Common Welding Defects: Porosity, Cracks and Lack of Fusion
A weld that looks sound may hide an internal defect leading to sudden failure under load. Conversely, many defects leave a visible signature that can be read and diagnosed before it worsens.
1. Porosity
Porosity is gas bubbles trapped in the metal after solidification, either breaking the surface or hidden entirely at depth.
Root causes: surface contamination (oil, grease, paint and rust vaporise in arc heat releasing gases); moisture (water dissociates into hydrogen and oxygen — hydrogen is welding's primary enemy); inadequate gas shielding; damp electrodes, as basic electrodes absorb atmospheric moisture quickly; and excessively rapid cooling that gives gases no time to escape.
Prevention: clean thoroughly — remove rust, paint and oil for at least 25 mm around the weld line. Store electrodes in a drying oven or sealed container. Inspect gas hoses periodically.
2. Cracks
Cracks are the most dangerous defect because they propagate under cyclic load and cause sudden failure.
Hot cracks
Occur during solidification. Impurities — particularly sulphur and phosphorus — concentrate in the last regions to freeze, forming weak films that separate under shrinkage stress. The signature is a longitudinal crack along the bead centre, usually visible immediately after the pass.
Prevention: use low-sulphur wire, avoid deep narrow beads (depth-to-width ratio should stay near 1:1), and reduce travel speed for more balanced solidification.
Cold cracks
Appear hours — up to 48 — after work finishes, which is what makes them dangerous: the job may be handed over before they show. They require three factors together: hydrogen in the metal, a hardenable microstructure, and residual stress. Removing any one prevents them.
Prevention: preheat thick or high-carbon sections, use low-hydrogen electrodes stored dry, and control cooling slowly.
3. Lack of Fusion and Lack of Penetration
Two distinct defects often confused. Lack of fusion: filler has not bonded with the base metal or previous pass — contact exists but no metallurgical union. Lack of penetration: metal has not reached the joint root, leaving a gap at the bottom.
Shared causes: low current, excessive travel speed, wrong gun angle, insufficient edge preparation, narrow root gap. Both form what amounts to a flat crack — a severe stress concentrator under cyclic loading.
4. Slag Inclusions
Slag residue trapped inside the metal, common in multi-pass welding when slag is not cleaned between passes. Clean completely between every pass with a wire brush and chipping hammer, avoid heavily concave beads that trap slag at the edges, and maintain a gun angle that pushes slag behind the pool rather than ahead of it.
5. Undercut
A groove along the bead edge that reduces effective base metal thickness and acts as a stress concentrator. Caused by excessive current, high travel speed, wrong gun angle, or wide weaving without pausing at the edges. Reduce current slightly, pause momentarily at each weave edge to let the groove fill, and review gun angle.
6. Distortion
Shrinkage during cooling pulls the parts and distorts the final shape.
- Alternating intermittent welding. Distribute passes across sides alternately rather than one continuous run.
- Clamping. Restrain parts during welding and cooling.
- Pre-setting. Tilt the work against the expected distortion by a calculated amount.
- Lower heat input. Smaller faster passes distort less than one large pass.
- Symmetrical sequence. Start at the centre and work alternately outward.
7. Quick Diagnostic Table
| What you see | Likely defect | Check first |
|---|---|---|
| Small round surface holes | Porosity | Surface cleanliness and gas |
| Hairline along bead centre | Hot crack | Bead profile and wire type |
| Crack appearing next day | Cold crack | Hydrogen and preheat |
| Groove at the edge | Undercut | Current and speed |
| Dark residue within weld | Slag inclusion | Inter-pass cleaning |
| Gap at joint root | Lack of penetration | Current and edge prep |
| Work bowed after cooling | Thermal distortion | Sequence and clamping |
8. Pre-Weld Preparation
- Clean rust, paint and oil for an adequate distance around the weld line
- Ensure the surface is completely dry
- Prepare edges with the correct angle and gap for the thickness
- Clamp parts firmly to prevent movement during cooling
- Verify electrodes are dry and properly stored
- Check gas connections for leaks
- Set current on a test coupon of the same material and thickness
Conclusion
Welding defects are not bad luck — each has an identifiable physical cause. The key rule: treat the cause, not the symptom. Re-welding a porous area without cleaning the surface simply produces fresh porosity.
