Weld imperfection numbers to ISO 6520-1

401, 5011, 2011 — on a visual, radiographic or ultrasonic inspection report an imperfection is recorded by number rather than by name. The full table below, in six groups, with the name behind every number and a link to the article wherever the defect is covered in detail.

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Short answer

The numbers that reach a report most often:

• 100 — crack, 104 — crater crack
• 2011 — isolated pore, 2012 — uniformly distributed porosity
• 3041 — tungsten inclusion (the classic TIG defect)
• 401 — lack of fusion, 402 — incomplete penetration
• 5011 — continuous undercut
• 507 — linear misalignment
• 510 — burn-through
• 602 — spatter

How to read the number

The first digit is the group: 1 cracks, 2 cavities and pores, 3 solid inclusions, 4 lack of fusion and penetration, 5 imperfect shape and dimensions, 6 miscellaneous. The following digits narrow the type, and the last one gives the position or the form.

Take 5011: 5 the shape group, 501 undercut, 5011 continuous undercut. In the same way 1013 is a longitudinal crack (101) lying in the heat-affected zone. The longer the number, the more precisely it states what the inspector saw and where.

The table

1 — Cracks

2 — Cavities and pores

3 — Solid inclusions

4 — Lack of fusion and penetration

5 — Imperfect shape and dimensions

6 — Miscellaneous imperfections

The number does not say the weld is rejected

This is the commonest misunderstanding around the standard. ISO 6520-1 only names and numbers — it is a dictionary, not a criterion. The judgement comes from ISO 5817, which for steel splits the requirements into three quality levels: B (the strictest), C and D. For aluminium welds ISO 10042 plays the same part.

So the same entry "5011, depth 0.4 mm" can be acceptable and unacceptable at once: at level D an undercut of that depth on 10 mm plate passes, at level B it does not. The quality level comes from the fabrication documentation, not from the inspector's judgement. Limits for a given thickness and level are worked out by the ISO 5817 acceptance limits calculator.

Planar defects are a separate matter — lack of fusion (401) and cracks (100). Their tolerance is zero at levels B and C not because they are large, but because they act as a notch: they concentrate stress and grow under fluctuating load. A pore of the same size is far less dangerous than a lack of fusion.

What these defects look like, where they come from and how to avoid them — the weld defect catalogue.

Author: SpawBaza Updated: 24 August 2026