Quality with FransWhat visual testing covers
ISO 17637 sets out how visual testing of fusion welds is done: good light, clean surfaces, the right gauges and a qualified inspector. It does not say how many welds to check; that comes from the specification or inspection and test plan. The defects you find are then judged against the acceptance standard, usually an ISO 5817 quality level (B is the strictest, D the most lenient) or the visual criteria table in AWS D1.1. Visual testing also comes first: radiography and ultrasonic testing standards expect the weld to have passed visual inspection before NDT starts.
The six defects at a glance
| Defect | What you see | Typical cause | Usual fix |
|---|---|---|---|
| Crack | A fine line in the weld or heat-affected zone, often at the crater or the toe | Hydrogen (damp consumables), high restraint, no preheat, or a crater left unfilled | Reject. Find the cause, grind out fully, confirm with MT/PT, re-weld to the procedure |
| Undercut | A groove melted into the parent metal along the weld toe | Current too high, travel too fast, wrong electrode angle or too much weave | Measure depth with a gauge. Over the limit: blend by grinding or add a repair run |
| Surface porosity | Small round holes or "pin holes" on the weld face | Lost gas shielding (draughts, leaks), damp electrodes or flux, oil, paint or zinc on the joint | Grind out local pores and re-weld; widespread porosity means cut out and re-weld |
| Overlap | Weld metal rolled over the parent plate at the toe without fusing | Travel too slow, current too low, wrong angle, especially in fillet welds | Grind back to sound metal and check the toe has fused |
| Underfill / incomplete fill | Weld face below the plate surface or a fillet smaller than the drawing | Too few runs, travel too fast, poor sequence | Add runs to the size on the drawing; check with a fillet or bridge gauge |
| Arc strikes and heavy spatter | Small burn marks outside the weld; beads of metal stuck to the plate | Striking the arc on the plate, poor earth clamp, wrong settings | Grind arc strikes smooth and check for cracks; remove spatter before coating |
Some defects, such as lack of fusion inside the joint, lack of root penetration and buried porosity, are often not visible from the face. That is why the specification may call for ultrasonic or radiographic testing on a share of the welds after visual inspection.
Why this matters for QA/QC
Most weld rejects I see on site are not a mystery. They come from the same few things: welding in the wind without screens, electrodes left out of the oven, galvanising or primer not removed from the joint, or a welder working outside the approved procedure (WPS). If you only reject the weld and do not fix the cause, the next ten welds come out the same.
So treat a visual defect as a question, not just a red mark. Check the welder's qualification and the WPS they should be following. Ask to see the electrode oven log. Look at the joint before welding, not only after. And make sure your inspection and test plan says which code and quality level apply, because the same undercut can pass at ISO 5817 level D and fail at level B.
Check on your site tomorrow
- Confirm the specification names the acceptance code and level (for example ISO 5817 level C, or AWS D1.1) and that inspectors have a copy of the limits.
- Check inspectors have working tools: a weld gauge, a torch or good lighting, a mirror for awkward joints and a marker for rejects.
- Walk the welding area: wind screens up, electrode oven on and logged, earth clamps close to the joint, joints clean of paint, oil and zinc.
- Match each welder on shift to a valid qualification and the WPS for the joint they are welding.
- For every reject, record the defect, the cause and the repair, and re-inspect the repaired weld before it is coated or covered.
The 31-point checklist I use before every pour: bar sizes, spacing, cover and laps against the approved drawing.
Download the checklist
Sources: TWI Job Knowledge: Defects/imperfections in welds, porosity · ESAB University: The welder's guide to defects and remedies · EN ISO 5817:2023, quality levels for imperfections (scope) · ISO 17637, visual testing of fusion-welded joints (preview) · ISO 10675-1:2016 (visual testing before radiography) · Virtual Labs (IIT): welding defects, causes and remedies. Summary in my own words; your project specification and the code it names set the acceptance limits.