What's inside
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Scope and safety
A TIG root pass on thin-wall stainless pipe is a fit-up and heat-control job. The goal is a continuous, fully fused root with an even internal bead—not a large, shiny deposit. A small gap change or a few seconds of excess heat can turn a sound joint into a burn-through or a sugared, contaminated root.
The steps below assume a fixed pipe joint welded by a qualified person using a procedure appropriate to the material, service, and applicable code. Pipe carrying pressure, food, pharmaceuticals, or corrosive fluids may require specified purge levels, inspection, and qualified procedures. Do not rely on appearance alone. Wear a properly rated welding helmet, gloves, and protective clothing; control fumes and fire hazards, and keep combustible purge gas safely vented.
Equipment and consumables
A DC TIG machine with a stable low-amp arc, remote amperage control, and high-frequency start is convenient. A foot pedal is useful on a bench; a fingertip control can be easier when access is tight. An older scratch-start machine can do the work, but starting without touching the tungsten takes practice. For a machine purchase, compare DC TIG welders with foot-pedal control by low-current stability, duty cycle, and available service—not peak amperage alone.
Use clean argon for the torch and, when the back of the root is exposed, a separate argon purge. A 2-percent lanthanated tungsten is a practical general-purpose choice; 1.0 or 1.6 mm diameter suits many small-bore, low-current jobs. Keep it sharply ground lengthwise. Use matching stainless filler, commonly 308L for 304 and 316L for 316, unless the drawing or welding procedure specifies otherwise. Filler diameter around 0.8–1.2 mm is manageable on thin wall.
Use a dedicated stainless brush and clean, lint-free wipes. Carbon-steel residue can rust stainless later. A small, properly shielded cup is often easier to place on pipe than a large gas lens. The small-cup TIG gas-lens kit is worth considering if the torch needs more gas coverage at low flow; the included cup sizes and torch compatibility matter.
Fit-up and purge
Cut both ends square, deburr inside and out, and clean the joint at least 20–25 mm back from the edge. Solvents must be fully evaporated before welding. For thin wall, avoid a heavy land: a nearly square edge or a small, consistent bevel is usually easier to control than a broad V-groove. Follow the joint design in the procedure if one is specified.
Clamp the pipe so it cannot move, align the ends, and check the gap all the way around. A useful starting target for many thin-wall joints is a uniform 0–0.5 mm gap, but the correct fit-up depends on wall thickness and procedure. Tack at four points around the pipe, then check alignment again. A tack that bridges an uneven gap can hide a poor fit until the arc reaches it.
If the inside will see corrosive service or the root must be oxidation-free, purge the pipe with argon. Seal the ends with purge dams or tape and provide an outlet so gas can escape; never pressurize a sealed pipe. Allow enough time for air to leave the volume. A purge meter is more useful than guessing from regulator flow. Many procedures specify oxygen below 50 ppm before welding; the job specification takes precedence. Excessive purge flow can create turbulence and pull air into the joint.
Starting settings and trade-offs
There is no universal amperage for “thin-wall” pipe: diameter, wall, joint access, and travel speed all matter. Use these figures only as a test-coupon starting point, then adjust on matching scrap.
| Wall thickness | Starting peak current | Practical note |
|---|---|---|
| About 1.0 mm | 35–55 A | Fast travel and a tight gap help prevent burn-through. |
| About 1.5 mm | 50–75 A | Use pedal control to reduce heat near tacks and stops. |
| About 2.0 mm | 65–90 A | A brief pause may be possible, but do not dwell on the root. |
Start with DC electrode negative, a small sharpened tungsten, and roughly 6–10 L/min (about 13–21 CFH) torch argon as a baseline. Drafts, cup size, and stickout change shielding needs. Too much flow can cause turbulence; too little or poor gas coverage can turn the bead gray or black. A smaller, less expensive TIG machine can be fine for this work if it runs steadily at low amperage and has suitable controls. Higher output is not an advantage if the low end is unstable.
Welding the root
Make a short test weld on matching pipe or a representative coupon and inspect both faces. Set the torch angle close to 90 degrees to the joint, with a slight push angle, and keep the arc short—about 1–2 mm. Position the filler at the leading edge of the puddle. Add small, regular dabs; do not let the filler wire sit outside the shielding gas between additions.
Start on a tack and establish a small puddle that just wets both sides. Move steadily, adding filler only as needed to bridge the gap. On a fixed pipe, the puddle behaves differently overhead than at the top: keep the pool small and reduce current slightly as needed when gravity starts to pull it out of shape. Use the pedal or fingertip control to taper heat as the pipe warms. Pause only long enough for fusion; excessive dwell is a common cause of a sagging root, enlarged hole, or internal icicle.
At each tack, remove any visible contamination or high spot before welding through it. If you stop, clean the crater, feather the tie-in, and restart with a controlled overlap. A cold restart can leave a pinhole; a long restart pause can leave a large crater. Keep the tungsten out of the puddle. If it touches filler or metal, stop, regrind it, and continue with a clean electrode.
Inspection and corrections
After the pipe cools, inspect the outside for a continuous seam, consistent width, and smooth tie-ins. Where accessible, inspect the inside with a mirror or borescope. A modest, even root bead with full fusion is preferable to a tall, lumpy bead. Root color ranges from bright silver to light straw depending on the material and procedure; heavy blue, gray, or black oxide signals excessive oxidation or poor shielding, not merely a cosmetic issue.
Common problems point to specific causes. Burn-through usually means the gap was too wide, current too high, or travel too slow. Lack of fusion often comes from moving too quickly, a dirty edge, or aiming the arc at one side. Porosity can follow oil, moisture, contaminated filler, or disturbed shielding. A sugared root indicates inadequate purge or a leak; for critical service, stop and correct the purge rather than burying the damage with a fill pass. Repair requirements depend on the job specification, so do not grind out and reweld a code joint without the approved repair procedure.
If you are still learning, spend the money on fit-up tools and clean purge equipment before buying a higher-output welder. A basic pipe purge kit with an oxygen meter helps make root conditions repeatable. For noncritical practice, a simple purge arrangement may be adequate; for specified low-oxygen service, verify the reading rather than assuming the purge is good.