ISO 17637 Visual Weld Inspection: Procedure & Acceptance Criteria

Visual testing (VT) is the oldest and most universally required non-destructive examination method for fusion welds. Despite its apparent simplicity, it is also the most abused: rushed, underdocumented, and systematically under-specified. ISO 17637 exists to close that gap — transforming an informal glance at a weld bead into a repeatable, auditable procedure with defined conditions, qualified personnel, and traceable acceptance decisions.

This guide covers what ISO 17637 requires, how to structure a compliant examination across all three inspection stages, how to map findings to ISO 5817 quality levels, and where visual inspection reaches its fundamental limits.

What ISO 17637 Covers

ISO 17637:2016 (Non-destructive testing of welds — Visual testing of fusion-welded joints) defines the requirements for visual examination of:

• All fusion welding processes: MIG/MAG, TIG, SAW, SMAW, FCAW, laser beam, electron beam, plasma arc
• All joint configurations: butt, fillet, T-joint, corner joint, lap joint
• Three examination stages: pre-weld, inter-pass, post-weld

The standard applies to metallic materials only. It does not govern thermoplastic welding or brazing.

Standards Relationship

ISO 17637 defines how to perform visual examination. It does not define acceptance criteria — those come from the product or application standard:

An examination performed per ISO 17637 but evaluated against an unspecified or wrong quality level produces a non-conforming inspection record. Both the method standard and the acceptance standard must be cited in the report.

Personnel Requirements

ISO 17637 requires the visual examiner to hold a recognised qualification in Visual Testing (VT). Accepted routes include:

• ISO 9712 Level 1, 2, or 3 — VT: internationally recognised; Level 1 can perform and record, Level 2 can evaluate and sign
• CSWIP 3.0 / 3.1 / 3.2: UK-centric, widely accepted in oil and gas and offshore
• AWS CWI / SCWI: standard in North America and global projects under ASME or AWS jurisdiction
• EN 473: predecessor to ISO 9712, still valid under many customer approval schemes

Level 2 qualification is required to evaluate findings against acceptance criteria and issue a signed examination report. Level 1 can assist and record under Level 2 supervision.

Equipment and Examination Conditions

Lighting Requirements

ISO 17637 specifies a minimum illuminance of 500 lux at the examination surface for standard VT. For enhanced visual testing (EVT) where finer surface detail must be resolved, 1,000 lux minimum is required.

Practical compliance:

• Use a calibrated illuminance meter with a current calibration certificate
• Avoid direct glare on reflective weld surfaces — angle the light source appropriately
• Use supplementary LED wands or fibre-optic probes for restricted-access joints
• Record the measured illuminance value in the examination report

Viewing Geometry

Examinations conducted at angles below 30° or distances beyond 600 mm must be explicitly justified and documented. Remote visual examination using borescopes, camera crawlers, or pan-tilt units constitutes a separate technique that must be agreed with the responsible welding coordinator prior to use.

Gauges and Measurement Tools

Standard VT equipment for weld examination includes:

• Weld gauges (bridge cam gauge, Hi-Lo gauge): fillet throat, leg, reinforcement, and linear misalignment
• Pit gauge or depth micrometer: surface imperfection depth
• Straight edge: linearity, distortion, crown profile
• Flexible rule: crown height on curved pipe sections

All measurement equipment must be within calibration validity. Calibration traceability records must be available for audit.

Three-Stage Examination Sequence

ISO 17637 structures VT into three mandatory stages. Missing an inter-pass inspection cannot be compensated by a more thorough post-weld check — the audit trail is missing and the examination is non-compliant.

Stage 1 — Pre-Weld Inspection

Before arc strike, verify and record:

1. Joint geometry vs WPS: root gap, bevel angle, included angle, land dimension, Hi-Lo alignment
2. Base material identity: heat number, material grade, thickness match to procedure
3. Surface preparation: joint faces and adjacent base material free from scale, moisture, paint, grease, and oil within the specified clean zone
4. Fit-up and tacking: tack welds within procedure limits; no excessive restraint; tack cracks absent

Stage 2 — Inter-Pass Inspection

After each pass (or per the approved inspection and test plan):

1. Slag and spatter completely removed from the deposited pass surface
2. No cracks, cold laps, or surface porosity visible
3. Undercut on previous passes within acceptance limits
4. Interpass temperature measured and within WPS limits — cross-reference with real-time thermal monitoring data where inline systems are installed
5. Pass geometry compatible with subsequent passes: excessive convexity on root passes can trap slag in fill and cap passes

Stage 3 — Post-Weld Inspection

Final examination after all passes are complete, slag removed, and surface cleaned:

1. Weld geometry: width, reinforcement, throat (fillets), straightness, angular distortion
2. Surface imperfections: cracks, surface porosity, undercut, overlap, arc strikes, excessive spatter, craters
3. Dimensional verification: weld meets drawing tolerances for position, length, and geometry
4. Weld run verification: confirm sequence and number of passes where specified

Acceptance Criteria — ISO 5817 Decision Framework

Findings are evaluated against the applicable quality level from ISO 5817 (steel) or EN ISO 10042 (aluminium). The quality level is specified by the application standard — EN 1090, EN 15085, AWS D1.1, ASME BPVC, or customer specification.

t = material thickness, b = weld width. This table is a simplified reference — always consult the current edition of ISO 5817 for authoritative limits. See our ISO 5817 quality levels guide for a complete breakdown including defect-specific tables.

Documentation Requirements

A compliant ISO 17637 visual examination report must include:

• Reference to ISO 17637 edition (year) and applicable acceptance standard + quality level
• Weld identification: joint ID, drawing number, weld type, base material, thickness
• Welding process and position (per ISO 6947)
• Examination stage covered (pre-weld, inter-pass, post-weld — list separately)
• Lighting: measured illuminance value, instrument used, calibration reference
• Equipment: gauges used with calibration certificate references
• Personnel: name, qualification standard, level, certificate number, certificate expiry date, vision check date
• Environmental conditions (if significant: outdoor, confined space, low ambient light)
• Findings: imperfection type per ISO 6520-1, location, dimensions, severity
• Accept / Reject verdict per specified quality level with justification for borderline cases
• Examiner signature and date; countersignature of Level 2 or above if Level 1 performed the work

Reports missing illuminance data, personnel certificate traceability, or ISO 6520 imperfection codes are non-conforming. They do not satisfy ISO 3834 documentation requirements.

ISO 17637 Compliance Checklist

The Detection Limit of Visual Inspection

ISO 17637 covers the outer surface of the weld. Subsurface defects — the category most responsible for catastrophic in-service failures — are outside its scope by definition. A weld with root porosity, inter-run lack of fusion, or a solidification crack sealed below the cap surface will routinely pass VT.

This creates a systematic gap for:

• Subsurface porosity not open to surface
• Root cracks in single-sided full-penetration welds
• Lack of fusion between weld passes or at fusion line
• Hot cracking that occurs during solidification and closes before the inter-pass inspection window

Inline thermographic monitoring systems capture the thermal signature of each weld bead at the moment of deposition — before slag is removed, before the surface cools, and before any manual inspection is possible. The thermal record provides objective evidence of process-parameter compliance per pass, complementing the ISO 17637 VT record without replacing the mandatory examination stages.

For high-volume production or safety-critical weld joints, combining ISO 17637 VT with inline monitoring addresses both the surface and the real-time process layers. The two methods are not competitive — they close different detection windows.

Read our comparison of weld inspection methods — VT, RT, UT, PAUT, and real-time monitoring for a structured breakdown of detection capability and cost per method.

Key Takeaways

• ISO 17637 defines how to perform VT; ISO 5817 defines what to accept — both must be cited.
• Three mandatory examination stages: pre-weld, inter-pass, post-weld. Missing any stage is a non-conformance.
• Minimum illuminance: 500 lux standard VT, 1,000 lux enhanced VT. Measure and record.
• Viewing: maximum 600 mm distance, minimum 30-degree angle to the examined surface.
• Level 2 examiner required to evaluate findings and sign the report.
• Report must include illuminance data, personnel certificate traceability, and ISO 6520 imperfection codes.
• VT cannot detect subsurface defects — inline thermal monitoring closes this gap as a complementary method.