If your team has ever argued over whether a weld is acceptable (not “pretty”, not “perfect”, but acceptable), ISO 5817 is often the standard that ends the debate—provided you implement it correctly.
This guide explains:
- what ISO 5817 covers (and what it doesn’t),
- how quality levels B / C / D actually work,
- how to connect ISO 5817 to visual inspection, NDT selection, and documentation,
- and how to build an audit-proof acceptance workflow you can run across shifts, stations, and suppliers.
Important: This article is implementation guidance, not a substitute for the official standard or your governing code/customer specification.
Table of contents
- What ISO 5817 is (and isn’t)
- Quality levels B, C, D in plain language
- ISO 5817’s “secret weapon”: ISO 6520-1 imperfection codes
- How ISO 5817 fits with ISO 17637 and ISO 17635
- A practical acceptance workflow you can run in production
- How to make acceptance traceable and audit-ready
- Common mistakes (and how to avoid them)
- FAQ
What ISO 5817 is and isn’t
ISO 5817 defines quality levels for imperfections in fusion-welded joints (beam welding excluded) for steels, nickel, titanium, and their alloys. In other words: it’s a structured way to say “this imperfection is acceptable at this quality level.”
Official reference: ISO 5817:2023 on ISO.org
ISO 5817 does not:
- tell you which NDT method to choose,
- replace product codes (ASME, EN 1090 project requirements, customer specs, etc.),
- guarantee fitness-for-service by itself (that depends on loads, environment, design, and code).
Think of ISO 5817 as a workmanship acceptance framework. Your governing code/spec decides when you must use it and what level applies.
Quality levels B, C, D in plain language
ISO 5817 uses three quality levels:
- B = highest requirement (tightest acceptance)
- C = medium requirement (typical industrial baseline)
- D = moderate requirement (most permissive)
How to choose the level (a practical decision rule)
Pick the level based on consequence, not vibes:
- Level B: safety-critical, fatigue-sensitive, high liability, high load cycling, “failure is catastrophic or very expensive”
- Level C: general structural and industrial fabrication where reliability matters but consequences are manageable
- Level D: non-critical components where function is tolerant and risk is low
If you’re under EN/ISO compliance regimes (e.g., EN 1090 execution classes), your project spec often implies a level—but always confirm in the contract and ITP (Inspection & Test Plan).
ISO 5817’s secret weapon: ISO 6520-1 imperfection codes
One reason ISO 5817 works so well in real life is that it leans on ISO 6520-1, which standardizes how imperfections are classified and named.
Official reference: ISO 6520-1:2007 on ISO.org
That matters because audits and supplier disputes usually fail on language:
- “lack of fusion” means one thing to QA and another to production… until you label it consistently.
- an NCR that says “bad weld” is useless; an NCR that says “imperfection per ISO 6520-1 classification” is actionable.
Best practice: Put the ISO 6520-1 imperfection categories/codes into your inspection forms and digital travelers so every call is consistent.
How ISO 5817 fits with ISO 17637 and ISO 17635
ISO 5817 answers: “What is acceptable?”
You still need standards that answer:
-
“How do we visually inspect it?” → ISO 17637 (visual testing of fusion welds)
Official reference: ISO 17637:2016 on ISO.org -
“Which NDT method and acceptance levels should we use?” → ISO 17635 (general rules for NDT selection and evaluation)
Official reference: ISO 17635:2025 on ISO.org
The clean mental model
- ISO 5817 = acceptance levels (workmanship quality levels)
- ISO 17637 = rules and conditions for visual testing
- ISO 17635 = how to choose NDT methods and evaluate results (aligned to quality requirements)
This is also why “we did 10% UT” can be a trap: sampling finds defects after the process drifted. If you want an efficient layered strategy, see:
A practical acceptance workflow you can run in production
Here’s a workflow that scales across shifts, lines, and suppliers.
Step 1) Define the acceptance rule hierarchy (no ambiguity)
Write down the hierarchy, in this order:
- Governing code / customer specification / contract
- Project ITP + weld map
- ISO 5817 quality level (B/C/D) where invoked
- Internal work instructions (tools, measurement method, hold points)
If two documents conflict, you resolve it here—not on the shop floor.
Step 2) Classify welds into “acceptance classes”
Most plants have different risk levels across parts. Create 2–5 classes, for example:
- Class A: fatigue/safety critical → typically tighter acceptance
- Class B: structural baseline
- Class C: non-critical
Assign each class a default ISO 5817 level only if allowed by the governing spec.
Step 3) Build a defect catalogue your inspectors can actually use
Your catalogue should include:
- imperfection name + ISO 6520-1 reference
- what it looks like (photo/sketch examples)
- how it’s measured (gauge, template, microscope, etc.)
- how it’s recorded (joint ID, location, length reference)
- what happens next (accept / repair / NDT escalation)
This is where quality goes from “tribal knowledge” to a repeatable system.
Step 4) Decide when VT is enough—and when it isn’t
Visual testing is fast and powerful, but it’s not magic. When VT is inconclusive or you’re dealing with subsurface risk, you escalate per ITP and ISO 17635 guidance.
A good layered approach is:
- VT everywhere
- targeted NDT where required by code/spec
- process monitoring to reduce drift and concentrate NDT where it matters
(If you’re exploring process monitoring, start here:
Infrared Thermography for Welding: Real-Time Quality)
Step 5) Close the loop with data (or you’ll re-learn the same lessons forever)
Acceptance criteria are not just pass/fail—they’re feedback signals.
If a defect repeats:
- route it into CAPA,
- quantify it (Pareto),
- link it to process variables and shift/station.
Useful reading:
- CAPA for Welding: 8D, 5-Why, and Data-Driven Root Cause
- SPC for Welding: X-bar/R Charts, Cp/Cpk, and Thermal Limits
How to make acceptance traceable and audit-ready
Audits rarely fail because someone used the “wrong” quality level. They fail because:
- the level wasn’t documented,
- evidence isn’t linked to the joint,
- acceptance decisions can’t be reproduced.
What “audit-ready” looks like
For every weld (or weld segment), you can produce:
- Joint ID / traveler ID
- WPS/PQR reference
- operator identity / qualification link
- acceptance criteria reference (e.g., “ISO 5817 Level C, project class B”)
- inspection record (VT + NDT if required)
- evidence attachments (photos, measurements, frames, notes)
- disposition + sign-off
If you want a strong blueprint for this, read:
- Digital Welding Quality Records: WPS/PQR, Continuity, and Traceability
- ISO 3834 & EN 1090 Compliance: Digital Welding Traceability
- Welding QMS Software: ISO 3834, EN 1090, ASME — Practical Guide
Where real-time monitoring fits (without overclaiming)
Real-time monitoring (thermography + AI) is best treated as:
- a 100% coverage process stability layer
- a way to flag drift early
- a way to generate time-stamped evidence linked to weld IDs
It doesn’t automatically replace code-mandated NDT—but it can reduce surprises and focus expensive inspection where it’s actually needed.
Background on Therness’ in-line monitoring approach:
Politecnico di Torino: Safer Welding with In-line Temperature Monitoring
Common mistakes and how to avoid them
-
Choosing B/C/D without defining consequence
If you can’t explain why a weld is Level B, you’ll apply Level B to everything and burn money. -
Not standardizing imperfection naming
Use ISO 6520-1 terminology so NCRs are unambiguous. -
No measurement method agreement
Acceptance depends on how you measure (tooling, angle, access). Define the method in the work instruction. -
Treating VT as “quick look” instead of a controlled test
ISO 17637 exists because VT must be performed under defined conditions, with defined records. -
No traceability chain
If evidence isn’t linked to joint IDs and acceptance criteria, your “system” is a collection of screenshots.
FAQ
Is ISO 5817 mandatory?
Only if your governing code/spec/contract says so. Many projects reference it explicitly as the acceptance framework for imperfections.
What’s the difference between ISO 5817 and EN ISO 5817?
“EN ISO” is the European adoption of the ISO standard (same technical content, adopted into the EN system). Your contract usually states which applies.
Can I copy ISO 5817 acceptance tables into my procedure?
Be careful: ISO standards are copyrighted. Many companies reference the standard, summarize the logic, and keep the official document controlled internally.
Which standard tells me how to do visual inspection?
ISO 17637 covers visual testing of fusion welds and how VT should be carried out and recorded.
Official reference: https://www.iso.org/standard/67259.html
Where does NDT method selection fit?
ISO 17635 provides general rules for selecting NDT methods for welds and evaluating results based on quality requirements.
Official reference: https://www.iso.org/standard/85705.html
References (official)
- ISO 5817:2023 — Quality levels for imperfections: https://www.iso.org/standard/80209.html
- ISO 6520-1:2007 — Classification of imperfections: https://www.iso.org/standard/40229.html
- ISO 17637:2016 — Visual testing of fusion welds: https://www.iso.org/standard/67259.html
- ISO 17635:2025 — General rules for NDT selection/evaluation: https://www.iso.org/standard/85705.html
Next step (templates)
If you’re building or updating your acceptance system, use a structured RFP and workflow template so procurement, QA, and production stay aligned:
- Weld Monitoring System RFP Template (see latest resources)
- Or start with the inspection strategy overview:
Welding Inspection Methods Compared: VT, RT/UT/PAUT, and Real-Time Monitoring