Finally got a 316L TIG job I'm actually proud of

I had a similar run with 316L piping last month, but my issue wasn't the root pass contamination like yours. Mine was more about maintaining consistency across the entire bead width. Even when the purge looked perfect, I was seeing slight variations in bead profile from one inch to the next, wich is frustrating when you are trying to keep the travel speed uniform. I found that the biggest difference for me was moving beyond just the gas flow setup. I started cleaning the exterior of the joint more rigorously before I even hit the arc. Using a small brass wire brush and then a swipe of acetone on the outside of the pieces always helped. Getting rid of any oils or fingerprints before starting really seemed to stabilize the melt pool and gave me a much more consistent ripple pattern on the face of the weld. For the actual gas, I ran into problems with too much flow creating turbulence in the joint. I found that dropping my setup from 35 cfh down to 22 cfh really improved the tightness of the shielding around the weld area. It makes a difference in how clean the sidewalls stay. It felt like a small tweak that made all the difference in my final quality.

I've found that even when the purge is clean and the bead looks good, 316L can still be finicky if you don't manage the heat input correctly. When I was running fittings last spring, I struggled with stress cracking on the face of the weld, especally near the edges. It wasn't related to the root pass at all. I realized I was setting the heat too high, which is common when you're trying to get nice ripples. I shifted my process to dial back the voltage slightly, maybe by just 2 volts, and focusing more on a slow, steady wire feed rate. This kept the overall heat in the puddle lower, which helped the material relax as it cooled. It made a big difference when I tested samples for ductility.

My shop prefers using a constant voltage setup because it gives me that fine control over the actual heat going into the metal. It's less of a 'rip or tear' situation compared to the pulsed options, which I found me a bit too aggressive for these specific pipe dimensions. It just allows for a smoother transition into the finished weld.

Nice when the lathe does the quality control for you, ha. One thing I'd add on the purge, I started using a dam in the pipe instead of just taping the ends off. Some aluminum foil balled up a few inches back from the joint keeps the argon from just escaping down the bore. Less gas wasted and the root stays shielded way longer while you're working. Made my purge setup time way shorter too.

Not sold on the brass brush advice upthread. Brass is soft enough that it can smear copper onto stainless, and copper traces in the weld zone can cause issues down the road, especially on anything meant for service where corrosion matters. I keep a brush with stainless bristles that only ever touches stainless, and I mark it so nobody grabs it for carbon steel work. A contaminated brush will undo all the acetone wiping you did.

Funny how the parts that get machined away are the ones we obsess over, but I'd add one caveat to that. If the back side gets faced down in the lathe, anything sketchy hiding at the root like lack of fusion or a concave spot can suddenly get exposed mid-cut, and then you're scrambling to explain it to the customer instead of catching it on the bench. I've taken to snapping a flashlight shot down the bore before anything goes near the machine tool, just so I have a record of what the underside actually looked like before it vanished.