Pipe Welding Essentials

Pipe welding is one of the most demanding and well-compensated specialties in the welding trade. Pipe welders build and maintain the piping systems that carry oil, gas, water, steam, and chemicals in refineries, power plants, chemical plants, and industrial facilities. The work demands precision, consistency, and the ability to weld in every position around a fixed joint. This guide provides the detailed technical knowledge you need to begin your career in pipe welding.

Why Pipe Welding Is Different

Unlike flat plate welding, pipe welding presents unique challenges:

• You weld all positions in a single joint. On a horizontal fixed pipe (5G), you start at the top in the flat position, move through vertical, pass through overhead at the bottom, come back up through vertical on the other side, and finish at the top. Your technique must adapt continuously as gravity changes its effect on the puddle.
• The joint geometry is curved. You are welding on a round surface, which means your torch angle, filler rod position, and travel path all change constantly.
• Root pass quality is critical. The inside of the pipe must be smooth with full penetration and no suck-back, icicles, or lack of fusion. On process piping, the root is inspected by radiograph (X-ray) and any defect means cutting out and rewelding.
• Code compliance is mandatory. Pipe welds on pressure systems must meet ASME Section IX, API 1104, or AWS D1.1 code requirements. Every welder must be individually certified.

Pipe Welding Positions

Pipe welding positions differ from plate welding positions:

• 1G (Rolled) - The pipe is horizontal and can be rotated during welding. The welder stays in the flat position. This is the easiest pipe position and is used when the pipe can be put on rollers. Ideal for prefabrication in a shop.
• 2G (Horizontal Fixed) - The pipe axis is vertical and the pipe is fixed. The welder welds horizontally around the joint. The puddle tends to sag downward, requiring careful heat control and a slightly uphill torch angle.
• 5G (Horizontal Fixed) - The pipe axis is horizontal and the pipe is fixed. The welder must weld around the full 360-degree circumference, passing through flat, vertical, and overhead positions. This is the most common field welding position.
• 6G (45-Degree Fixed) - The pipe axis is fixed at 45 degrees from horizontal. This combines elements of all other positions and is the most difficult. A welder who passes a 6G certification test is qualified for all positions. If you can weld 6G, you can weld anything.
• 6GR (6G Restricted) - A 6G test with a restriction ring placed near the joint to simulate a real-world obstruction (such as welding a pipe at a vessel nozzle). This is the most advanced qualification.

Joint Preparation

Pipe joints must be prepared to very tight tolerances for a sound root pass.

Standard V-Groove Preparation

• Bevel angle - 37.5 degrees on each side for a total included angle of 75 degrees. This is the industry standard for most carbon steel pipe welding. The bevel can be cut with a beveling machine, a pipe beveler, or by hand grinding.
• Root face (land) - The flat portion at the root of the bevel. Typically 1/16 to 3/32 inch wide. The land provides material for the root pass to fuse into. Too thin a land burns through. Too thick a land prevents full penetration. Measure with a fillet gauge or rule.
• Root gap (root opening) - The space between the two pipe ends. For a TIG root pass, set the gap at 3/32 to 1/8 inch using gap gauges, welding rod spacers, or specialty root gap tools. Consistent gap around the entire circumference is essential.

Beveling Methods

• Pipe beveling machine - A clamping tool that mounts on the pipe end and rotates a cutting tool around the circumference. Produces the most consistent bevel. Common brands: EH Wachs, ESCO, Reed.
• Hand grinding - Use a 4-1/2 inch angle grinder with a grinding disc. Mark the bevel angle with a bevel gauge or protractor. Grind evenly around the circumference. Check your angle frequently.
• Oxy-fuel torch - For large diameter pipe, an oxy-fuel cutting torch with a beveling attachment can cut the bevel. The cut surface must be ground smooth to remove the oxidized layer.
• Pipe cutting and beveling machine (lathe type) - Clamped internally or externally, these machines cut and bevel in one operation. Used in pipeline construction.

After beveling, clean the bevel face and at least 1 inch of internal and external surface back from the joint with a grinder or wire wheel to remove rust, coating, oil, and mill scale.

Fit-Up

Proper fit-up determines root pass quality.

Step-by-Step Fit-Up Procedure

1. Check pipe ends - Verify that both pipe ends are beveled correctly and are round (not ovalized from handling). Out-of-round pipe creates variable gap and hi-lo.
2. Align the pipe - Use clamps or a pipe alignment tool (internal or external lineup clamp) to hold the two pieces in alignment.
3. Set the root gap - Use gap gauges, filler rod pieces, or special root gap spacers to set a consistent gap around the entire circumference. Check the gap at the 12, 3, 6, and 9 o'clock positions at minimum.
4. Check hi-lo (mismatch) - Hi-lo is the mismatch between the inside diameters of the two pipes at the joint. Use a hi-lo gauge to measure. Maximum allowable hi-lo is typically 1/32 inch. If the hi-lo exceeds tolerance, rotate one pipe relative to the other to find a better alignment, or use a lineup clamp to pull the pipe into alignment.
5. Tack weld - Place at least three tack welds evenly spaced around the circumference (120 degrees apart) for small diameter pipe. Use four tacks (90 degrees apart) for larger pipe. Tacks should be 1/2 to 3/4 inch long, made with the same process and filler as the root pass.
6. Feather the tack welds - Grind the start and stop of each tack to a thin, tapered edge so the root pass transitions smoothly over the tack without creating a defect.
7. Remove the clamp - After tacking, remove the external lineup clamp. If using an internal clamp, it can be removed after the root pass is completed if accessible.

Fit-Up Tools

• Internal lineup clamps (for pipe 6 inches and larger)
• External chain clamps or cage clamps
• Hi-lo gauges
• Root gap gauges or feeler gauges
• Pipe jack stands and V-head pipe stands
• Levels and squares for checking pipe alignment
• Wrap-around pipe marking templates for squaring cut lines

The Root Pass

The root pass is the first and most critical weld pass. It must achieve full penetration through the root gap and produce a smooth, uniform bead on the inside of the pipe.

TIG Root Pass (GTAW)

TIG is the preferred process for root passes because of its precision and control over the puddle.

Setup:

• Tungsten: 3/32" 2% lanthanated, ground to a point with a 2x taper
• Filler rod: ER70S-2 or ER70S-6, 3/32" diameter
• Gas: Pure argon, 18 to 22 CFH
• Cup: #6 or #7 alumina, or a gas lens with a #8 cup
• Amperage: Start at 70 to 90 amps for 3/8" wall pipe. Adjust as needed.

Back Purge:
Before striking the arc, purge the inside of the pipe with argon. Seal the open ends of the pipe with purge dams (water-soluble paper dams, foam plugs, or inflatable bladders) and flow argon inside until the oxygen level drops below 1% (use an oxygen analyzer). Maintain the purge throughout the root and hot pass. Without purge, the inside of the root weld oxidizes and forms "sugar" - a porous, brittle, black crust that is an automatic reject on code work.

Technique - Walking the Cup:

• Rest the ceramic cup on the bevel face and use a rocking, walking motion to advance the torch. Walk the cup alternately on the left and right bevel face, moving forward with each step.
• The tungsten tip stays about 1/8 inch from the root face.
• Watch for the keyhole - a small, teardrop-shaped opening at the leading edge of the puddle. The keyhole indicates full penetration through the root. If you lose the keyhole, you are losing penetration. Slow down or increase amperage.
• Dip the filler rod into the leading edge of the puddle, then pull it back. The rhythm is: walk, dip, walk, dip. Each dip adds a small amount of filler that freezes as a ripple in the bead.
• Move at a pace that keeps the keyhole consistent and the bead width uniform.

Technique - Freehand:

• On smaller diameter pipe or when the cup cannot rest on the bevel (tight clearance), hold the torch freehand.
• Brace your hand against the pipe or use your pinky as a guide.
• The technique is the same: maintain the keyhole, dip filler into the leading edge, and advance at a steady pace.

Starting and stopping on the root pass:

• Start at the top (12 o'clock) on a 5G or 6G joint. Weld downhill to the 6 o'clock position on one side, then return to 12 o'clock and weld the other side down to 6 o'clock.
• At the tie-in (where the two halves meet at 6 o'clock), grind the end of the first bead to a taper and overlap the second bead onto it by about 3/8 inch. Feather the start of the second bead into the taper of the first for a smooth tie-in.
• Alternatively, some welders work from 6 o'clock up to 12 o'clock on each side (uphill root). This gives deeper penetration but requires more skill.

Common Root Pass Defects

• Lack of penetration - No keyhole visible. Root bead does not push through to the inside. Cause: insufficient heat, too much filler, or excessive land thickness. Fix: increase amps, use less filler, or reduce the land.
• Burn-through (melt-through) - Excessive hole in the root. Cause: too much heat, too wide a gap, or too slow travel speed. Fix: reduce amps, decrease gap, or increase travel speed. Use the foot pedal to modulate heat.
• Suck-back (concavity) - The root bead is concave on the inside rather than slightly convex. Cause: too much heat or too little filler. Fix: add more filler and reduce amperage slightly.
• Wagon tracks - Lines of lack of fusion along the edges of the root bead on the inside. Cause: insufficient heat at the edges or feeding filler directly into the center of the puddle. Fix: ensure the puddle wets into both sides of the bevel before adding filler.
• Sugar - Black, porous oxidation on the inside of the root. Cause: no purge or inadequate purge. Fix: always purge and verify oxygen levels before welding.

Hot Pass, Fill Passes, and Cap Pass

After the root pass, additional passes build up the joint.

Hot Pass

• The hot pass is applied immediately after the root pass (while the joint is still warm) to reinforce the root and burn out any minor slag or contamination.
• Can be welded with TIG or switched to SMAW (stick) with E7018 electrodes.
• Run slightly hotter than the fill passes to ensure complete fusion with the root.
• Clean the root pass with a wire brush or light grinding before making the hot pass.

Fill Passes (SMAW with E7018)

• E7018 is a low-hydrogen electrode that produces strong, ductile welds with excellent mechanical properties. It is the standard electrode for pipe fill and cap passes.
• Electrode storage - E7018 electrodes are moisture-sensitive. Store in a rod oven at 250 to 300 degrees F. Exposure to moisture causes hydrogen cracking (also called cold cracking or underbead cracking). Never use electrodes that have been exposed to moisture for more than 4 hours unless re-dried per the manufacturer's instructions.
• Technique - Use stringer beads (straight passes with no or minimal weave). Overlap each pass by about 50%. Maintain a short arc length (about 1/8 inch) and a 5 to 10 degree drag angle.
• Interpass temperature - Check interpass temperature with a contact thermometer or temperature-indicating crayon (Tempilstik). Maximum interpass temperature for carbon steel is typically 500 degrees F per the WPS (Welding Procedure Specification). If the joint is too hot, let it cool before the next pass.
• Number of passes - Depends on pipe wall thickness. A Schedule 80, 2-inch pipe might need 3 to 4 fill passes. A 1-inch wall heavy-wall pipe could need 10 or more.

Cap Pass

• The cap (cover) pass is the final, visible weld. It must meet both structural and visual requirements.
• The cap should be slightly convex (not flat or concave) with even, consistent reinforcement across the full width of the bevel.
• Cap reinforcement height is limited by code - typically 1/16 to 3/32 inch maximum above the pipe surface.
• Edges of the cap must tie into the base metal smoothly with no undercut. Undercut is a groove melted into the base metal along the toe of the weld and is a rejectable defect.
• Maintain a consistent bead width and ripple (wash) pattern for visual quality.

Welder Certification

Pipe welders must hold current certifications to work on code jobs. Certification is not a one-time event. It must be maintained by continued work or retesting.

Codes and Standards

• ASME Section IX - Boiler and Pressure Vessel Code. Governs welder qualification for pressure piping and vessels. The most common code for plant and refinery work.
• ASME B31.1 - Power piping. For steam, water, and other piping in power plants.
• ASME B31.3 - Process piping. For chemical, petrochemical, and refinery piping.
• API 1104 - Standard for welding pipelines. Covers cross-country pipeline construction and in-plant piping.
• AWS D1.1 - Structural welding code. Sometimes applied to pipe supports, structural pipe columns, and hangers.

Certification Test Process

1. You are given a WPS (Welding Procedure Specification) that defines the process, filler metals, joint design, positions, preheat, and interpass temperature requirements.
2. You weld a test coupon - typically two pieces of pipe welded together in the 6G position.
3. The completed coupon is inspected visually per the applicable code.
4. The coupon is then tested by one or more methods:
   • Bend tests - Strips cut from the coupon are bent in a guided bend fixture. Root bends and face bends must show no cracks or open defects exceeding 1/8 inch.
   • Radiographic testing (RT) - X-ray examination of the weld. The radiograph is reviewed by a certified interpreter (Level II or III RT). Any porosity, lack of fusion, cracks, or inclusions exceeding code limits result in failure.
   • Tensile tests - Sometimes required to verify the weld meets minimum tensile strength.
5. If you pass, you receive a welder qualification record (WQR) documenting the process, position, material, and thickness range you are qualified for.

Maintaining Certification

• Most codes require that you have used the welding process within the last 6 months to maintain your qualification.
• If you have not welded pipe with that process in 6 months, you must retest.
• Some employers require annual recertification regardless of activity.
• Keep records of every certification test, including employer, date, code, process, position, material, and test results.

Essential Tools for Pipe Welding

• TIG torch (air-cooled or water-cooled depending on amperage)
• Tungsten electrodes (multiple diameters)
• Tungsten grinder (dedicated diamond wheel)
• Filler rods (ER70S-2 and ER70S-6 for carbon steel)
• SMAW electrode holder (stinger) and E7018 electrodes
• Angle grinder with grinding discs, flap discs, and cut-off wheels
• Wire brushes (carbon steel and stainless steel, dedicated)
• Pipe bevel gauge
• Hi-lo gauge
• Root gap gauge and feeler gauges
• Pipe wrap-around template
• Levels and squares
• Welding blankets and heat shields
• Purge dam materials and argon purge setup
• Oxygen analyzer (for monitoring purge gas)
• Temperature-indicating crayons (Tempilstiks) or contact thermometer
• Chipping hammer
• Fillet weld gauges

Tips from Experienced Pipe Welders

• "The root pass is where you make your money." A clean root that passes X-ray on the first try means no cut-outs, no repairs, and no lost time. Spend the time on fit-up and purge to get it right.
• "Get your body position figured out before you start welding." Walk around the pipe, figure out where you will stand, kneel, or lie down for each portion of the weld. On a 5G joint, you may need three or four different body positions to get around the full circumference.
• "Never assume the purge is good. Verify it." Use an oxygen analyzer. Even a small leak in your purge dam can let enough air in to cause sugaring. If the back side of a root is not bright silver, something is wrong.
• "Keep your E7018 rods dry." Moisture in the electrode coating causes hydrogen cracking that may not show up until hours or days after welding. Always use a rod oven on the job.
• "Practice tie-ins until they are invisible." The tie-in where two beads meet is the hardest part of pipe welding and the place where most defects occur. Feather your starts and stops and overlap by at least 3/8 inch.

Key Takeaways

• Pipe welding requires mastering all positions in a single joint. The 6G position qualifies you for all others.
• Joint preparation and fit-up are precise. Measure the bevel angle, root face, root gap, and hi-lo before tacking.
• Always purge the inside of the pipe with argon. Monitor oxygen levels. Sugaring is an automatic code reject.
• Use TIG for root passes and SMAW (E7018) for fill and cap passes on carbon steel.
• Keep E7018 electrodes in a rod oven at 250 to 300 degrees F. Never use damp electrodes.
• Welder certifications are process-, position-, and material-specific. Maintain them by welding regularly and keeping records.