be used? I have searched and cannot find this guidance anywhere. AWS D1.1 commentary C-2 make mention of the above concern but offers no guidance on utilizing root opening or what size it shoud be.

So I am thinking a simple linear expansion calculation is all that is needed to determine a logical size of the root opening. By choosing as delta T, the difference between molten steel temperature (~2800 deg F)and minimum service temperature (say 70 deg F) and plug this into:
dL = L x dT x alpha (where alpha = expansion coef. of steel)

we get dL = .75 x (2800 - 70) x (.00000633) for a specified fillet weld size of 3/4-inch leg.

Therfore dL = root opening = .013" = ~1/8"

Does this make sense? I don't know. We wouldn't place a 3/4 inch fillet weld at one shot, it would be built up in several passes probably using ~3/16" thick beads. Should the 3/16" bead size be used for the above calculation? Then we would get dL of .003" and that doesn't seem like much of a root opening an why bother?

So what thoughts does anyone have on how to determine root opening size?

 

[metengr]

tc7;
What you have done and explained makes perfect sense. By the way, you will not find any specific Code information related to root opening because this is a process function. The proof for process functions related to welding is in the qualification of the weld procedure with the stated root opening range.

 

[HgTX]

You can't safely assume the root opening will shrink shut, so whatever you do, you'll have to bump up your fillet weld size to accomodate it.

Hg

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[tc7]

There is a slight irony here which is that we usualy qualify weld procedures based on CJP plate tests. This permits us to apply the qualified procedure for PJP fillet welds also however it reveals nothing about the contribution (or lack thereof) of root openings for a fillet welded T-joint.

Met, what is your rule of thumb for root openings in this type of T-joint?

 

[tc7]

HgTX-
If the root opening doesn't shrink shut - that is a good thing because then we wouldn't have the joint restraint condition that I am worrying about. Did you mean to say "we can't safely assume the root opening WON'T shrink shut"?

How are you specifying root opening size?
Thanks

 

[GRoberts]

While you can't assume that a root opening will shrink shut, AWS D1.1 & D1.5 welding codes don't require any additional weld size until the gap between the parts is greater than 1/16". The additional weld size is always based on the gap before welding, not after. By the way, there is code limitations to the maximum allowable gap size.

 

[HgTX]

GRoberts basically said what I was trying to say. From a stress standpoint you might not want it to shrink shut, but from a geometry standpoint, greater gap for same fillet weld size means reduced throat. You have to assume that you're ending up with the same gap you started out with.

D1.1 section 5.22.1 controls fillet weld root opening and adjustment of fillet weld size to accomodate the root opening.

Hg

Eng-Tips policies: faq731-376

 

[tc7]

I didn't mean for this to be a Code issue question. I have been looking for technical guidance on how best to determine root opening size when dealing with restrained joints, such as a Tee-joint.

I understand that once a root opening is incorporated, either by design or as a consequence of mill irregularities then Codes require fillet weld sizes to be adjusted.

But for the purpose of offsetting shrinkage stresses in a Tee-joint, do you folks not require an intentional root opening? If you do, what is your practice or method to determine size of the root gap?
Thanks

 

[gtaw]

My experience is that a 1/16 inch root opening work well. Once the root bead and a couple of added beads are in place, they will resist further contraction forces and further reduction of any root opening present. I usually use a couple of pieces of annealed "tie" wire as the spacer so that it will crush and offer minimal restrain for the root bead.



Best regards - Al