The most common structural weld defects are undercut, porosity, incomplete fusion, incomplete penetration, slag inclusions, overlap, cracks and welds of the wrong size or profile. In Canada, structural welds are made and judged against CSA W59, the structural welding code. Welds are inspected visually, and where the specification calls for it, with non-destructive testing (NDT). Anything outside the acceptance criteria is removed and re-welded to an approved procedure, then inspected again.
This post covers what each defect is, how inspectors find it, how acceptance works and what a proper repair looks like. For the code itself, see our guide to CSA W59.
Discontinuities vs defects
Inspectors make a distinction that matters:
- A discontinuity is any interruption in the normal structure of the weld or base metal: a pore, a small undercut, a ripple in the profile.
- A defect is a discontinuity that exceeds what the code and specification accept, because of its type, size, location or how often it occurs. Defects must be repaired.
Not every imperfection is a reason to reject a weld. Acceptance limits allow for the fact that real welds are not perfect. Cracks are the exception that inspectors treat most seriously, because a crack can grow under load.
Common structural weld defects
| Defect | What it is | Typical causes | Found by |
|---|---|---|---|
| Undercut | A groove melted into the base metal along the weld toe and left unfilled | Too much current or travel speed, wrong electrode angle | Visual |
| Porosity | Gas pockets trapped in the weld, at the surface or inside | Moisture, contamination, poor shielding gas coverage, drafts | Visual (surface), UT or RT (internal) |
| Incomplete fusion | Weld metal not fused to the base metal or to a previous pass | Low heat input, poor technique, contaminated or poorly prepared joint | UT, RT; sometimes visual at the surface |
| Incomplete penetration | Weld metal does not reach through the joint as designed | Root opening too small, wrong joint preparation, low current | UT, RT, visual at an accessible root |
| Slag inclusions | Non-metallic slag trapped between passes or at the sidewall | Poor cleaning between passes, poor bead placement | UT, RT |
| Overlap (cold lap) | Weld metal rolled over the base metal surface without fusing | Slow travel, excessive metal, poor angle | Visual |
| Cracks | Fractures in the weld, heat-affected zone or base metal | Hydrogen, high restraint, missing preheat, unsuitable consumables, high carbon steel | Visual, MT, PT (surface); UT, RT (internal) |
| Undersized or short welds | Weld smaller or shorter than the welding symbol requires | Poor technique, misread drawings | Visual with weld gauges |
| Poor profile | Excessive convexity, concavity or reinforcement | Technique, parameters | Visual |
| Arc strikes | Spots where the arc touched the base metal outside the weld | Careless handling | Visual |
| Spatter | Metal droplets stuck to the surface | Parameters, process | Visual |
Why some defects matter more
- Cracks and incomplete fusion are planar: they are thin and sharp, and they concentrate stress. They are the most serious.
- Undercut reduces the base metal's section at the toe and creates a notch, which matters most where loads cycle.
- Undersized welds are simply weaker than the design assumed.
- Porosity and slag, within limits, are less critical in statically loaded welds, but heavy or clustered porosity indicates a process problem that needs fixing.
- Arc strikes can harden and crack the steel locally. They are removed and checked.
How structural welds are inspected
Inspection happens before, during and after welding, not just at the end.
Before welding: confirm the welding procedure data sheet for the joint, welder qualification, joint preparation and fit-up, cleanliness, and preheat where the procedure requires it.
During welding: check consumables and their storage, interpass cleaning, technique and parameters, and that the sequence follows the procedure.
After welding: visual inspection of every weld, then NDT where specified.
| Method | What it finds | Notes |
|---|---|---|
| Visual testing (VT) | Surface defects, size, length, profile | Done on every weld; needs good access and light, plus weld gauges |
| Magnetic particle (MT) | Surface and near-surface cracks | Works on ferromagnetic steel; quick and common on fillet welds |
| Liquid penetrant (PT) | Surface-breaking defects | Works on any non-porous metal; needs clean surfaces |
| Ultrasonic testing (UT) | Internal defects such as lack of fusion, cracks, slag | The usual method for complete joint penetration welds in structural steel |
| Radiographic testing (RT) | Internal defects, especially porosity and slag | Requires radiation safety measures and access to both sides; used less on structural joints |
The project specification sets which welds get NDT and how many. Complete penetration groove welds in moment connections and splices are common candidates. Fillet welds are usually accepted on visual inspection, with MT if the specification calls for it.
Acceptance criteria and who decides
CSA W59 sets the quality requirements for structural welds, including visual acceptance limits and the levels used to judge NDT results. The acceptable size and frequency of a discontinuity depend on the type of weld, the direction of stress, and whether the structure is subject to fatigue (cyclic) loading. The specification and the engineer can add requirements for particular joints.
We don't reproduce W59's numbers here. They depend on the edition named in the specification, the joint and the loading, and they are applied by qualified inspectors with the standard in hand. If you need to know whether a particular weld is acceptable, ask the inspector or the engineer of record rather than a summary.
Who inspects
- Fabricator QC. A CWB-certified company's welding supervisor and quality control personnel inspect their own welds as part of production.
- Certified welding inspectors. The CWB Group certifies welding inspectors to CSA W178.2. It describes Level 1 as an entry level working under Level 2 or 3 supervision, Level 2 as competent to inspect without supervision, and Level 3 as having additional technical knowledge for tasks such as inspection and test plan reviews.
- Inspection companies. Organizations can be certified to CSA W178.1, which covers visual and NDT inspection services.
- NDT technicians, certified for each method they perform.
Third-party inspectors are usually hired by the owner when the specification calls for them. Our steel inspection and QC checklist covers how weld inspection fits with the rest of the job.
Why structural welds fail
Weld failures in service are rare when the steel is welded by certified companies to approved procedures and inspected. When they happen, the cause is usually one of these:
- Design or size: the weld was too small for the load, or the wrong type for the joint.
- Hydrogen cracking: moisture in consumables, missing preheat, or high restraint in thick material, often showing up hours after welding.
- Lamellar tearing: separation inside thick plate loaded through its thickness by large restrained welds.
- Fatigue: cracks growing from weld toes or defects under repeated loading, such as crane runways or vibrating equipment supports.
- Welding to unknown steel: field welding to existing members whose grade and weldability were never checked. See can you cut or weld an existing steel beam.
A cracked or failed structural weld is an engineering question, not just a repair. Stop using the area if there's any doubt and get an engineer to look at it.
Repairing defective welds
A proper repair follows the same discipline as the original weld:
- Mark and record the defect from the inspection report.
- Remove it by grinding or air arc gouging, far enough to reach sound metal.
- Confirm removal, often with MT or PT for cracks.
- Prepare the groove to a shape that can be welded properly.
- Re-weld to an approved procedure, with preheat if required.
- Re-inspect the repair with the same method that found the defect, or the one the specification requires.
Simple surface items, such as excessive convexity or minor undercut within the allowed repair methods, may be ground or built up. Cracks need their cause found, or they tend to come back. Repeated repairs in the same spot can damage the base metal, so the welding engineer or the engineer of record may need to approve the approach. CSA W59's workmanship requirements cover how defects are removed and repaired.
Certified welding at Procut
Our shop welding is certified by the Canadian Welding Bureau, and we fabricate to shop drawings prepared in-house, so welds are made to the details and the code from the start. If your specification calls for third-party inspection or NDT, send it with your drawings and we'll confirm how we'll accommodate it. See structural welding services for how shop and field welding compare.
Request a quote with your drawings, or see our fabrication and welding services.
Frequently asked questions
What are the most common structural weld defects?
Undercut, porosity, incomplete fusion, incomplete penetration, slag inclusions, overlap, cracks, and welds that are undersized or have a poor profile. Arc strikes and excessive spatter are also flagged on visual inspection.
What is the difference between a weld discontinuity and a defect?
A discontinuity is any interruption in the weld or base metal. It becomes a defect, which must be repaired, when its type, size or frequency goes beyond what the applicable welding code and specification accept.
Who sets the acceptance criteria for structural welds in Canada?
CSA W59, the structural welding code, sets the quality requirements for welds in steel structures. The project specification can add inspection and testing requirements, and the engineer may set stricter limits for critical joints.
What is a CWB certified welding inspector?
An inspector certified by the CWB Group to CSA W178.2. Level 1 is entry level and works under supervision, Level 2 can inspect independently, and Level 3 has additional technical knowledge for tasks such as reviewing inspection and test plans.
