Despite what the codes may allow, some customers want more than the minimum ASME requirement. Stick is used for root passes in the field, out of position roots and manual pipeline welding. MIG root passes wouldn't be possible in globular or spray transfer. Usually around 18 volts and about 110 to 125 amps which is for sure short circuiting transfer. I think MIG short circuit transfer is allowed for root passes when it is used in conjunction with another process such as stick, sub-arc or flux-core. MIG root is fairly fast when roll welding and goes in almost as nice as TIG roots. On large diameter, thick vessels, you spend way more time grinding the tacks out and feathering the root than you do putting the root in.
I did a root pass for the head of a 2 3/4" vessel that had a zillion tacks to hold everything in position. Some of them were under a lot of force in order to match the shell to the head. They aren't just tacks though. They were pieces of 1/2" round bar in the bevel about 2" long that are welded on each side that have to be ground out. They are done this way so you don't burn the edge of the bevel prior to the root pass. Preheat was also used because of the thickness. There is generally no land for MIG roots but for a test guys will often use about a 1/16" to 3/32" land. 3" and above uses a 3/16" gap rod and smaller uses 5/32" gap rod. Big vessels sometimes use a 1/4" gap rod. On the same vessel, they ordered the wrong size repad for a nozzle. The nozzle was 20" dia. but the repad was cut for a 24" nozzle. Being that the repad was 2 1/2" thick, they didn't want to pay for another one so we had to fill it up with 7018, lots and lots of 1/4" 7018!!! It had to be done with stick. The vessel was 2 1/2" thick beveled for full penetration and the repad was also beveled for pull penetration. That's 5" thick of weld averaging about 3" wide around a 20" diameter. It took 2 welders on 2 shifts over 14 hours to weld it up. Once the shell portion was welded, it had to be ground flat for the repad. After the repad was welded, it had to be ground flat with only a 3/8" fillet weld showing on the nozzle. You sure didn't want to build it too much or you'd have a lot of grinding to do. When it was finished, you couldn't tell the weld went out almost 4" at the top. That was the biggest weld I ever worked on and it was hot sitting up there for hours on end. The vessel weighed 50 to 60 tons.
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