Welding Lap Joints

[JXWu]

Dear experts,
I am working on a storage tank and the project spec. calls for minimum 2 passes welding on the bottom lap joints.
Bottom plates is 8mm thick.
I am considering using SAW with auto downflat tractor but not sure if it is problem trying put 2 passes in a 8mm lap joint.
Please advise.
Many thanks.

 

SAW tractors are ideal for making 8mm fillets in a single pass. Quality is excellent and distortion is minimized. Using them to do the task in two passes can be done but defeats the advantages of the tractor.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[JXWu]

Thanks for your response. I am also considering SAW/FCAW combo. Maybe weld the 1st pass with SAW with a 6mm vertical leg length and top it off with FCAW. Thanks.

 

[kingnero]

You'll want three passes if it needs to be multi layer.
Two passes is possible, but it will probably require an oscillating welding torch mount (which is $$$) if you're going to weld this using the tractor. The reduction in travel speed to accomplish this means a (much) higher heat input, leading to higher distortions.
Three passes is common, and can be done faster and with a lower total heat input.

But you need a 8mm leg length weld, which is (in theory) a 6mm throat thickness, and this is still in the realms of SAW. I'd qualify this (a single pass weld), and ask for approval / make a change request to your customer.

 

[dhengr]

JXWu:
I think you should talk with the original spec. writer, because there are several things which need clearing up before you proceed with your work. The spec. does not seem to be particularly well written, or say what it probably really means. You say that the floor pls. are 8mm thk., and everyone seems to assume that your fillet weld is also 8mm. Many codes have a reqrm’t. that the fillet weld be 1 or 2 mm smaller than the pl. thk. This is to assure that you are not burning off the upper edge of the pl. and, in effect, reducing its thickness in that immediate area. What is the spec’ed., req’rd., fillet weld size? That’s what should be spec’ed. The idea that you need a two pass weld is questionable and I’d get that cleared you. At an 8mm fillet you are right on the boarder line of needing at least two passes, depending on the methods and process which you employ. A SAW weld using a tractor would normally allow a 7 or 8mm weld to be made in one pass, and would seem to be a good process for this application.

 

[JXWu]

I have submitted a deviation request to the client to use single with SAW process and they approved!
Thanks to everybody for the inputs and advice.