What's inside
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Porosity is one of the most common defects in flux-core welds. It appears as small holes or cavities in the bead, either on the surface or hidden inside the joint. A little pinholing can weaken a weld; widespread porosity can make a structurally important weld unacceptable.
Flux-core welding is not automatically dirty or unreliable. When the wire, machine settings, shielding, and joint preparation are right, it produces sound welds quickly. Troubleshooting works best when you change one variable at a time instead of immediately increasing amperage or replacing the wire.
What Porosity Looks Like
Surface porosity is visible as round pinholes, pits, or small craters in the bead. Cluster porosity looks like several holes grouped in one area. Wormhole porosity forms elongated tracks and is often associated with trapped gas from contaminated steel, excessive voltage, or poor gas coverage.
Some porosity is hidden below the surface. If a weld looks unusually rough or collapses during grinding, do not assume it is solid. A failed bend test, a broken weld, or an inspection method such as radiography may reveal internal voids. On noncritical work, carefully grinding a test weld can still provide useful evidence, but it is not a substitute for qualified inspection.
First Check Wire Type and Shielding
Flux-core wire is not one single process. Self-shielded flux-core wire, often marked FCAW-S, produces its own shielding gas and is commonly used outdoors. Gas-shielded flux-core wire, or FCAW-G, needs an external gas supply, usually a carbon dioxide and argon mixture. Connecting the wrong setup can create severe porosity.
| Wire type | Shielding requirement | Common porosity causes | Best use |
|---|---|---|---|
| Self-shielded flux core | No cylinder; flux creates shielding | Incorrect polarity, wind, damp wire, excessive stickout | Outdoor repairs and general fabrication |
| Gas-shielded flux core | Usually 75/25 argon-CO2 or a specified gas | Gas leaks, blocked nozzle, low flow, drafts | Cleaner indoor welds and higher deposition work |
| Dual-shield wire | Wire shielding plus external gas | Wrong gas, poor flow, contaminated base metal | Heavy fabrication and production work |
Read the wire label before changing settings. Polarity is especially important. Many self-shielded wires run on DC electrode negative, while many gas-shielded wires run on DC electrode positive. The wrong polarity can produce spatter, unstable arc behavior, slag problems, and porosity even when everything else is correct.
Check Wind and Gas Coverage
Wind is a leading cause of porosity in flux-core work. A light breeze can disturb the shielding around the arc, particularly with gas-shielded wire. If welding outdoors, use a windbreak and keep the opening around the joint as small as practical. Do not rely on a welding curtain that flaps directly across the arc.
For gas-shielded flux core, start near the wire manufacturer’s recommended flow rate. A typical range is about 25 to 35 cubic feet per hour, but the correct value depends on the nozzle, joint, and surrounding airflow. Too little gas leaves the weld exposed. Too much can create turbulence that pulls air into the shielding envelope.
Check the cylinder valve, regulator, hose, fittings, solenoid, and gun connection for leaks. Inspect the nozzle for spatter buildup. A partially blocked nozzle can divert gas away from the arc. Hold the gun close enough for proper coverage, but not so close that spatter restricts the nozzle. If you need reliable flow measurement, a MIG welding gas flow meter is more useful than guessing by sound.
Dry and Protect the Wire
Moisture in flux is a frequent cause of pinholes and wormholes. Store opened spools in a dry cabinet or sealed container with desiccant. Do not leave a spool overnight in a damp garage, especially if the wire will be used for a critical weld.
Do not bake flux-core wire unless the manufacturer provides a specific procedure. Excessive heat can damage the flux formulation. If a spool has visibly rusted wire, swollen flux, or a history of poor storage, replacement is usually cheaper than trying to rescue it. For occasional repairs, a reasonably priced self-shielded flux-core wire can be perfectly adequate, provided it matches your machine’s polarity and material thickness.
Clean the Joint Thoroughly
Oil, paint, mill scale, rust, moisture, cutting fluid, and galvanized coatings can all introduce gas into the weld pool. Flux can handle some surface contamination, but it is not a replacement for preparation. Grind or wire-brush at least 1/2 inch on both sides of the joint, and remove oil with a suitable solvent before welding.
Pay special attention to trapped moisture. Damp plate, wet gloves, condensation, or water in a joint gap can generate porosity immediately. If welding thick or cold steel, warm the workpiece enough to remove condensation and follow the applicable welding procedure for preheat.
Adjust Settings and Technique
Use the wire manufacturer’s voltage, wire-feed-speed, and travel recommendations as your starting point. Excessive voltage can make the arc long and unstable, while too little voltage can cause a harsh, short arc and poor tie-in. Both conditions can trap gas.
Maintain a consistent contact-tip-to-work distance. A common starting point for self-shielded flux-core is about 3/4 inch, though the wire label takes priority. Excessive stickout preheats the wire and reduces arc energy. Keep the gun angle and travel speed steady. Moving too quickly can leave a narrow, poorly protected bead; moving too slowly can create a large, turbulent pool.
Use the correct travel direction for the wire. Many self-shielded wires are designed for a drag angle, with the gun pulled along behind the arc. A push technique can allow shielding to run ahead incorrectly and may increase defects. Remove slag between passes with a welding chipping hammer and wire brush. Trapped slag is not technically porosity, but it can look similar after grinding and creates another serious weld defect.
Use a Logical Test Sequence
Make a short test weld on clean scrap of the same thickness. First verify wire type and polarity. Next clean the joint and check for wind. Then inspect gas flow, nozzle condition, and leaks if using gas-shielded wire. After that, adjust voltage, wire speed, stickout, and travel technique.
If porosity remains, replace the contact tip and inspect the liner for drag or contamination. Check that the spool feeds smoothly and that the drive rolls match the wire diameter. A worn tip or erratic feed can make the arc unstable enough to produce defects. Only after these checks should you suspect a bad spool or a machine fault.
When to Stop and Rework
Do not simply cap a porous weld with another pass. Grind out the defective area until sound metal is visible, clean it, and reweld using corrected settings. For load-bearing, pressure-containing, lifting, or code-regulated work, follow the required welding procedure and inspection standard. A clean-looking bead is not proof of a sound weld, and porosity hidden under slag or a cover pass can remain a serious failure point.