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If I'm getting a good mental picture here, you're basiclly doing a socket weld, correct? If this is correct then I normally light up on the fitting (usually heavier) and as you said just let the pipe be washed in to the mix. You'll have to add filler to the pipe side of the puddle to keep it from undercutting and this helps keep the pipe on the cooler side of things. It also helps to have things set up so you can start welding and then get moving. Keep your heat input as low as possible by moving as fast as possible.
As a side note, whyu use a 309 rod instead of a 308/308L? The 309 is usually reserved for dis-similar metals, but will work in your case, not a terrible thing, just wondering.
To the question of does 308L weld better, not better just different, but I think that you'd find that it welds easier on your project because it flows a tiny bit better than 309L. Stainless is inherently sluggish and requires a little manipulation to get it to go where it needs to go as opposed to mild steel. Maybe that's because it's non-magnetic, not sure, just a guess.
Although not required, it wouldn't hurt to have a backpurge anytime you're welding stainless because cooler is better most times, and the Argon or Nitrogen backpurge helps cool the weld as well as sheilding it. Header pipes aren't critical as far as the backside sugaring, but the purge would help for cooling. You can also cool it with compressed air before moving to your next weld to limit heat input.
Using a heavy chunk of metal for a heat sink will also help. If you already have your v-band welded to a plate, weld that plate to a heavier plate, even if it's just carbon steel, your 309 will work perfectly for that.
Oscar wrote:I would ditch the 1/16" filler in favor of 0.045". To tack, use a super quick burst aimed mostly at the thick side. With enough amperage and a sufficiently quick arc on-time, you don't have to "wash" anything. The key is a SUPER tight arc length, around 0.5mm.
That is a 1.6mm tungsten, so you can have a frame of reference.