welding long seams

[hoddykubody]

I am going to be making some grain wagons next month (gravity flow). They will be brought to my shop all tackes together with stiffeners inside put on. The seams are to be welded inside and out . I do believe there is some overlap on the seams except on the corners. I"ll try to post some pics soon. Question I have is being a certified stick welder I am not used to mig welding since long ago . When doing a long seam do you backstep it that is to say you start a seam go as far as you can than go ahead (not welding) of the seam and start there and weld back to where you stopped . All this to avert warping the sheet metal (12 gauge). Looking for a little headsight please. Hoddykubody

 

[ray66v]

Working on sheet metal in the body shop, we weld in short increments, spacing them evenly across the seam.

Then come back to the first weld, and add to it.

Repeat the process down the line, until the weld is complete.

 

[CharlieTR]

Yes, you must step the welds as you think and like Ray said.
If working inside MIG is good but if working outside you need Flux Core (FCAW). Wind blows mig gas away.

 

[Transit]

Yes, skip weld to control warping, alternate sides too. Tack in some temporary bracing if needed.
12 Ga. [0.100 in] stock calls for 100 amps and 0.030 wire, C-25 gas.
Flux core will run hotter and you will have to cope with chopping slag.
Run a test coupon to fine tune your settings.

 

[Gary Fowler]

Since you said you have to weld inside and out, you may want to think about stitch welding one side like 3 " and skip 12" then go outside and weld the same way but weld in between the areas that you welding on the inside. Keep swapping this back and forth welding 3" or so at a time until you have it completed. This will help equalize the drawing of the weld metal and help keep the seam from buckling. Keep your weld beads as small as you can. There is no need to have a bead. Any more is a waste of weld wire and just puts in excess heat that will cause warpage. Weld it horizontal also will make it easier to carry a smaller weld puddle.