Learn about the most common welding defects including porosity, cracking, undercut, and incomplete fusion. Discover causes and prevention strategies for each.
Introduction
Welding defects compromise joint integrity and part quality. Understanding causes and prevention is essential. At Fulei Metal, our quality system catches and prevents defects.
Defect 1: Porosity
Gas trapped in the weld pool. Appears as small holes or bubbles.
Causes: contaminated material, insufficient gas coverage, wet or oily surface, wrong gas type, excessive gas flow causing turbulence, dirty filler wire.
Prevention: clean material thoroughly, verify gas flow and type, store filler properly, check for drafts, clean all surfaces.
Defect 2: Cracking
Cracks in or near the weld. Most serious defect.
Hot Cracking
Occurs during solidification. Caused by high sulfur or phosphorus, excessive concavity, rapid cooling. Prevention: use correct filler, maintain proper bead shape, control cooling rate.
Cold Cracking
Occurs after cooling. Caused by hydrogen, residual stress, martensitic structure. Prevention: use low-hydrogen consumables, pre-heat, control heat input, post-weld heat treatment.
Defect 3: Undercut
Groove melted into the base metal adjacent to the weld. Reduces cross-section. Stress concentrator.
Causes: excessive amperage, excessive travel speed, incorrect torch angle, excessive arc length.
Prevention: reduce amperage, slow travel speed, correct torch angle, maintain short arc.
Defect 4: Incomplete Fusion
Weld metal does not fuse with base metal or previous passes.
Causes: insufficient amperage, excessive travel speed, incorrect torch angle, dirty surfaces, improper joint preparation.
Prevention: increase amperage, slow travel, correct angle, clean surfaces, prepare joints properly.
Defect 5: Incomplete Penetration
Weld does not reach through the full joint thickness.
Causes: insufficient amperage, excessive travel speed, excessive root gap, wrong joint preparation, wrong bead sequence.
Prevention: increase amperage, reduce speed, correct root gap, prepare joint correctly, use proper sequence.
Defect 6: Spatter
Metal droplets expelled from the weld. Stick to surrounding surface.
Causes: excessive amperage, low voltage, wrong gas mixture, dirty wire, wrong wire feed speed.
Prevention: optimize parameters, use correct gas, clean wire, consider pulsed transfer.
Defect 7: Overlap
Weld metal flows onto the base metal surface without fusing.
Causes: excessive deposition, slow travel speed, incorrect torch angle.
Prevention: increase travel speed, correct torch angle, reduce deposition rate.
Defect 8: Distortion
Excessive warping of the part.
Causes: excessive heat input, poor sequence, inadequate clamping, wrong joint design.
Prevention: control heat input, proper sequence, adequate clamping, consider joint design.
Inspection and Detection
Visual inspection: undercut, overlap, surface porosity, bead shape, size. Dye penetrant: surface cracks and porosity. Radiography: internal defects, porosity, lack of fusion. Ultrasonic: internal cracks, lack of fusion. Magnetic particle: surface and near-surface cracks (ferromagnetic only).
Defect Repair
Assess extent of defect. Remove defect by grinding or gouging. Clean the repair area. Re-weld with correct parameters. Re-inspect the repair. Document the repair.
Conclusion
Welding defects are preventable with proper procedures, parameters, and skill. At Fulei Metal, our quality system and experienced welders minimize defects and ensure weld integrity.
Defect, First Root Cause to Check, and the Preventive Action
Most weld defects have one dominant cause and several minor ones. Working through the dominant cause first is what keeps repair time down.
| Defect | How it presents | First thing to check | Preventive action |
|---|---|---|---|
| Porosity | Surface pits or internal worm tracks | Gas coverage and contamination | Shield flow in range, no draught across the arc, joint faces free of oil, paint, rust and moisture, wire dry |
| Lack of fusion | Linear indication along the fusion line | Heat input and torch angle | Raise current or slow travel, grind starts and previous beads, correct the work angle so the arc points at the root |
| Slag inclusion | Elongated indication between passes | Interpass cleaning | Chip and grind between passes; the defect is almost always cleaning discipline rather than a setting |
| Undercut | Notch along the weld toe | Voltage and travel speed | Reduce voltage, moderate travel speed, limit weave width; the notch is a stress raiser and is often rejectable on its own |
| Centreline cracking | Crack along the weld centre | Restraint, bead profile and filler choice | Use a crack-resistant filler, fill the crater at the end of the run, keep the bead slightly convex, reduce restraint where the design allows |
| Hydrogen cracking | Crack in the heat-affected zone, often delayed | Hydrogen source, hardness and restraint | Preheat to the level the carbon equivalent calls for, use low-hydrogen consumables held in a heated quiver, control interpass temperature |
| Burn-through | Hole or excessive penetration on thin sheet | Heat input versus section thickness | Pulse transfer, copper backing, higher travel speed, or a smaller root gap |
| Excessive spatter | Metal droplets around the weld | Voltage to wire-feed mismatch, stick-out | Retune the arc, shorten stick-out, clean or replace the contact tip |
The distinction worth holding onto is between defects caused by settings and defects caused by preparation or discipline. Slag inclusion, porosity from contamination and lack of fusion at a start are preparation failures and no amount of parameter tuning will cure them. That is where our in-process inspection concentrates, because it is also where the cost of finding the defect late is highest.
Frequently Asked Questions
Can porosity be repaired by welding over it?
Sometimes, but it is the wrong default. Grinding out to sound metal and re-welding is reliable; welding over porosity frequently re-forms it because the source, usually gas coverage or contamination, has not changed.
Which defects are rejectable rather than repairable?
That is set by the acceptance level in the applicable standard, most commonly ISO 5817 for steel, with quality levels B, C and D. A defect that is acceptable at level D can be rejectable at level B, so the level needs to be on the drawing rather than assumed.
Why do cracks sometimes appear hours after welding?
Hydrogen induced cracking is delayed by nature: hydrogen diffuses to the hard heat-affected zone over time. That is why inspection timing matters, and why a weld that looked sound at the end of the shift should still be re-checked later on crack-sensitive material.
Questions about a specific part are usually faster to answer against the drawing — send it through the route below.
Next step. Send the drawing for a sheet metal welding service. We confirm the process route and the achievable dimension before quoting, so the numbers describe this part rather than an average. Useful background: send the drawing for review, weld quality standards and inspection and welding inspection methods.