Welding 3/16" 304 Stainless

[gocirino]

I have a job coming involving welding 3/16" 304 Stainless tanks with some stainless fittings like the one in this pic. I called the manufacturer of the tank and they suggested using 308LSi on my Lincoln 350mp mig. Anyone have any other suggestions on hoe I might attack this? Like process or pre-heat or just using straight 308?
Thanks

[sn0border88]

Pre or post head would be a bad decision, as its will further warp the material and likely contribute to carbide precipitation.

I dont know what kind of advice you want, it can be done just as well with MIG or TIG. I would feel equally comfortable doing it with either one, so just pick what works best for you. As far as wire choice is concerned 308 is fine of this application. If you dont like or have 308 for some reason, 316 would also work well.

[gocirino]

Thanks!

[MoonRise]

As said by the tank maker and by snoborder, and now by me with references from Lincoln and ESAB and others, 308 filler is the 'standard' to use when welding 304 stainless.

The call-out of 308LSi is a variant of 308 with a slightly lower carbon amount (hence the L for "low") than 'plain' 308 and a little bit more silicon (hence the Si for "silicon") than 'plain' 308. Some 308 filler is dual-classed as 308L/308.

309 is also listed as a suitable filler for welding 304 stainless. Again, variants such as L and Si exist. 316 may or may not work.

But if the tank maker has already told you that they recommend 308LSi filler, just get that if you don't already have some. It's pretty common as solid wire filler goes AFAIK. I don't remember off-hand if the 308 wire I have is just 308L/308 classified or if it has the extra silicon for the Si class as well.

Double check the recommended/required shielding gas from the filler maker. Common gases would be 98-2 Argon-O2 for spray transfer and tri-mix for short-circuit transfer.

Running 0.035 solid wire with tri-mix will top out at about 170 amps in short-circuit mode, which seems just a little low for 3/16 material. The 0.045 solid wire with tri-mix gets you up to about 185 amps in short-circuit mode, which is pretty close for the material thickness. If you run the 0.035 wire with 98-2 in spray mode (pulsed or plain), then you can run from 180 to 210 amps, which should cover the 3/16 thickness nicely. Again, amperage recommended depends a bit on the joint configuration but a tank fitting looks like a fillet joint so that usually takes a little more amps than say a butt joint.

Skip-weld and/or using copper chill bars (nickel-plated to minimize possible copper pick up if the arc can reach the chill/backing bars) is usually recommended for welding stainless to keep the heat of the welding zone down and promote rapid cooling of the metal. Time spent at 1200F is most critical as regards carbide precipitation and sensitization, so try and minimize the time the metal is at that temperature. Only a few seconds at 1200F is more damaging from a sensitization/carbide-precipitation standpoint as several minutes at 800F or 1450F. So minimize the heat input into the weld and do NOT use preheat.