GTAW Root Pass on Pipe – Introduction to TIG Pipe

What Is GTAW Pipe Welding?

Gas Tungsten Arc Welding (GTAW), commonly called TIG (Tungsten Inert Gas), uses a non-consumable tungsten electrode to create an arc that melts both the base metal and a separately fed filler rod. Unlike SMAW or GMAW, the welder holds filler in one hand and the torch in the other, demanding coordination and patience. On pipe, GTAW is the preferred process for root passes because it produces the smoothest, most controllable inside-diameter (ID) bead — a quality that matters enormously when pipe carries high-pressure fluid or gas.

This lesson focuses on the 1G position, meaning the pipe rotates on stands while you weld from the top (flat position). This is the most forgiving pipe position and the right starting point before you move to fixed positions.

Equipment and Consumable Selection

Tungsten: For carbon steel with DC electrode negative (DCEN), use 2% thoriated (red band) or 2% ceriated (gray band) tungsten, 3/32-inch diameter. Thoriated tungsten is a mild radioactive material; do not grind it in an unventilated space and dispose of grinding dust per your shop's hazardous-waste procedure. Ceriated tungsten is a common non-radioactive alternative with similar performance.

Prepare the tungsten tip to a tapered point for DCEN. Grind the taper lengthwise — not in circles — so the grain runs with the electrode axis. A contaminated or balled tip destabilizes the arc immediately.

Cup (gas lens and nozzle): A gas lens collet body dramatically improves shielding gas coverage and puddle visibility by producing a laminar (smooth, non-turbulent) flow. Use a cup size large enough to flood the joint — a No. 6 to No. 8 ceramic nozzle is typical for 3/32-inch tungsten on 2-inch pipe.

Filler rod: ER70S-2 is the standard choice for carbon-steel pipe root passes. It contains deoxidizers (titanium, zirconium, aluminum) that help produce clean welds even on mildly contaminated base metal. Start with 1/16-inch diameter for tight root openings and switch to 3/32-inch for wider gaps or fill passes.

Shielding gas: Pure argon (99.99% minimum purity) at approximately 15–20 cubic feet per hour (CFH) at the torch. Confirm the exact flow rate from your Welding Procedure Specification (WPS); that document controls your setup.

Argon Back-Purge: Why It Is Non-Negotiable

When you weld the root pass, the back side of the joint is exposed to atmosphere. At welding temperatures, oxygen combines with the hot steel to form black, porous, brittle iron oxides on the ID surface — the trade calls this sugaring. Sugared root beads fail bend tests, corrode faster, and restrict flow in process pipe.

A back-purge (also called purge backing) floods the inside of the pipe with argon before and during welding, displacing oxygen. Use purge dams (foam, tape, or commercial plugs) to seal both ends of the pipe section, leaving a small vent hole on the exit end. Connect argon supply tubing to the inlet end. A typical back-purge flow rate is 5–15 CFH — enough to displace oxygen without creating backpressure that blows out the puddle. Allow sufficient pre-purge time (at least 30 seconds to 1 minute for short spools) before striking the arc.

You will verify this principle directly: today's back-purge experiment lets you see and document the difference between a purged and an unpurged root bead side by side.

Setting Up the GTAW Workstation

1. Inspect the power source — set polarity to DCEN (work lead to positive terminal, torch lead to negative).

2. Install tungsten — verify diameter, grind taper, and seat it in the collet with approximately 3/8-inch stick-out.

3. Connect and leak-check gas lines — open the argon cylinder slowly, set regulator to the WPS-specified flow, and verify no hissing fittings.

4. Set back-purge — install dams, route purge gas tubing, confirm vent hole, and begin pre-purge flow.

5. Check pipe fit-up — root opening and land dimensions must match the WPS. Tack welds should be ground feathered at their ends so the root pass blends through them smoothly.

6. Sign off the GTAW setup checklist before you strike an arc. This is a professional habit; in industry, a WPS sign-off protects both the welder and the employer.

Welding the Root Pass

Position yourself so you can see directly into the joint. For 1G, the pipe rotates toward you; weld in the 12 o'clock to 2 o'clock zone so gravity helps the puddle lay flat.

Arc length: Keep the tungsten tip approximately 1/16 to 1/8 inch from the base metal — roughly equal to the tungsten diameter. A longer arc widens and flattens the puddle, reducing penetration control. A shorter arc risks tungsten contamination.

Feeding filler: Dip the rod into the leading edge of the puddle in a steady, rhythmic motion — never wave it through the arc. The arc melts the base metal; filler fills the joint. Keep the rod angle low (roughly 15° off the pipe surface) so it does not block your view.

Puddle visibility: You should see a clear, bright keyhole at the leading edge of the puddle — a small, elongated opening at the root that confirms full penetration. If the keyhole closes, slow travel or add more heat. If it grows too large, speed up or reduce amperage slightly.

Travel speed and amperage: Typical starting parameters for 3/32-inch tungsten on 2-inch Sch-80 pipe with a 1/16-inch root opening are in the range of 60–90 amperes DCEN. Your WPS specifies the qualified range — use it.

Safety on This Lesson

• PPE is mandatory: full leather gloves, leather jacket or sleeves, welding helmet at minimum shade 10–11, and safety glasses under the helmet.
• Ventilation: GTAW produces less fume than SMAW, but fumes from hot metal and any coatings are still hazardous. Use local exhaust ventilation at the workstation.
• Thoriated tungsten grinding: use a dedicated grinder with a local exhaust hood; collect and label dust.
• Compressed gas cylinders: chain or strap all cylinders upright to a fixed support. Open valves slowly. Never use cylinders without a regulator.
• Hot metal: pipe stays hot long after welding. Use pliers or tongs; mark hot work with soapstone.
• Lockout/tagout: if the power source or gas system requires maintenance during class, follow your shop's LOTO procedure before any work on equipment.