Weaving on pipe tie-ins - overkill or good practice?

For thick wall carbon pieces like the 6 inch pipe fittings I run at the yard, my process is always stick to the stringer method on the root and the cap. When you are dealing with that much heat soak, letting the puddle move around unnecessarily just guarantees you're going to burn thru the root or have lack of penetration by the end. I ussually start with a slow, deliberate progression along the weld line, moving maybe six inches at a time before stepping up to the next pass. I found that trying to weave when the plate thickness is over three-quarters of an inch just spreads the thermal energy too thin across the joint. You end up with inconsistent fusion and alot of slag to clean later. For my usual work on steel supports, I just run straight lines all the way around and then ramp up the amperage slightly on subsequent passes to ensure full weld saturation. It has nothing to do with the shielding gas. wheather you are purging or not, the physics of the molten metal don't change. You need that heat concentrated to push the weld profile out correctly. If you are running stringers into a tie-in, make sure your travel speed is low enough that you don't undersize the fillet before you even start the full joint.

I get where the stringer method is coming from when you're dealing with that massive heat input on a 6 inch piece, but I’ve found that absolute straight stringers on these tie-ins can sometimes leave a wierd undercut on the inner face if you aren't hyper focused on your fit-up. When I run these through my shop, I’ve learned that a true full weave isn't the answer, but a little controlled overlap is. It keeps the weld from getting too narrow at the joint face while still managing the heat soak. My personal trick is to treat the tie-in like a controlled series of tiny, overlapping scallops. Instead of a full circular weave that forces you to pause and wait for cooling on each section, you run the bead for maybe six or seven inches, then slightly angle the next bead to overlap the tail of the previous one by about half an inch before running it back along the original line. This gives the puddle the chance to spread a little and fill any micro-gaps without you spending forever waiting for thermal dissipation.

This approach lets the puddle move enough to make sure the toe of the weld is properly blended into the existing metal, which prevents those stress concentrations you want to avoid in critical structural joints. It’s about letting the heat envelope move slightly into the tie-in area during the run, rather than just trying to keep the weld perfectly confined to the centerline. It’s just a bit of a compromise that works for the specific thickness I’ve been running lately.

I fought sagging tie-ins on heavy wall for years til I switched to a bigger gas lens cup on my torch. That was the thing that actually fixed it for me. I could walk a stringer into the tie-in cold and the puddle would just sit there tidy instead of dropping in on me. On carbon I don't bother purging, just a dam and decent fit-up does it for me. Curious what cup size and gas flow you're running now, because bumping mine up made more difference than anything I changed with technique.

With the guy who walks a stringer into the tie-in cold, that's how I've done it on heavy wall too. The difference for me was adding a cool-down pause right before I close it up. I'll stop a couple inches short, let the pipe cool till I can rest a bare knuckle near the joint, then come back and finish it in one steady pass. On 6 inch that last inch used to turn grey on me every time until I started doing that. Purged or not it works the same for me.

One thing that helped me close heavy wall tie-ins cleaner was dropping my amps about 10 percent for the last inch instead of changing my technique. I run around 115 on 6 inch sch 80 normally, then ease off the pedal when I'm coming up on the stop. The puddle tightens up and you don't have to rush it. I still do a short little weave on the close, maybe two filler widths wide, just to wash into the crater. Try it on a scrap coupon first and watch how the color comes out compared to your normal setting.