TIG Welding Pipe: Root Pass Technique, Keyhole Method & Purge Setup

A TIG root pass on pipe is the highest-skill manual weld in the trade. You’re welding an open-root groove joint with full penetration, maintaining a keyhole at the leading edge, feeding filler into a tiny puddle, and adjusting for position changes as you move around the pipe circumference. Every variable matters: root gap, land thickness, bevel angle, amperage, travel speed, filler timing, and gas coverage.

The reward for getting it right is a smooth, fully penetrated root bead that passes X-ray inspection. The root pass sets the foundation for every subsequent fill and cap pass. A bad root causes defects that can’t be fixed from the outside.

Joint Preparation

Standard V-Groove for Pipe

The standard pipe bevel for TIG root welding is:

• Bevel angle: 37.5 degrees per side (75 degrees included angle)
• Root face (land): 1/16" to 3/32" (the flat area at the root of the bevel)
• Root opening (gap): 3/32" to 1/8"

Beveling Methods

Machine bevel: Pipe beveling machines produce consistent angles. This is the standard for code work.

Grinding: A flap disc on an angle grinder works for small-diameter pipe and repair work. Mark the bevel angle with a protractor and grind to the line. Less consistent than machine beveling.

Cutting torch with bevel attachment: Common in the field. Produces a rough bevel that needs grinding to clean up the oxide and smooth the cut face.

Root Gap Consistency

Consistent root gap is critical. Variations in the gap cause inconsistent penetration and keyhole behavior. If the gap narrows, the root closes up and you get lack of fusion. If the gap widens, you burn through.

Set the root gap with spacer wire or welding rod of the correct diameter. Tack with the spacer in place, then remove it before final welding. Use four tacks evenly spaced around the circumference, each about 1/2" to 3/4" long.

The Keyhole Technique

The keyhole is a small opening at the leading edge of the root weld puddle. It appears when the arc melts completely through the root face and root gap, creating a visible hole that travels ahead of the solidifying weld metal.

Establishing the Keyhole

1. Strike the arc on a tack weld (never start in the root gap itself).
2. Build up amperage gradually using the pedal or fingertip control.
3. Watch the root area. As the heat builds, the root face edges melt and a small opening appears. This is the keyhole forming.
4. The keyhole should be about the diameter of your filler rod (3/32" to 1/8").

Maintaining the Keyhole

Once established, the keyhole moves ahead of the puddle as you advance along the joint. Your job is to keep it a consistent size:

• Keyhole too large: Too much heat. Reduce amperage or speed up travel. If the keyhole grows to more than twice the filler rod diameter, you’re about to burn through.
• Keyhole too small: Not enough penetration. Increase amperage slightly or slow down. If the keyhole closes completely, you’ve lost full penetration and the root won’t pass X-ray.
• Keyhole disappearing and reappearing: Travel speed is inconsistent. Smooth out your advance rate.

Filler Addition in the Keyhole

Feed filler into the top (trailing edge) of the keyhole. The filler melts and flows into the keyhole from behind, closing it as you advance. The timing is: advance the torch slightly, the keyhole opens at the leading edge, dip filler to close the trailing edge. The result is a smooth, fully penetrated root bead.

Don’t dip filler into the center of the keyhole. This blocks the arc and closes the keyhole before it’s ready, creating a cold lap or lack of fusion on the root face.

For detailed filler dipping rhythm and coordination, see TIG dip technique.

Pipe Welding Positions

1G Roll

The pipe rotates while the torch stays stationary at the 12 o’clock position. The easiest position because you always weld flat. Used for shop fabrication where the pipe can be mounted on rollers.

Amperage stays constant. Travel speed stays constant. It’s the best position for practicing root passes because the only variable is your technique.

2G Fixed (Horizontal Pipe, Vertical Axis)

The pipe is positioned with its axis vertical. You weld around the circumference in a horizontal plane. Gravity pulls the puddle downward, so angle the torch slightly uphill (10-15 degrees push angle from the weld direction).

Amperage stays relatively constant around the circumference because gravity effects don’t change. This is the first fixed-position pipe test most welders attempt.

5G Fixed (Horizontal Pipe)

The pipe axis is horizontal. You weld from the top (12 o’clock) down each side to the bottom (6 o’clock). This is the most common field position for pipeline work.

The position changes from flat (12 o’clock) to vertical downhill (3 and 9 o’clock) to overhead (6 o’clock) as you progress. Amperage adjustments are continuous.

6G Fixed (45-Degree Pipe)

The pipe is fixed at 45 degrees from horizontal. Every position on the circumference is a unique combination of angles. This is the hardest pipe welding position and the most respected certification test.

6G root passes require constant adjustment of amperage, travel speed, torch angle, and filler timing. There’s no “set it and forget it” section. Many welders use a fingertip torch control instead of a foot pedal for 6G because they need to position their body in ways that make a foot pedal impractical. See foot pedal vs. fingertip TIG.

Back Purge Setup for Pipe

Back purging floods the inside of the pipe with argon to protect the root side of the weld from oxidation. On carbon steel, purging produces a cleaner root. On stainless steel, purging is mandatory.

Simple Dam Method

1. Cut two pieces of cardboard or foam slightly smaller than the pipe ID.
2. Tape or friction-fit them inside the pipe, 4-6 inches on each side of the weld joint.
3. Poke a hole in one dam for an argon inlet fitting.
4. Poke a small vent hole in the other dam.
5. Flow argon at 5-10 CFH until the cavity purges (several minutes for initial fill, depending on pipe diameter and cavity volume).
6. Reduce flow to 3-5 CFH during welding to maintain positive pressure.

Inflatable Purge Dams

Commercial purge dams are inflatable bladders that seal the pipe bore precisely. They’re faster to set up and create a more reliable seal than cardboard. Available in standard pipe sizes from 1" to 60"+.

Water-Soluble Purge Paper

For stainless and alloy pipe, water-soluble purge paper creates dams that dissolve when the system is flushed after welding. No need to remove dam remnants from inside the pipe. Required on pharmaceutical, food-grade, and some chemical piping systems.

Oxygen Monitoring

For critical work, use an oxygen analyzer to verify purge quality before striking the arc.

• Carbon steel general: Below 1% O2 (10,000 ppm) produces a clean root
• Stainless structural: Below 500 ppm
• Stainless food-grade / pharmaceutical: Below 100 ppm, preferably below 50 ppm
• Titanium: Below 50 ppm, preferably below 20 ppm

For detailed purge techniques, dam construction, and oxygen threshold explanations, see back purging stainless TIG welds.

Root Pass Settings

These are flat-position starting points. Reduce amperage for overhead sections as described in the 5G position table above.

Fill and Cap Passes

After the root pass, the groove is filled with additional passes and capped with a final cover pass.

Hot Pass

The first fill pass after the root, sometimes called the “hot pass.” Its purpose is to wash out any minor root pass defects and build up a convex base for subsequent fill passes. Run slightly hotter than the root (10-15% more amperage) with wider oscillation to fuse into both bevel faces.

Fill Passes

Build up the groove with stringer beads or slight oscillation. Each pass should fuse into the previous pass and both bevel faces. Stringer beads produce lower heat input than wide weave passes, which matters on stainless and alloy pipe.

TIG fill passes on thick-wall pipe are slow. Many shops switch to MIG (GMAW), stick (SMAW), or flux-core (FCAW) for fill and cap passes after a TIG root. This combination is standard in pipeline and power plant work.

Cap Pass

The final visible pass. On pipe, the cap pass is often applied using the walking the cup technique for consistent appearance. The cap should extend 1/16" to 1/8" beyond the bevel edges on each side and show uniform width and height around the circumference.

Tack Welding on Pipe

Tack placement and quality matter on pipe because every tack becomes part of the root pass. A bad tack creates a defect that propagates through the entire root bead.

Tack Procedure

1. Set the root gap with spacer wire (3/32" or 1/8" diameter)
2. Align the pipe so the bevels match and the high-low (mismatch) is less than 1/32"
3. Place four tacks at 12, 3, 6, and 9 o’clock positions
4. Each tack should be 1/2" to 3/4" long with full root penetration
5. Feather (grind) the starts and stops of each tack so they blend into the root pass

High-Low Misalignment

High-low (also called mismatch or offset) is when one pipe edge is higher than the other at the root. More than 1/32" of mismatch makes the root pass inconsistent because the keyhole forms unevenly. Fix high-low before welding by adjusting the pipe alignment and re-tacking.

Common Root Pass Problems

Incomplete Penetration

The root bead doesn’t fully fuse through the joint. Causes:

• Root gap too narrow (increase to 1/8")
• Root face (land) too thick (grind to 1/16" to 3/32")
• Amperage too low (increase 5-10A)
• Travel speed too fast
• Filler rod diameter too large (absorbing too much heat)

Excessive Penetration (Icicles)

The root bead sags through to the inside of the pipe in droplets or “icicles.” Causes:

• Root gap too wide
• Root face too thin (or nonexistent from over-grinding)
• Amperage too high
• Travel speed too slow
• No back purge (argon pressure inside the pipe supports the root slightly)

Root Concavity (Suck-Back)

The root bead is pulled inward, creating a concave root surface inside the pipe. Caused by excessive heat pulling the puddle through surface tension, insufficient filler, or no back purge pressure. Add more filler at each dip and reduce amperage slightly. Back purge gas pressure helps support the root.

Wagon Tracks

Two lines of lack of fusion running along the root, visible on X-ray as parallel linear indications. Caused by the arc not fusing into both root faces. The keyhole was present (full penetration) but the puddle didn’t wet into the sidewalls. Widen the keyhole slightly by slowing travel, and direct the arc more toward the root faces rather than straight through the gap.

Porosity at Tack Locations

Tacks that weren’t properly cleaned or feathered trap contamination that creates porosity when the root pass melts over them. Grind tack starts and stops smooth, and wire brush the tack surface before welding over it.

Safety for Pipe Welding

Pipe welding often involves confined spaces, elevated work, and awkward body positions. Ensure adequate ventilation in all pipe welding environments. When working inside vessels or enclosed spaces, forced air ventilation and gas monitoring are required.

Pipe welding in the 5G and 6G positions puts you in sustained awkward postures. Take breaks to prevent repetitive strain. Knee pads and welding mats make floor-level pipe work more sustainable over long shifts.

When back purging, be aware that argon displacing air inside a pipe or vessel creates an oxygen-deficient atmosphere. In enclosed spaces, this is a suffocation hazard. Never put your head inside a purged pipe or vessel without confirming oxygen levels are safe (19.5-23.5% O2).