How big and/or how many tack welds are needed to keep a part in place
so it can be moved, to allow it to be pre-heated and to allow a root pass to be placed?As a possible example to use,say a one inch thick plate of A514 material twelve inches square preheated to remove the moisture welded to a A514 plate one inch thick length and width over three feet.The final fillet weld is to be one half inch and the root pass is to be one fourth of an inch.The filler material is .052" wire Lincoln Outershield MC-900,Metal Core.The same filler metal used to weld the fillet is the same filler metal used to weld the tacks.
The good engineering practice says that once you've set the two parts in position, you tack weld them so they won't move off position before making the first (root) pass.
The same good engineering practice says that the tack welded parts can not be moved from one place to another, and even less lifted up by means of a hoist or crane.
In other words, the tacks are not supposed to support the stress imposed by moving or lifting the piece. To do this, at least the root pass should have been completed.
This is how I've done in many years of erection work.
Did I understand your question correctly? Has someone got a difference experience?
Giovanni S. Crisi
Sao Paulo - Brazil
No, that was not what I asked.My apologies.I asked how big and or how many tacks are needed not the techniques or purposes of tack welds; your information is helpful and much appreciated though.
In any case, and in my opinion based on many years of field experience, you shouldn't rely on the strength of tack welds to move from one place to another or to lift two pieces that are just tack welded. For this, the first (or root) pass should be complete.
Giovanni S. Crisi
I would be very cautious of "Tacked up" Q&T steel especially with the preheat you describe. I have tacked up similar steel and had tacks crack later.
I noticed the problem of tacks breaking when I am preheating the mentioned Q & T steel.The setup welders usually use E7018 stick when doing the initial setup,but tack on parts with FCAW/metal core wire when the setup is after this such as putting a plate on after the welding inside the area is done.I guess it's more convenient just to use the wire to tack at that stage instead of using stick which takes getting the cable,rods etc.which isn't usually available where the weldment is getting finished.Probably I should put in more spot welds until it "looks good."Guessing isn't a great way to do things,I would think.
knightour,
Most of my clients using A514 are careful to preheat the items before tacking (in your example of 1", this would be 200°). SMAW electrodes are 1/8 or 5/32, FCAW wire used is 1/16. Tacks on single pass welds are the same size as the weld. Tacks on mutiple pass welds are no smaller than the root pass of the weld. Number of tack welds and spacing are dependant on the part configuration taking into account movement, weight, type of restraint, or the use of bracing (many times the tacking sequence is not called out on the drawings). The length of tack welds is no less than 1". In many large groove welds, blocking is also used.
~thirdeye~
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