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Up Topic Welding Industry / General Welding Discussion / Parrallel and tandem subarc question
- - By dmilesdot (**) Date 09-05-2007 18:20
I understand that with parrallel sub arc the kilajoules of the two arcs are added together because they contribute to the same puddle.  With a tandem set up, with 4 inches of seperation, would you add the kilajoules together or would they be considered seperate and is there any place where this is written and could be referenced?

Dave
Parent - - By 803056 (*****) Date 09-05-2007 18:49
That's a good question to which I have no answer. Any ideas on this one?

Al
Parent - - By dmilesdot (**) Date 09-05-2007 18:59
I posted this same question on another forum, but only got one reply from js55.  Im not doubting what he wrote im just looking for more input and a source to reference.
Dave
Parent - By jon20013 (*****) Date 09-05-2007 19:12
And here I was thinking that I was seeing double, lol!

I agree completely with js55.  While I'm uncertain if there's anything in writing, stop and think about it, if your trailing arc is 4 inches behind your leading arc, why in the world would you try combining them to calculate joules?  You may try contacting one of the suppliers of subarc equipment to verify; such as Lincoln, ESAB, Euroweld or Miller but my very best guess is you'll be told exactly the same thing js55 mentioned.
Parent - - By js55 (*****) Date 09-05-2007 20:34
No problem Dave. My wife doubts me more than anybody. And she knows me best.
There is always greater wisdom in numbers. Unless you're on a committee.
Parent - - By js55 (*****) Date 09-05-2007 20:37
It all depends upon puddle solidification. A 4" spread seems to me pretty much just an efficient multipass weld.
Parent - - By js55 (*****) Date 09-05-2007 20:50
Actually, to my knowledge there is nothing in any code that addresses this specifically. And not to make this too technical but,,,,I suppose the real threshold point could theoretically be below the last transformation. If the trailer arc impinges after the leader deposit solidifies through the last transformation you figure them seperately. If not, you somehow have to combine them, since you have added recrystallization energy, and additional weld metal to a deposit that hasn't completely manifested its first dose of energy. Does this make sense? Or maybe I'm over thinking this.
For carbon steel this would be the final cooling to ferrite, for CrMo's perhaps bainite finish, and for say Grade 91 martensite finish (Mf). If the trailer impinges the weld deposit before the final transformation some kind of additive equation would be more accurate. Though I don't know that anybody has done this. And i haven't a clue as to how it would be implimented.
Parent - - By js55 (*****) Date 09-05-2007 21:05
And Jon is right, to be code compliant, as far as I know, count em seperate.
But if you really wanna represent whats happenin, take the novel approach. Engineer it.
Think about it, Grade 91 for example. The WM HAZ, if transformation is allowed (additional passesd after Mf), as with a real multi pass application, will be tempered martensite. If transformation is not complete (if you run up on the leader before Mf), you will breeze right by it before its transformed and it will be be untempered martensite. A real variance in properties. Similar to interpass max's.
Not saying they dont' exist of course, but I've never seen any papers dealing with this issue. Great question.
Parent - - By petty4345 (**) Date 09-06-2007 14:09
This is a little off point, but what about max interpass temperature from one arc to the other?
You may be far from exceeding it, but in the world I'm in I would certainly be asked to verify it.
Parent - - By js55 (*****) Date 09-06-2007 14:32
I think thats a good question. And it fits in with the thread.
From a code compliance standpoint I don't believe there is any justification for considering tandem SAW in terms of interpass violation.
From a metallurgical standpoint it might be an issue depending upon alloy.
As in the Grade 91 example I used, if your single passing an alloy that requires a martensitic transformation (generally lower in temp than bainitic or ferritic transformations) to obtain its strength often additional tempering from subsequent passes is beneficial. Yes, these alloys most often receive PWHT to temper. Or at least they should. But what about moving untempered assemblies from welder station to furnace? Do you really want the viability of your assembly-with such a critical type alloy as we are considering-in the hands of what quite often is one of the lowest paid and least experienced people in your shop-material handlers-without some additional measures?
For the great bulk of alloys I would think consideration of such would be unnecessary. Stupid even. But there could certainly be exceptions.
Parent - - By petty4345 (**) Date 09-06-2007 17:01
Would you agree that a less technical or metalergical explaination would be that if the second arc followed the first one far enough behind that if the max interpass temp was not reached behind the first arc, that it would then be considered a different weld pass and that the heat input would not be added together?
Parent - - By js55 (*****) Date 09-06-2007 17:10
My own personnal opinion, stepping away from the too technical, is that if the weld deposit is solidified by the time the trailer arrives then the heat inputs should be considered seperate. This, in my opinion is code compliant, since the code doesn't really address it anyway, and generally metallurgically sound.
Heat input as well as Charpies, are an approximation, good ones to be sure, in my opinion, anyway, so there seems to me at least for now, little point in gettin too picky about exactly when to add em or seperate em. The solidification point is a good as any I suppose.
I would certainly welcome disagreement here. This is a good discussion.
Parent - - By petty4345 (**) Date 09-06-2007 17:31
My comments have not really been based on my somewhat limited knowledge of D1.1 but come more from what I've gathered from working with ASME and Military codes.Which I deal with much more.
My interpretation of the Mil specs (and respective engineers) is that they tend to lead me to the max interpass temp to make these assumptions.
I certainly wouldn't consider myself a D1.1 expert,and don't mean to lead someone down the wrong path, just thought this would be a different way to arrive maybe the same conclusion for someone (like me without a metalurgy background)
Thanks, this has been interesting.
Parent - By js55 (*****) Date 09-06-2007 17:46
The problem with using max interpass to determine heat input segregation is that interpass temps are not always generated from the same metallurgy that is important to heat input.
For example, how does the heat input segregation idea based upon interpass temps work when with some alloys the issue is interpass mins (CrMo's) as opposed to interpass max's (SS's)?
My thought wouldbe that if some specific segregatioj point was to be argued, that it be based upon an actual state of the material, not a procedure related to achieving a state.
Now I'll let this post cool a bit and reread it to see if I actually said something.
Parent - - By HgTX (***) Date 09-06-2007 17:53
...and what about welding two sides simultaneously (say a T joint) on relatively thin plate?

Hg
Parent - By petty4345 (**) Date 09-06-2007 18:19
I tried that a while back just to see what would happen, to eliminate back grinding (FCAW on 1/4" A516-70 test plate- 3/32" root opening) and got some wicked cracking. Didn't do any mechanical testing.
Also we did not try a "cooler" process.

As far as min interpass, I should have said within preheat and interpass of what was proven on the PQR.
Parent - By js55 (*****) Date 09-06-2007 19:04
I would certainly think that welding opposite in such a manner would change cooling rate on both sides and have a definate effect metallurgically. And a heat input calculation could be developed. Its a different application but the same basic reasoning. Combining them would not be accurate, IMO, and calculating them seperately would not be accurate either.
And again, it would depend on the alloy as to its importance.
AWS D1.1 oriented people have a tendency to consider heat input when its not necessary due to the fact that its somewhat codified through current, voltage, wire speed, and travel speed limitations. How critical is heat input on the great bulk of carbon steel?
ASME people don't emphasize it at all unless there are impacts involved, or of course some metallurgical specific reason to do so. And even then not in most instances of thicknesses below 5/8".
Up Topic Welding Industry / General Welding Discussion / Parrallel and tandem subarc question

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