I am planning to weld a heavy frame using "I" beams (size W12 X 31). Wondering if it can be welded with regular 0.35 mig welding wire or should I be sticking to (no pun intended) rods like 7018. Any help would be appreciated. Thank you Rob
I would without a doubt stick weld it with large electrodes and using high amperage. I'd probably borrow or rent a big machine that can run 300A all day at 100% duty cycle.
You can certainly weld that stuff with wirefeed but to my knowledge it should be done with dualshield, and with heavy wire at that. And not with some little MIG machine, either, it takes a large beast to weld stuff like that.
If I had to do that job I'd go buy some Excalibur 5/32" rod and weld it up with that stuff. That is the sweetest running 7018 rod I've ever used, certainly not the cheapest though.
As the other poster said, a lot also depends on what kind of welds. If you're just going to cope 45s and weld up a box frame then sure, stick welds. If you are doing anything with plate being welded to the flat flange, you might consider burning some holes and plug welding with a big dualshield machine.
It depends, Thickness of the flange and web? Open root or backing bar? Back gouged root? What type of loads will the weldment bear? Is this an overhead crane or other high capacity and consequence structure? What type of engineering and QC will be required? What procedure does the certifying engineer require? What joint prep is required? What does the job spec require?
Given enough time and use of external heat $$$ource to preheat and maintain proper interpass temp you can weld any thickness of steel with a (too) small welder, but you will experience increased levels of internal stress buildup and external distortion due to the cumulative effects of the shrinkage of each individual pass. Failure to preheat will be more problematic with a (too) small welder. You will also increase the cost of flux cleaning between passes and a (too) small welder needs better interpass cleaning and may require additional peening to relieve stress. All this costs $$$$$$ and lots of unproductive time. In the real world there are lots of good reasons for big welders and big electrodes.
My personal favorite procedure for slicing beams would be to use a flame cut bevel of about 30 degrees or less depending on plate thickness (to reduce the width of the cap and the quantity of weld required) then a quick pass with a big grinder to clean the slag. I like a very small land just to straighten the edge and fit to the mating beam with very small gap but with a 1/8" backing bar of scrap strip. Depending on the weather I use a big torch to dry and take the chill off the beams before beginning any weld. I can then use (dry) 3/16" 7018 DCRP and do not use any special root pass, this will result in penetration that can readily be seen on the back side of the backing bar. For a crude (non visible) job, the bar does not need to be removed, but for high quality or visible work, after the first couple of passes I use an ArcAir (@ big amps) to remove the backing bar and back gouge the root pass which can be then capped easily. I then complete the cap on the open side, alternating sides in this way tends to equalize shrinkage and limit distortion. Back gouging confirms the quality of the root and simplifies joint fitup and prep. This work is a lot easier if the beam can be rolled and I like to put the beam on an angle so the weld can be made uphill which helps the flux flow away behind the arc.
A level yard and long timber blocks make leveling and rolling much easier. For large beams it is helpful to weld some scrap pipe sections on the beam (over the timbers) to make it easier to roll. For a frame that is too big to roll you will want a crane to help position your welds for welding. This work can be done in position but it is a lot easier, faster and cheaper if the work can be positioned.
I always use an air powered needle scaler to remove flux and peen the beads between passes. IMHO this is a required tool for this type of work.
You will experience arc blow at the edges of the flange and in the corners which depending on the source of the beam can be extreme. The use of a piece of scrap (remove after welding) at the edges will make this area easier but the corners are mostly a fight using different rod angles. Many will suggest using AC to limit arc blow but I only have DC so I just fight it. Some will suggest other tricks like wrapping your ground cable around the beam which will help but IMHE never eliminate the blow.
Fabing the framework will also require other joints and probably fillet welds which are a lot more straight forward to fit and easier to do out of position.
If the beams are less than 1000lbs, or so, Harbor Freight sells 24" adjustable wrenches that fit the bill nicely for rolling. Just dial in to the flange side and heeeeeeeeeeeeave! I picked up a few last year for $20 a piece, and have found all kinds of uses.