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Welding defects are one of the first things inspectors learn to recognize, but understanding them properly takes more than memorizing a list of names. A weld may look clean from a distance and still contain internal defects. Another weld may show a small surface imperfection that is not automatically rejectable unless it exceeds the applicable acceptance criteria.
For welding inspectors, QA/QC inspectors, NDT technicians, vendor inspectors and fabrication quality teams, the real task is not only to “find defects.” The task is to understand what the defect may indicate, how it could affect quality or safety, which inspection method can detect it, and whether the weld meets the approved code, drawing, ITP and project specification.
This article explains the most common welding defects inspectors should know, including cracks, porosity, lack of fusion, lack of penetration, undercut, slag inclusion, overlap, excess reinforcement, burn-through and distortion.
For the broader inspection workflow, including welding stages, NDT methods, WPS/PQR/WQT, report review and inspector duties, see the complete welding and NDT inspection guide.
Professionals who want structured training on this subject can also review NTIA’s Welding and Non-Destructive Testing Training Course, which covers welding processes, inspection, NDT methods, codes, quality control and safety.
Welding defects are imperfections in a weld or welded joint that make the weld unacceptable against the applicable acceptance criteria.
They may be found:
A welding defect can be visible on the surface, hidden inside the weld, related to geometry, or connected to documentation and qualification control.
For example, undercut may be seen during visual inspection. Lack of fusion may require ultrasonic testing. Porosity may be visible on a radiographic film or digital image. A weld made without the correct WPS or welder qualification may also become unacceptable even if the weld appearance looks acceptable.
Inspectors should be careful with the difference between a discontinuity and a defect.
A discontinuity is an irregularity in the weld or base material. A defect is a discontinuity that exceeds the allowed acceptance limit.
| Term | Meaning |
| Discontinuity | An imperfection or irregularity in the weld or material |
| Defect | A rejectable discontinuity that does not meet acceptance criteria |
This distinction matters because not every indication is automatically rejectable. Acceptance depends on the applicable code, standard, project specification, weld type, material, thickness, service condition and inspection method.
A professional inspector should not accept or reject a weld based only on personal judgment. The decision must be traceable to the approved acceptance criteria.
The most common welding defects include:
Each defect has different causes, inspection concerns and acceptance implications.
Cracks are linear fractures in the weld metal, heat-affected zone or base material. They are usually among the most serious welding defects because they can grow under stress, pressure, fatigue, vibration or thermal cycling.
Cracks may appear as longitudinal cracks, transverse cracks, crater cracks, toe cracks or root cracks.
Common causes include:
Cracks may be detected by visual testing if they are visible at the surface. PT can help find surface-breaking cracks on clean, non-porous materials. MT is useful for surface and near-surface cracks in ferromagnetic materials. UT may be used for internal or subsurface planar indications.
Because cracks can be critical, inspectors should document them clearly and verify repair, retest and final acceptance records.
Porosity is caused by gas trapped in the weld metal during solidification. It may appear as isolated pores, clustered porosity, linear porosity or surface-open pores.
Common causes include:
Porosity may be visible during visual inspection if it is open to the surface. Internal porosity is commonly detected by RT, and in some cases by UT depending on size, distribution and procedure.
Small isolated pores may be acceptable in some cases, but clustered or excessive porosity can lead to rejection. The inspector should always compare the finding with the applicable acceptance criteria.
Lack of fusion occurs when the weld metal does not properly fuse with the base metal or with a previous weld pass.
It may occur at the sidewall, between passes, at the root area or near the weld toe.
Common causes include:
Lack of fusion can reduce the effective strength of the weld. It is especially important because it may not be visible from the surface.
UT is often useful for detecting lack of fusion, especially when the indication is planar and internal. RT may detect some fusion-related defects, but detectability depends on orientation and technique.
Lack of penetration, also called incomplete penetration, occurs when the weld does not fully penetrate the joint root as required.
Common causes include:
This defect is a strong reminder that many welding problems begin before welding starts. Fit-up inspection, root gap, bevel angle and joint preparation are critical.
Depending on the joint type and access, lack of penetration may be detected by visual inspection, RT or UT. If full penetration is required by design, incomplete penetration can affect strength, pressure integrity and fatigue performance.
Undercut is a groove melted into the base metal beside the weld toe or root and left unfilled by weld metal.
Common causes include:
Undercut can reduce effective thickness and create stress concentration. It is usually detected during visual inspection and may be measured using weld gauges.
A small amount of undercut may be acceptable within specified limits, but excessive undercut is usually rejectable. The inspector should not rely on appearance alone; measurement and acceptance criteria matter.
Slag inclusion occurs when non-metallic slag is trapped inside the weld metal or between weld passes. It is common in processes that use flux, such as SMAW, FCAW and SAW.
Common causes include:
Slag inclusion may be detected by RT or UT, depending on the weld configuration and inspection procedure.
For inspectors, interpass cleaning is important. If slag is not removed before the next pass, it may become buried and later appear as an internal defect during NDT.
Overlap occurs when weld metal flows over the base metal surface without proper fusion.
Common causes include:
Overlap is usually detected during visual inspection. It may look like extra weld metal, but the problem is that the metal has not fused correctly with the base material.
Inspectors should not confuse overlap with acceptable weld reinforcement. The key issue is proper fusion and acceptable weld profile.
Excess reinforcement occurs when too much weld metal is deposited above the required weld profile.
Common causes include:
Excess reinforcement can create stress concentration, poor profile transition, unnecessary distortion and acceptance problems. Bigger welds are not automatically better welds.
Inspectors should check weld size and profile against the approved drawing, weld symbol, WPS and acceptance criteria.
Burn-through occurs when excessive heat causes the weld or base metal to collapse, leaving a hole or excessive penetration.
Common causes include:
Burn-through is especially important in thin materials, root passes, piping and pressure-retaining welds. It may be detected visually if the root side is accessible, or through RT, borescope inspection or pressure/leak testing where required.
Distortion is unwanted deformation caused by welding heat and shrinkage.
It may appear as angular distortion, longitudinal shrinkage, transverse shrinkage, misalignment, buckling or out-of-roundness.
Common causes include:
Distortion may not always be classified like an internal weld defect, but it can still cause rejection if dimensional tolerances are not met. Inspectors should check alignment, straightness, dimensions and fit-up to the next assembly stage.
Welding defects can be grouped into surface defects, internal defects, geometric defects and documentation-related issues.
| Defect Group | Examples | Typical Detection |
| Surface defects | Surface cracks, undercut, overlap, arc strikes, crater defects | VT, PT, MT |
| Internal defects | Porosity, slag inclusion, lack of fusion, lack of penetration | UT, RT |
| Geometric defects | Excess reinforcement, distortion, misalignment, incomplete filling | VT, dimensional inspection |
| Documentation issues | Missing WPS, unqualified welder, incomplete repair record | Document review |
This is why welding inspection and NDT inspection must work together. Visual inspection controls what can be seen directly, while NDT helps provide evidence for surface-breaking, near-surface or internal discontinuities.
For a comparison of the main methods, read VT, PT, MT, UT and RT in weld inspection.
No NDT method is best for every weld defect. Method selection depends on material, thickness, weld geometry, expected defect type, access, safety requirements and project specification.
| Method | Useful For | Main Limitation |
| VT | Visible surface defects and weld profile | Cannot detect hidden internal defects |
| PT | Surface-breaking defects on clean, non-porous materials | Cannot detect subsurface defects |
| MT | Surface and near-surface defects in ferromagnetic materials | Limited to ferromagnetic materials |
| UT | Internal planar defects, lack of fusion, lack of penetration | Requires skilled operator and suitable geometry |
| RT | Internal volumetric defects such as porosity and slag inclusion | Radiation control, access and orientation limitations |
For a practical selection workflow, see how to choose the right NDT method for welds.
A welding defect should never be judged only by its name. Acceptance depends on the applicable code, standard, project specification and service condition.
Before accepting or rejecting a finding, the inspector should confirm:
A common mistake is to check only whether the NDT report says “accepted.” A stronger inspection review confirms that the correct weld was tested, the correct method was used, the required coverage was achieved and the result is traceable.
For document control, use an NDT report review checklist before final release.
Use this compact checklist during welding inspection and report review.
For a more detailed inspection workflow, see the visual welding inspection checklist.
Inspectors should avoid these common problems:
A strong welding inspection process connects the physical weld condition with approved procedures, inspection points, NDT results and final documentation.
For wider QA/QC workflow control, NTIA also covers inspection planning, reporting and acceptance logic in its technical inspection training courses.
Common welding defects include cracks, porosity, lack of fusion, lack of penetration, undercut, slag inclusion, overlap, excess reinforcement, burn-through, distortion, arc strikes, crater defects and incomplete filling.
A discontinuity is an irregularity in the weld or material. A defect is a discontinuity that exceeds the applicable acceptance criteria and makes the weld unacceptable.
No. Not every discontinuity is rejectable. Acceptance depends on the applicable code, standard, project specification, service condition and acceptance criteria.
Cracks are usually considered among the most serious welding defects because they can grow under pressure, stress, vibration, fatigue or temperature change.
No. Visual inspection can detect visible surface defects and weld profile problems, but it cannot detect all internal defects. Internal defects may require UT, RT or another suitable NDT method.
There is no single best method for all weld defects. VT, PT, MT, UT and RT each have different strengths and limitations. The correct method depends on material, weld geometry, defect type, thickness, access and project requirements.
Welding defects are not just names on an inspection checklist. They are evidence of what may have gone wrong before, during or after welding.
For inspectors, the key is to understand the defect type, likely cause, detection method and acceptance criteria. A weld should not be accepted only because it looks clean, and it should not be rejected only because an indication exists. The decision must be based on the approved code, project specification, WPS, ITP, NDT report and final documentation.
A disciplined welding defect review helps reduce repair cost, prevent repeated nonconformities, improve NDT reliability and support smoother project handover.
For structured learning, NTIA’s Welding and Non-Destructive Testing Training Course covers welding inspection, NDT methods, defect recognition, quality control and documentation. You can also check upcoming dates in the NTIA training calendar.
At NTIA (Norwegian Technical Inspection Academy), we specialize in delivering world-class training programs in vendor inspection, quality surveillance, and technical compliance – specifically tailored for professionals in the oil, gas, and energy industries.
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