Why Knowing Welding Defects Matters
Welding is a critical process in countless industries — from construction and manufacturing to pipelines and automotive. A seemingly small flaw in a weld can compromise the strength, durability, and safety of an entire structure. That’s why being able to recognize and prevent defects is essential.
The image above highlights many of the most common welding defects — from visible surface issues like cracks or porosity to deeper problems like incomplete fusion or overlap. Understanding these defects helps engineers, inspectors, and welders ensure quality and avoid costly failures.
What Is a Welding Defect?
A welding defect is any flaw, imperfection, or discontinuity in a weld that degrades the quality, strength, or reliability of the welded joint. These defects can be visible on the surface or hidden internally, and they arise when the welding process deviates beyond acceptable tolerances.
Defects compromise structural integrity — sometimes mildly (a cosmetic blemish), other times severely (risk of breakage, leaks, or collapse). Because of this, many industry standards (e.g. ISO 6520, ISO 5817) define allowable limits for imperfections depending on application and material.
Classification of Welding Defects
Welding defects are often grouped into major categories:
| Category | What It Means | Typical Defects |
| External (Surface) | Visible or detectable on the weld’s surface | Cracks, porosity, undercut, overlap, spatter, excessive reinforcement |
| Internal (Sub-surface/Volumetric) | Hidden inside weld or joint, not visible superficially | Slag inclusions, lack of fusion, lack of penetration, internal cracks, porosity |
| Dimensional / Geometric | Deviation in shape, size, alignment due to welding heat or technique | Distortion, misalignment, underfill, excessive convexity/concavity |
Common Welding Defects (with Examples & Causes)
Here are many of the defects shown in your image — along with an explanation of how and why they occur, and what to watch for.
Porosity
- What it looks like: tiny holes or pores in the weld bead — often giving a “blown-hole” or “swiss-cheese” appearance. (Visible in your image.)
- Why it happens: When gas (e.g. hydrogen, oxygen, nitrogen) gets trapped in the weld as it cools and solidifies. Contaminated metal surfaces, moisture in electrode or shielding gas, wrong gas shielding, or incorrect welding parameters (current, arc length) can cause porosity.
- Problem: Reduces weld density and strength; may lead to corrosion or leak paths in pressure-containing welds.
Cracks
- What it looks like: Fractures on or within the weld or in the base metal adjacent to the weld — can be surface or internal cracks. The image shows a clear example labelled “Cracks.”
- Why it happens: Cracks often result from high thermal stresses during cooling, improper weld design, incorrect heat input, or poor technique. They may also arise due to hydrogen embrittlement, poor post-weld treatment, or material issues.
- Problem: Among the most serious defects — even small cracks can propagate under stress, leading to catastrophic failure.
Undercut
- What it looks like: A groove or depression along the weld toe or edge, where the base metal has melted away but no filler metal was deposited. (One image area is labelled “Undercut.”)
- Why it happens: Due to excessive heat, incorrect electrode angle, too long arc length, wrong travel speed, or using inappropriate welding parameters.
- Problem: Reduces cross-sectional thickness of base metal and weakens the joint.
Slag Inclusions
- What it looks like: Non-metallic remnants (slag, flux, oxides) trapped inside the weld bead — not visible on the surface but revealed in cross-section or via non-destructive tests. The image references “Slag Inclusion.”
- Why it happens: Poor cleaning between passes, incorrect electrode manipulation, low amperage, too slow travel rate, or improper flux removal and surface preparation.
- Problem: Slag pockets act as stress concentrators and potential initiation sites for cracks, especially in harder steels.
Lack of Fusion
- What it means: The filler metal fails to bond properly (or at all) with the base metal or between weld passes. In the image, this is shown as “Lack of Fusion.”
- Why it happens: Often due to insufficient heat or current, wrong torch angle, incorrect welding technique, or contaminated / unclean surfaces.
- Problem: Creates weak interfaces, which can lead to delamination, cracking, or joint failure under load.
Incomplete Penetration
- What it means: The weld bead does not fully penetrate the joint depth (root), leaving under-filled zones. The image labels “Incomplete Penetration.”
- Why it happens: Low welding current, inappropriate joint preparation, incorrect weld parameters, or insufficient passes.
- Problem: Creates internal crevices, lowers joint integrity, and can lead to leaks or structural failure — especially critical in pipe and pressure-vessel welds. Overlap
- What it looks like: The weld metal rolls over the base metal without fusing properly — often seen as a bulging bead extending beyond weld toe. The image shows “Overlap.”
- Why it happens: Poor technique, excessive filler metal, large electrode, too high current, or incorrect arc length.
- Problem: Creates weak or non-fused zones, stress concentrations; reduces load-bearing strength and may initiate cracking.
Spatter & Excess Reinforcement / Convexity / Concavity
- What it looks like: Spatter — small metal droplets around the weld bead; Excessive reinforcement — weld bead overly tall or convex; Concavity — bead curves inward.
- Why it happens: Improper welding parameters (current, arc length, speed), wrong electrode, or poor technique.
- Problem: While spatter may be more of a cosmetic issue (though annoying and requiring cleanup), excessive reinforcement or poor bead profile can create stress concentrators or reduce weld quality.
Why These Defects Appear
At a high level, defects often arise from a handful of root causes:
- Poor welding technique or operator error — wrong electrode angle, too long arc, incorrect travel speed.
- Wrong machine settings — improper current, voltage, or heat input.
- Contaminated materials or surfaces — rust, oil, moisture, flux residue, or dirty base metals.
- Incorrect consumables or shielding gases — wrong filler, degraded electrode, poor gas flow or gas mix.
- Inadequate preparation and joint cleaning — leftover slag, scale, oxide, or improper joint fit-up.
- Rushed / improper welding procedure — not following welding procedure specifications, multi-pass welds without cleaning between passes, incorrect sequencing.
In short: welding defects often reflect a breakdown in process control.
How to Prevent Welding Defects (Best Practices)
Based on welding-industry guides and standard recommendations: Ensure clean, properly prepared base metal and filler — remove rust, oil, moisture, scale, flux residue before welding.
- Set correct welding parameters — use the right current, voltage, arc length, travel speed, and electrode angle.
- Maintain proper technique & operator skill — especially in torch manipulation, bead control, and joint execution.
- Use appropriate consumables and shielding gas — choose correct filler metal, keep electrodes dry, ensure adequate gas flow and quality.
- Perform interpass cleaning (for multi-pass welds) — remove slag and residues before depositing next pass to avoid inclusions or lack of fusion.
- Use proper joint design and fit-up — correct groove design, joint alignment, and gap to allow full penetration if required.
- Inspect welds carefully — both visually and with NDT (non-destructive testing) — Visual inspection for surface defects; Radiographic, ultrasonic, or magnetic-particle testing for internal flaws.
Relating to the Photo: What the Visual Shows
The image you provided is an excellent visual summary of many common welding defects. For instance:
- The “Good Weld” section shows what a proper bead should look like — smooth, even, full penetration, no voids or irregularities.
- “Porosity” — shows multiple gas-trap points along the weld bead, indicating serious trapped gas.
- “Cracks” — a stark example where the weld metal or heat-affected zone has visibly fractured.
- “Slag inclusion,” “Lack of fusion,” “Incomplete penetration/overlap” — these show welds with poor bonding, material overlap without fusion, or insufficient root penetration.
- “Undercut,” “Overlap,” “Incomplete penetration” — all defects that compromise the weld’s strength, as described above.
This type of visual reference is incredibly useful for engineers, inspectors, and welders — offering a quick “defect cheat-sheet” to compare actual welds against.
Why Engineers & Inspectors Should Care
- Safety & Reliability: Welded structures — buildings, bridges, pressure vessels, pipelines — must bear loads or contain pressure safely. Defects weaken joints and may cause catastrophic failure.
- Quality Assurance: Consistent defect-free welding ensures longevity, reduces maintenance costs, and avoids early failures or expensive rework.
- Regulatory Compliance: Many standards (e.g. ISO 6520, ISO 5817) require welds to meet certain defect thresholds. Knowing defects ensures compliance and reduces rejection risk.
- Cost Efficiency: Detecting and avoiding defects early saves material, labor, and downstream repair costs.
