Never welded anything close to 12mm myself before....would be better to have confirmation from someone who has.....I have a 200A machine but only 2.5mm rods so I can't even have a go without getting another box of rods!
I did come across this thread when I was looking for my MMA machine....he seems to be using 130A on 10mm plate....
It’s certainly pushing your machine to its limit but if you’re doing it you’re probably going to want to use argoshield heavy for the little extra co2 content, I wouldn’t use pure co2 as spatter would be significant.
I have welded many pieces together that have been thicker,
The 5 inch riser on my mill consists of two pieces of 30mill thick ring, with a top and bottom piece welded on, all done with a 215A mig, welded both inside and outside, both chamfered first,
For the use I am putting on it, it’s stronger that the cast iron base it’s bolted to.
for 8mm to 12mm plates, to get the penetration you need with MIG you'll want 200 amps at a minimum, 250 to really make sure it's in there at the root, and proper fusion is attained consistently. It's just science... see the Miller weld calculator.
You'll probably get away with 200amps because there will be a safety factor in the weld and material, but if it's for something structural that your ass is on the line for, you need a 250amp machine or bigger. It's not just the amps, its making sure the machine has the voltage to drive those amps to get the weld bead profile in there properly. If it doesn't have the volts, then you'll get a ropey bead which is no good to anyone.
You could definitely do that with a 160amp MMA source, 4mm rods in 1 pass would do it, you'll be shooting for a 8mm weld leg length. It will be slower than MIG, of course.
It depends... you maybe have a feeling of how important a job this is, and how happy you are with your MIG and skills to put a decent quality weld in there... it's on the edge of doable so no-one can definitely say whether it's ok or not ok, without seeing some proper weld tests
My two cents worth.
Most of the above comments are aimed at one pass only, then I agree, well almost. 8mm SHS to 12mm plate, to get full penetration in one pass you will need quite a powerfull machine, and not 250amps. However if you 'V' prep the SHS and have a multi run fillet there is absolutely no problems. You can do this with .8mm wire no problems, but multi runs. The best process will be to get a sample of what you are expecting to do, do the prep, the multi runs, cut, check and etch the sample then you will know how good it is. Most of us that weld a bit do this from time to time to confirm what we are doing is good and correct. Without a sample check no one can tell you what you are doing is 'good'. Just to give you an idea, I weld 10mm to 10mm and the machine is showing on the amps scale, average 330amps on 1.2mm wire. Hope this helps
I've read of some people getting really good results with some pre-heat on steel - basically everything flows smoother, wetting of weld toes is better, and penetration increases. I do it myself on aluminium MIG because cold alum sucks the heat out of your weld starts so fast that you get a horrible lumpy cold start.
My two centimes..
I would get a 200 amp stick welder and some practice. I expect you could pick up the basics pretty quickly and add to your skill set for the future. I have an old 160amp transformer MMA that I used for exactly that. The 150 amp mig did all the light stuff, up to 4mm with prep. Above that, stick.
I now have a bigger Mig, but the MMA still gets used occasionally.
KF
If getting a bigger set or moving to a stick set is out if the question, make sure all the milk scale is off both pieces of metal, at 200amps and in 0.8mm wire you should get it running quite hot so on a bevelled edge and multi passes you should be fine. A bit of pre heat won’t hurt either if you have access to oxy/acet or propane