The question of root opening size gets a fair amount of converstaion in my shop and typical practice more or less accepted for CJP's is to match the opening to the size of electrode, or if GTAW then it "depends" on access, cup size, weld size, etc. Usualy everyone is easily agreeable.
The subject on root opening size for PJP T-joint fillet welds is a different story. Today I was attacked by fitters, welders and production managers because I called for a 1/16 -1/8 inch opening on roots of a heavy weldment (2 inch thk plates, 3/4" and 1" fillets, all material is A36). The problem is that pieces were all sized and fabrictated to exact dimension without consideration of root openings and so tolerances will accumulate or else the fab shop will have a lot of rework. My logic is that fillet welds of this size will be highly restrained UNLESS a root opening is provided to accomadate shrinkage stresses. I also called for a 2-hour pwht at 1100 deg for 2 hours.
Now I am having doubts about what I specifed and cannot find reference data to confirm or refute root opening rationale. Thanks if you can advise.
Hello Tom Cooper, I guess the terminology of your question has me wondering a bit. When you say root opening are you referring to spacing the joining members apart to allow for shrinkage of the fillet welds? I have been around many shops that use 1/16" as a gap to set base plates and caps. For instance if you set a base plate on a column and leave no gap between the plate and the end of the beam, the welding takes place and there will be an increase of stresses in the fillet welds and finished weldment. By leaving a space between the base plate and the column end as the fillet welding takes place there is a slight reduction in this stress. Is that what you are asking in your question? You talked about having an 1100 degree PWHT for 2 hours, I don't believe that A-36 grades steels require any PWHT. The thicknesses would indicate a preheat, but not anywhere near 1100 degrees. From what I have gotten from your post this would be my opinion. Best regards, aevald
By way of clarification,
Aevald -
yes, you have interpreted the question as I meant to ask it; the "root gap" that I mentioned is that space between two members being joined. Our practice of pwht on A36 is geared towards distortion control and it is a standard practice for us on heavy weldments that get finish machined later on, same as what Dave Boyer talks about below.
803056-
the job in question is a Mil standard weldment for some Navy machinery, and as far as standard joint details, either AWS D1.1 Section 3 prequalified joints or Mil-Std 22 standard joints will both show similar callouts for the root gap and that will be seen to be (0 - 3/16") with no guidance on when to use 0" or when to use 3/16" or anything in between. I had forgot about Blodgetts book - how do you suppose that 12% factor could be used for sizing the gap? Maybe 12% of the fillet weld leg? Say 12% of 3/4" fillet = .09" rounded up to 1/8" gap?
Well based on the general idea in most of the replies, I think insisting on the root gap is legitimate. My last question is, is it really necessary since we are also doing the pwht?
Thanks
Possibly D1.1:2006, paragraph 5.22.1 or C5.22.1 can help with sizing the root opening and fillet weld upsizing for the amount of gap?
As for the Stress Relieving, D1.1:2006, paragraph 5.8 or C5.8 gives some info and concerns that may help.
John-
Thankyou for pointing out the info in 5.22.1; By this paragraph it seems the intent of "root opening" is to accomadate irregularities and typical mill variation along the length of a member with "close contact" between surfaces being the preffered objective. BUT now this seems to conflict with the idea of an intentional root gap as a means to relieve weld shrinkage stresses!! I have reread Blodgetts book on welding as mentioned by Al and see that Blodgett strongly recommends (page 7.2-6) a 1/16" gap between fillet welded members for exactly this reason.
So I don't know what to think now, I suppose I should favor the language in a universally accepted Code like para 5.22.1 of D1.1 and worry about handling shrinkage stress by means of the pwht.
Hello again Tom, despite the slight conflicts that you have discussed here, it sounds like you've got a pretty good handle on your situation. I know I have learned a bunch and been reminded of a few past practices and the reasonings behind them, even if I didn't understand them at the time. Best regards, Allan
Tom: While I did mention that if there was any through thickness tearing from shrinkage stresses in the fully restrained large fillet welds We didn't test for them, I should elaborate. We used these tools in house, and in some cases some large welds failed. I NEVER saw any evidence of through thickness failure in the plate material. Perhaps using a moderate strength [70KSI] filler allows the filler to elongagate enough that this is not a problem. I don't think the material was anything special, it was refered to a "boiler plate" in the plant, I guess it was A36, We had it on hand to 12" thick.
personally I would fit to the demensions in the the drawing, bridge tack zip disc the preffered root and blaze away.
What welding standard is being used? Are there standard joint details? If there are, then work within those limits.
A 1/16 inch root opening between members to be fillet welded is not unusual for heavy members. It may not be advisable or necessary for thinner materials.
I believe Omer Blodgett suggested an allowance of about 12% for shrinkage and contraction based on the width of the CJP or PJP weld if there is no restraint. That allowance can be utilized to calculate the necessary root opening required to minimize joint restraint.
Best regards - Al
I can tell You what We did at the auto frame plant, but I would not go as far as to say it was the best method. We NEVER used any gap in building heavy weldments [plate 2" & up] As a rule, the parts were blanchard ground or machined on mating surfaces and put together with 3/4 & 1" fillet MIG welds, CO2 & globular transfer, & in the old days with 7024 "baseball bats". Then the weldments were stress relieved at 1400f. Mild steel was not preheated. If there was any through thickness tearing issues, We never knew it as nothing was x rayed or tested with any fancy stuff. Most of these assemblies would be loaded greatly in compression, and the idea was that the parts mated and caried the loads from surface to surface, not just in the welds. The stress relief was an attempt to remove stresses that would cause warpage as some of the stressed material would be machined away. These parts were components for large stamping dies, things that would have been cast prior to cost effective fabrication methods. We had a young engineer, fresh out of Lehigh University who designed a piece of equipment that was fabricated from 1/4 & 1/2" material. He specified a gap at the weld joints to reduce stress & warpage. The gaps were formed by inserting bailing wire before welding. To My knolege this was the only job welded in the tool & die shop that used weld gaps.
Hello Dave, funny you should mention baling wire for setting those gaps. Caterpillar, in one of their technical repair manuals, specifies what amounts to be tie wire, for the same purpose. Both annealed wires easily capable of compressing under force and stress. Regards, Allan
your right Allen, we just finished up a 39k lb weldment for CAT that specified using tie wire for allowing the shrinkage!
Allan, the wire as I remember was about 1/16" soft wire. This wasn't one of My jobs, but I saw the guy putting the wire in the joints and asked what the reasoning was. The welding in the tool shop was generally never engineered, this job was the exception, as the "new kid" engineer probably studied welding at the university, and was new enough to be interested in how His job was being built. These weldments due to thier large overall size and not needing extreme precision were not stress relieved. In another building there was an anealing oven that would acomodate huge items, but to My knoledge none of our large weldments were stress relieved in it, as it would have interupted production and caused a union pay rate problem with the operator. Weldments that would not fit in the garbage can size heat treat ovens wern't stress relieved in spite of it being specified on the prints.
This is not entirely relevent, but we used to build deck plates for industrial facilities by welding on the angle stiffeners with a 3/16" gap between the angle and the plates. Intermittent fillet welds were used to weld the angles to the plates, I don't remember the size. The whole idea was to enable the deck plates to be galvanized and get the galvanizing between the stiffeners and the plates. It worked.
Bob G.
Hello Bob, necessity is the mother of invention. That sounds like a reasonable way to allow for that situation. Kudos. Best regards, Allan
Remember to increase the fillet weld size accordingly to compensate for the gap.
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