Root Pass, Hot Pass, Fill & Cap: Multi-Pass Pipe Welding Sequence

Every pipe weld is a sequence of four pass types: root, hot pass, fill, and cap. Each pass has a specific job, and doing them right in order produces a weld that passes visual, bend testing, and NDE. The root establishes penetration. The hot pass cleans and reinforces the root. Fill passes build up the joint. The cap provides the finished surface. Skip or rush any pass and the whole weld suffers.

This sequence applies to all pipe welding positions (1G through 6G) and to most groove welds on plate as well. The technique adjustments described below are for positional pipe work, where gravity complicates every pass.

Root Pass

The root pass is the most critical pass in any pipe weld. It determines whether the joint has full penetration. Inspectors check the root first, and a bad root means a failed test or a cut-out in production.

Joint Preparation Review

Before the root pass, verify fit-up:

• Root opening: 3/32 to 1/8 inch, consistent around the circumference
• Root face (land): 1/16 to 3/32 inch
• Hi-lo mismatch: under 1/16 inch at any point
• Tack welds: feathered or ground to blend into the bevel

If any of these are off, fix them before welding. See the fit-up tolerance guide for acceptable limits.

TIG Root (GTAW)

TIG produces the cleanest root and is the industry standard for pressure pipe and code work.

Settings:

• Process: GTAW, DCEN
• Tungsten: 3/32 or 1/8 inch, 2% thoriated or 2% lanthanated
• Filler: ER70S-2 or ER70S-6, 1/16 or 3/32 inch diameter
• Gas: 100% argon, 15-20 CFH
• Amperage: 70-95 amps (adjust by position)

Keyhole Technique:

The keyhole is a small hole that forms at the leading edge of the weld puddle when you have full penetration. Molten metal fills the keyhole from behind as you advance. If you can see the keyhole, you know the root is fusing through to the inside of the pipe.

• Establish the keyhole at the start: hold the arc in one spot until the root melts through and a small opening forms
• Advance by feeding filler into the trailing edge of the keyhole
• The keyhole should be about 1/16 inch in diameter, roughly the width of the root opening
• If the keyhole grows: add filler faster, reduce amperage, or speed up
• If the keyhole closes: slow down, increase amperage, or add less filler

Position-specific adjustments:

Stick Root (E6010/E6011)

E6010 roots are standard for pipeline work and some structural pipe.

Settings:

• Electrode: E6010, 1/8 inch, DCEP
• Amperage: 75-90 amps

Whip and Pause:

Instead of a continuous keyhole, stick root welding uses a rhythmic whip-and-pause technique:

1. Strike the arc and establish the puddle in the root
2. Watch for the keyhole to form
3. Whip the electrode forward 1/8 inch (toward unwelded joint)
4. Pause: let the puddle fill the keyhole
5. Whip forward again
6. Repeat at a consistent rhythm

The whip length controls penetration. Longer whips reduce heat input and can cause lack of penetration. Shorter whips increase heat and can cause burn-through.

Root Pass Quality Checks

After completing the root:

• Check the inside of the pipe (if accessible) for uniform root reinforcement
• Root should be 1/32 to 1/8 inch of reinforcement on the inside
• No concavity (suck-back), which indicates excessive heat or thin land
• No icicles or melt-through drops
• No visible porosity
• No wagon tracks (two parallel beads with a valley between them)

Hot Pass

The hot pass gets its name from the timing: run it while the root is still warm. Don’t let the root cool to room temperature.

Purpose

1. Burns out contamination at the root toes where slag, oxides, or gas can get trapped
2. Re-melts the root toes to ensure complete fusion at the edges of the root bead
3. Builds the first substantial layer that fill passes will stack on
4. Refines the grain structure at the root boundary by providing a second thermal cycle

Technique

• Switch from TIG/E6010 to E7018 for a dual-process procedure
• Amperage: slightly higher than the root pass (95-115 amps for 3/32 inch E7018)
• Slight weave: 1/8 inch side to side, pausing at each toe
• Travel speed: moderate. Fast enough to prevent excessive buildup but slow enough for complete fusion
• Arc length: tight, about 1/8 inch for E7018
• Clean the root thoroughly before the hot pass: wire brush (TIG root) or chip and brush (E6010 root)

Common Hot Pass Problems

Slag inclusion between root and hot pass. Caused by incomplete cleaning. Chip every bit of slag from the root before the hot pass.

Undercutting the root toes. Too much heat or too fast. The hot pass should tie into the root toes, not erode them.

Delayed hot pass. If the root cools completely before the hot pass, you lose the metallurgical benefit. On a 6G test, this happens if you take too long on the root or stop for a break between passes.

Fill Passes

Fill passes build the groove up from the hot pass to within 1/16 to 1/8 inch of the pipe surface. This is the bulk of the weld volume.

Bead Placement

Run stringer beads across the groove, each one overlapping the previous pass by about 50%. Start each layer on the bottom bevel face and stack across:

1. First bead: along the bottom bevel face, angled into the bevel to ensure sidewall fusion
2. Second bead: overlapping the first, filling toward the top bevel face
3. Third bead (if needed): along the top bevel face, completing the layer
4. Repeat for additional layers until within 1/16 to 1/8 inch of the surface

Settings

• Electrode: E7018, 3/32 or 1/8 inch, DCEP
• Amperage: 90-130 amps depending on rod size and position
• Arc length: tight (one rod diameter or less)
• No weaving wider than 2.5 times the electrode diameter

Critical Rules

Clean between every pass. Chip all slag, wire-brush the bead, and inspect for defects before the next pass. Slag inclusions between fill passes are a leading cause of NDE rejection.

Keep beads flat to slightly convex. Concave fill beads create valleys that trap slag. Highly convex beads create undercut at the toes and make the next pass harder to place.

Watch interpass temperature. Maximum 600F for carbon steel. Let the joint cool between passes if it’s getting too hot. A contact thermometer is more accurate than feeling the pipe.

Stagger starts and stops. Don’t start every fill pass at the same clock position. Offset by at least 1 inch between passes to avoid stacking start/stop defects.

Fill Pass Quality Checks

• Each bead fuses into the sidewall (no undercut, no cold-lap)
• Consistent width and profile
• All slag removed before the next pass
• No visible porosity or cracks
• Fill level is uniform around the circumference

Cap Pass

The cap is the final pass. It determines the visual quality and the stress distribution at the weld toe.

Profile Requirements

Technique

Overhead section (6 o’clock): Narrow stringer beads, low heat, fast travel. The cap wants to drip in overhead. Keep beads narrow and build the cap from multiple stringers if needed.

Vertical sections (3-9 o’clock): Slight weave, pausing at each toe. The weave fills the groove width. Match the weave width to the groove width so the cap extends just past each bevel edge.

Flat section (12 o’clock): Wider weave, more filler. Gravity holds the puddle, so you can fill more aggressively. Watch for excessive buildup; it’s easy to overweld at the top.

Cap Weld Tie-In

Where the cap meets itself (the final tie-in), grind a gentle taper on the existing cap bead and weld over it. The transition should be smooth with no visible overlap or undercut. This is the most scrutinized spot on a test coupon.

Cap Quality Checks

• Uniform width all the way around
• Consistent reinforcement height (no lumps or thin spots)
• Smooth toe transition (no undercut at either toe)
• No surface porosity
• No cracks (check with a flashlight at a raking angle)
• Clean, uniform ripple pattern

Common Mistakes

Rushing the root. Speed on the root pass is not your friend. A clean root takes time. Five extra minutes on the root saves you from cutting out and re-welding the entire joint.

Skipping the hot pass. Some welders go straight from root to fill. This leaves contamination at the root toes that shows up on NDE. Run the hot pass.

Fat fill passes. Big weaves look productive but trap slag at the toes. Stringer beads, each no wider than 2.5 times the rod diameter.

Inconsistent cap profile. A cap that’s tall at 12 o’clock and thin at 6 o’clock shows the welder didn’t adjust for position. Aim for the same height and width all the way around.

Not cutting practice welds open. The only way to know if your root is good is to cut it and look. Surface appearance doesn’t tell you what’s happening on the inside. Cut, etch with nitric acid, and examine the cross-section.

For pipe position details, see pipe welding positions. For test-specific preparation, read the 6G pipe welding test tips. For groove weld dimensions on plate, see the single-V groove weld procedure. Return to pipe welding or the welding techniques pillar for the full topic list.