11.1 ISO 6520 Imperfection Classification

11.1 ISO 6520 Imperfection Classification

Quick Answer: ISO 6520 classifies weld imperfections by type and location (cracks, cavities, solid inclusions, lack of fusion/penetration, imperfect shape, miscellaneous). An imperfection is a discontinuity in the weld or joint; a defect is an imperfection that exceeds the acceptance standard (commonly ISO 5817 quality levels B, C, or D for steel fusion welds). Classification and acceptance are separate documents and separate inspector tasks.

Module WI1.7 requires IWI-S candidates to use ISO 6520 terminology correctly, recognise the main groups, and never confuse classification (what is it?) with acceptance (is it allowed?). Chapter 11 introduces the groups and deep-dives into cracks, porosity, and lack of fusion/penetration; Chapter 12 continues with shape imperfections, inclusions, and evaluation logic. Chapter 15 returns to ISO 5817 quality levels in detail.

Why Classification Matters for Inspectors

Fabrication drawings, NDT reports, repair instructions, and non-conformance reports must name the same thing the same way. Saying “porosity” is vague if the indication is a single gas pore versus a wormhole cluster. ISO 6520 (and its companion reference practices used by training bodies) supplies a shared vocabulary:

1. Name the imperfection type (and subtype where known).
2. Locate it (weld metal, fusion line, HAZ, parent metal, root, surface, internal).
3. Size and density it as required by the applicable acceptance standard.
4. Decide accept / reject / repair only against the contracted quality level, not against personal preference.

Without that sequence, two inspectors can disagree even when they see the same film or surface indication.

Imperfection Versus Defect (Hard Exam Distinction)

Exam logic: Every defect is an imperfection, but not every imperfection is a defect. A small gas pore may be an imperfection that is still acceptable at quality level C. A crack of any practical size is typically still an imperfection and a defect at B, C, and D for steel fusion welds under ISO 5817—because cracks are generally not permitted.

IWI-S language in reports should prefer precise classification names, then state the acceptance conclusion separately: “ISO 6520 gas pore (2011), diameter …, density … — exceeds ISO 5817 quality level B limits.”

The Six Primary Groups (ISO 6520 Framework)

ISO 6520 organises imperfections into major groups. Training syllabi and IWI examinations expect you to know the group identities and typical members—not every numeric code by heart, but the map:

Group 1 — Cracks

Discontinuities with sharp tips produced by local rupture under stress (hot, cold, crater, lamellar tearing, etc.). Subtypes include longitudinal, transverse, radiating, crater, and branching cracks; locations include weld metal, HAZ, and parent metal. Planar, high stress-raising, and usually zero-tolerance in common structural acceptance standards. Covered in depth in Section 11.2 and metallurgically in Chapter 6.

Group 2 — Cavities

Gas-related and shrinkage cavities: gas pores (single, uniform, clustered, linear), elongated cavities, wormholes (piping), crater pipes, and shrinkage cavities. Common production imperfections; often size- and density-limited rather than automatically rejectable. Section 11.3.

Group 3 — Solid Inclusions

Foreign solid material trapped in the weld metal: slag, flux, oxide, metallic inclusions (tungsten, copper), and other solids. Sources include incomplete slag removal between runs, oxidised surfaces, and consumable-related debris. Chapter 12.

Group 4 — Lack of Fusion and Lack of Penetration

Lack of fusion (LOF) between weld metal and parent metal or between weld beads; lack of penetration (LOP) / incomplete root penetration where the design requires full penetration. Planar or near-planar; typically not permitted at ISO 5817 levels B/C/D for the relevant joint types. Section 11.4.

Group 5 — Imperfect Shape and Dimensions

Undercut, overlap, excess weld metal, excess penetration, incorrect weld profile, linear misalignment, angular misalignment, incomplete filled groove, and related geometric departures from the specified joint. Often assessed by gauge, rule, and visual criteria against ISO 5817 tables. Chapter 12.

Group 6 — Miscellaneous Imperfections

Spatter, arc strikes, grinding marks, temper colours (where relevant), stray arc, underflushing, and other items that do not fit cleanly in groups 1–5 but still affect integrity, corrosion, fatigue, or appearance. Chapter 12.

Classification Codes and Reporting Practice

ISO 6520 uses a numeric reference system for types and subtypes (training materials often quote common codes such as cracks in the 100 series, cavities in the 200 series, solid inclusions in the 300 series, LOF/LOP in the 400 series, shape in the 500 series, miscellaneous in the 600 series). IWI-S candidates should:

• Prefer descriptive names + location in everyday shop talk and NCR narrative.
• Use standard codes when the client, procedure, or NDT report format requires them.
• Avoid inventing hybrid names (“porosity-crack”) when two imperfections coexist—report both.

When NDT reports only say “indication,” the welding inspector’s follow-up is to convert that into ISO 6520 type + location + dimensions before acceptance comparison.

ISO 5817 Is Acceptance, Not Classification

ISO 5817 Welding — Fusion-welded joints in steel, nickel, titanium and their alloys (beam welding excluded) — Quality levels for imperfections defines quality levels B, C, and D (stringent → intermediate → moderate) with limits for many imperfection types. Key inspector truths:

1. ISO 6520 tells you what you found; ISO 5817 (or the product standard) tells you whether it is allowed.
2. Quality level is a contractual choice (drawing, specification, EN product standard, client ITP)—not the inspector’s personal choice on the day.
3. Some imperfections have quantitative limits (height of undercut, pore diameter, excess weld metal).
4. Some imperfections are not permitted at any listed level (classically cracks, and typically lack of fusion / incomplete penetration for the joints covered).
5. Acceptance may also come from other standards (pressure equipment, pipelines, aluminium-specific standards such as ISO 10042 concepts, client ECA)—always follow the governing document hierarchy.

Do not use ISO 5817 tables as a classification dictionary, and do not use ISO 6520 as an acceptance code.

Related Standards Inspectors Meet Daily

IWI-S competence includes identifying which document governs acceptance for the job—not inventing hybrid criteria.

How Imperfections Arise (Systems View)

Every imperfection group maps to process, metallurgy, design, or workmanship causes:

• Cracks → hydrogen, solidification, liquation, restraint, brittle microstructure (Chapters 5–7).
• Cavities → gas entrapment, moisture, contamination, shielding, arc length, solidification shrinkage.
• Inclusions → slag/flux practice, oxidation, electrode debris.
• LOF/LOP → heat input, technique, joint design, access, root gap/face.
• Shape → parameters, position, travel speed, fit-up, welder skill.
• Miscellaneous → handling, grinding, arc strikes, poor housekeeping.

The inspector’s value is connecting observed classification → root-cause group → procedure or training corrective action, not only ticking a reject box.

Inspector Workflow Using ISO 6520

1. Prepare — know the specified quality level, joint type, material, and NDT plan.
2. Detect — VT first; surface NDT (PT/MT); volumetric NDT (RT/UT) as specified.
3. Classify — name using ISO 6520 groups/types; note orientation and location.
4. Measure — length, height, depth, diameter, density, spacing as required.
5. Compare — apply ISO 5817 (or governing standard) tables/rules.
6. Record — clear NCR/report language; photo/sketch where useful.
7. Disposition — accept, repair, or refer for engineering/ECA when the contract allows.

Common Terminology Traps

• Calling every indication a “defect” before acceptance comparison.
• Confusing undercut (shape group) with lack of fusion (group 4).
• Treating slag line as a crack on RT without secondary evaluation.
• Assuming ISO 5817 Level B is always required—level is specified, not assumed.
• Forgetting that multiple imperfections can interact (clusters, combined length rules in acceptance standards).

Link Forward

Section 11.2 focuses on cracks—the highest-severity group. Section 11.3 covers porosity and gas cavities. Section 11.4 addresses lack of fusion and penetration. Chapter 12 completes shape, inclusions, and evaluation practice; Chapter 15 locks acceptance tables and quality-system links (ISO 3834, procedure qualification). Master the vocabulary first: without ISO 6520 language, acceptance decisions and NDT reports become ambiguous under audit.