How Do I Weld a Steel Flange onto the End of a Pipe?

Welding a steel flange onto a pipe is a common fabrication task in workshops, farms, and industrial settings across New Zealand. Done correctly, it produces a strong, leak-free joint suitable for piping systems carrying water, air, hydraulic fluid, or other media. Done poorly, it can result in a weak weld, distortion, or a joint that fails under pressure — so preparation and technique matter enormously.

Understanding Flanges and When You Need One

A flange is a flat disc or collar welded to the end of a pipe, allowing it to be bolted to another flange, a valve, a pump, or a piece of equipment. The two most common types you'll encounter in NZ fabrication work are:

• Slip-on flanges – slide over the outside of the pipe and are welded with two fillet welds (inside and outside).
• Weld-neck flanges – butt-welded directly to the pipe end with a full-penetration groove weld; stronger and better suited to high-pressure applications.
• Socket-weld flanges – the pipe inserts into a socket in the flange and is fillet-welded around the outside.

For most DIY and light-industrial jobs, slip-on flanges are the most forgiving and practical choice.

Safety First

Before you strike an arc, take these precautions seriously:

• Wear an auto-darkening welding helmet (minimum shade 10), leather gloves, a welding jacket, and steel-capped boots.
• Ensure adequate ventilation — welding galvanised or coated steel produces toxic fumes. Work outdoors or use extraction. See WorkSafe NZ for guidance on welding fume hazards.
• Keep a fire extinguisher nearby and clear flammable materials from the work area.
• Never weld on pipe or vessels that have contained flammable liquids without thorough cleaning and purging.

What You'll Need

• MIG welder (GMAW) or stick welder (SMAW) — MIG is easiest for most DIYers
• Appropriate filler wire or electrodes (e.g. ER70S-6 MIG wire or E6013/E7018 stick electrodes for mild steel)
• Angle grinder with grinding and flap discs
• Steel wire brush
• Pipe clamps or a welding positioner
• Combination square and marker
• Weld-through primer or anti-spatter spray (optional)

Step 1 — Prepare the Pipe End

Square the pipe end using an angle grinder or pipe cutter so it is perfectly flat and perpendicular. Any angle or burr here will cause the flange to sit crooked. Clean back at least 25 mm from the end, removing mill scale, rust, paint, or grease with a flap disc and wire brush. Contamination is the number-one cause of weld defects.

Step 2 — Prepare the Flange

Check the flange bore matches your pipe's outside diameter (OD) for a slip-on, or matches the pipe schedule's wall thickness and OD for a weld-neck. Clean the bore and face of the flange the same way you cleaned the pipe.

Step 3 — Fit-Up and Alignment

This is the most critical step. For a slip-on flange:

1. Slide the flange onto the pipe until the pipe end sits approximately 1.5–3 mm back from the flange face (this protects the face from weld spatter and allows for a proper inside fillet).
2. Use a combination square to confirm the flange face is square to the pipe centreline — check at multiple points around the circumference.
3. Tack-weld in at least four equally spaced positions (12 o'clock, 6 o'clock, 3 o'clock, 9 o'clock) to hold alignment before running full welds.
4. Re-check squareness after tacking — heat can pull things out of alignment.

Step 4 — Welding the Flange

Outside fillet weld

Run a continuous fillet weld around the full circumference where the pipe meets the back face of the flange. Use a weaving or stringer bead technique depending on the gap and material thickness. Aim for a fillet size equal to the pipe wall thickness (or as specified by your application). Weld in segments, alternating sides to minimise distortion — e.g. weld 12 to 3, then 6 to 9, then fill the remaining quarters.

Inside fillet weld

For slip-on flanges, an inside fillet weld is strongly recommended (and often required for pressure applications). This can be tricky on small-bore pipe — use a short, angled electrode or MIG torch. This second weld significantly improves strength and leak resistance.

Weld-neck flanges

These require a full-penetration butt weld. Bevel the pipe end to match the flange hub, maintain the correct root gap (typically 1.5–3 mm), and use a backing ring or weld from both sides if possible. This is more demanding and is best left to an experienced welder.

Step 5 — Inspection and Finishing

• Allow the weld to cool slowly — don't quench with water.
• Grind off any spatter and inspect the weld visually for porosity, undercut, or incomplete fusion.
• For pressure-rated systems, consider a dye-penetrant test or have the weld inspected by a qualified inspector.
• Apply a suitable primer and paint to prevent corrosion — Resene and Wattyl both offer industrial metal primers suited to NZ conditions.

When to Call a Professional

Some applications require certified welding and are not suitable for DIY:

• Pressure vessels and pressure piping — these fall under the Health and Safety at Work (Pressure Equipment, Cranes, and Passenger Ropeways) Regulations. Welds may need to be performed by a coded welder and inspected by a third party.
• Gas piping — any work on gas lines is restricted work under NZ law and must be carried out by a licensed gasfitter registered with the Plumbers Gasfitters & Drainlayers Board.
• Structural applications — flanges used in structural steel connections may require an LBP or engineer involvement. Check with MBIE Building Performance if a building consent may be needed.
• If you're unsure of your skill level or the application is safety-critical, find a qualified local welder on tradedirectory.co.nz.

Sources & Further Reading

• WorkSafe NZ — Welding Hazards
• MBIE Building Performance — Building Code
• Plumbers Gasfitters & Drainlayers Board
• BRANZ — Good Practice Guides
• Standards NZ — NZS/AS Standards
• Resene — Industrial Primers & Coatings