Doing some heat coils at the moment nothing fussy no WPS, in 312 SCH40 2", was going to root them all in TIG but just wondering about the cap, go in with an MMA or all the way through with the TIG?
As Al says, bearing in mind your only talking around 4mm wall thickness I’d keep to TIG for all passes, you’ll have much more weld pool control than if you switched to MMA for fill/cap passes
Finally managed to make a start on these, I'm no pipe welder just a jack-of-all that gets by. Tightened the Root gap up from what id have in M/S to help with distortion and have been putting both cap and root in fast. new welder to boot so never the easiest work to get dialled in with but im getting there.
Once there done iv got to prep them into the tanks and make up the M/S saddles, more critical than the coils TBH as there have about 10T of live load each on them.
Lad who works on and off for me has ******** off skiing so I'm on my own, Clearly he's earning too much!.
The procedure we go by at work (i usually get lumbared with all the stainless jobs for the last few years) is 10lpm purge flow throughout procedure with 1.6mm filler rods for root, then step up to a 2.4 after the root if over 4-5mm wall thickness simply to fill out a bit quicker. Feathered edge (deburred) and a 3.2mm gap to allow for closure, 80-90amps for the root then 90-100 (higher range if using 2.4mm filler) for subsequent passes, with a 150deg max interpass temperature. After root and hotpass, number of runs depends on thickness, and single or split cap depends on width of prep and position, and a split cap if wider than 10mm.
As far as setup goes though, youll always want a wider gap with stainless as you cant really "push the root" through the gap using constant pressure on the rod like you can with carbon. It just doesnt work due to not being as fluid as carbon when molten. Youll end up with a flat root at best, but more often concave or not fully penetrated both sides.
You need to "keyhole" it, feeding the rod into the inside portion of the puddle.
That is about the best way of getting a good xray quality weld in stainless.