Before we get too deep into 3g mig welding test techniques...
A word about short circuit mig.
Short circuit mig is properly called GMAW-S.
GMAW stands for gas metal arc welding - and the s is for short circuit.
The term "short circuit" indicates the way the metal melts into the puddle....many times per second, the wire actually short circuits into the puddle before arcing back and then short circuiting again.
Most structural welding is done using flux core (fcaw), stick (smaw), or even gmaw spray transfer.
Thats because short circuit mig requires contractors to weld extra procedure qualification tests while smaw and fcaw allow for using pre-qualified procedures.
Its complicated, but the simple reason all the extra tests are required because short circuit mig is the most likely process to cause cold lap.
Short circuit mig is used for pipe welding more than for structural and in preparation for pipe welding, it makes sense to weld plate first because its easier and cheaper, and it is good practice especially when using the same bevel, gap, land, etc. because the exact same settings work.
A common Mig welding certification test is a 3g Mig along with a 4g Mig on 3/8" thick plate.
This video is about the 3g mig welding certification test...
the 4g overhead plate test is coming soon.
When a 3g vertical and 4g overhead plate are done together, all positions are qualified according to most AWS codes...thats a lot of bang for the buck.
2 plate test welds and you get qualified for flat, horizontal, vertical, and overhead positions.
So...Here are some tips for passing a 3g mig plate test...
• Clean the mill scale to 1/2" away from the weld...clean to bright shiny metal. (in the real world, you might not clean as much, but for a test, it helps)
• grind or file a 1/16" land on the bevels ( some prefer no land at all)
• gap the pieces 1/8" plus or minus 1/32"
• mig machine ( millermatic 250) settings go something like this...
• 19 volts , 245 ipm, 25 cfh of argon/co2 75/25
• 245 ipm wire feed speed ( turned it up to around 275 ipm for second pass)
• Make the tack welds large enough so that when you light up on the tack, you dont lose the gap.
• Stay on leading edge of puddle for all passes.
• Second pass technique is to "trace" the leading edge of the puddle.
• Second pass should be 1/16" below flush in order for the cover pass to go in well.
• let cool before cover pass
• Cover pass should be equal to or less than 1/8" above flush.