Catalogue of weld defects to ISO 6520-1
ISO 6520-1 sorts weld imperfections into six groups and gives each a number. Knowing the group matters more than memorising the numbers: the group tells you what went wrong and therefore what to change.
The six groups
100 — Cracks. The most serious group. A crack is a discontinuity with a sharp tip, and a sharp tip concentrates stress, so cracks grow. Rejectable at every quality level. See cracks in welds.
200 — Cavities. Gas porosity, wormholes, shrinkage cavities and crater pipes. Round pores are the mildest defect in the catalogue because the round shape does not concentrate stress much; they are still a sign the shielding failed. See porosity.
300 — Solid inclusions. Slag, flux, oxide and tungsten trapped in the weld. Slag inclusions come from a bead shape that lets slag run ahead of the pool, or from not cleaning between runs. Tungsten inclusions come from dipping the electrode in TIG.
400 — Lack of fusion and penetration. Metal that was never melted together, or a root that was never reached. Rejectable at every level and frequently invisible from the surface. See lack of fusion and penetration.
500 — Imperfect shape and dimension. Undercut, excess weld metal, incorrect toe angle, overlap, linear misalignment, sagging, burn-through, incompletely filled groove. Most of what visual inspection actually finds sits here. See undercut.
600 — Miscellaneous. Arc strikes, spatter, grinding marks, temper colours. Minor on paper, but an arc strike outside the joint is a hard spot in the parent metal and on some contracts it is a repair.
How each group is found
| Method | Finds | Misses |
|---|---|---|
| Visual (VT) | Shape defects, surface cracks, undercut, spatter | Everything internal |
| Penetrant (PT) | Surface-breaking cracks and pores | Anything below the surface |
| Magnetic particle (MT) | Surface and near-surface cracks in ferritic steel | Austenitic stainless, aluminium |
| Radiography (RT) | Volumetric defects: pores, slag, lack of penetration | Tight cracks parallel to the beam, planar lack of fusion |
| Ultrasonics (UT) | Planar defects: cracks, lack of fusion | Fine scattered porosity |
The last two rows explain a common misunderstanding. Radiography and ultrasonics are not interchangeable: RT is good at volume and poor at planar defects, UT is the reverse. A procedure that specifies UT for a joint prone to lack of fusion is not being awkward.
From defect to decision
Finding an imperfection is not the same as rejecting the weld. The drawing gives a quality level — B, C or D — and ISO 5817 gives the limit for that level and thickness. The acceptance limits calculator does that lookup for the imperfections that come up most often in visual inspection.
Frequently asked questions
Which defects are never acceptable?
Cracks and lack of fusion, at every quality level of ISO 5817. Everything else has a limit that depends on the level and the thickness — these two do not. That is why they are worth recognising before an inspector does.
Is an ugly weld a defective weld?
Not necessarily. Acceptance is measured against the quality level on the drawing, not against appearance. A rippled, uneven bead within the limits passes; a neat bead with a crack under it does not.
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