oSIST prEN ISO 11666:2026

• Abstract:

  ISO 11666:2018 specifies two ultrasonic acceptance levels known as acceptance level 2 (AL 2) and acceptance level 3 (AL 3) for full penetration welded joints in ferritic steels, which correspond to ISO 5817:2014, quality levels B and C. An acceptance level corresponding to ISO 5817:2014, quality level D is not included in this document, as ultrasonic testing is generally not requested for this weld quality.; These acceptance levels are applicable to testing carried out in accordance with ISO 17640.; ISO 11666:2018 applies to the testing of full penetration ferritic steel welds, with thicknesses from 8 mm to 100 mm. It can also be used for other types of welds, materials and thicknesses, provided the tests have been performed with necessary consideration of the geometry and acoustic properties of the component, and an adequate sensitivity can be employed to enable the acceptance levels of this document to be applied. The nominal frequency of probes used in this document is between 2 MHz and 5 MHz, unless attenuation or requirements for higher resolution call for other frequencies. It is important to consider the use of these acceptance levels in conjunction with frequencies outside this range carefully.

• Status: Not Published
• Public Enquiry End Date: 01-Nov-2026
• ICS: 25.160.40 - Welded joints and welds
• Technical Committee: VAR - Welding
• Current Stage: 4020 - Public enquire (PE) (Adopted Project)
• Start Date: 10-Sep-2026
• Due Date: 28-Jan-2027
• Ref Project: prEN ISO 11666 - Non-destructive testing of welds - Ultrasonic testing - Acceptance levels (ISO/DIS 11666:2026)

• Revises: SIST EN ISO 11666:2018 - Non-destructive testing of welds - Ultrasonic testing - Acceptance levels (ISO 11666:2018)
• Effective Date: 01-Oct-2026

Overview

oSIST prEN ISO 11666:2026 defines requirements for non-destructive testing of welds using ultrasonic testing, specifically establishing acceptance levels for full penetration welded joints in ferritic steels. The standard sets out two primary ultrasonic acceptance levels-Acceptance Level 2 (AL 2) and Acceptance Level 3 (AL 3)-which correspond to ISO 5817 quality levels B and C. These acceptance levels help ensure welded joints meet the necessary quality and safety requirements for critical applications in industries such as construction, shipbuilding, pipelines, and pressure vessels. The standard is maintained by SIST and aligns with international best practices for ultrasonic weld testing.

Key Topics

• Ultrasonic Testing Principles
  The standard covers pulse-echo ultrasonic testing of welds, outlining reference methods, evaluation and recording levels, and probe frequency guidance.

• Acceptance Levels

  • Acceptance Level 2 (AL 2): Associated with higher quality requirements (ISO 5817 level B).
  • Acceptance Level 3 (AL 3): Associated with standard quality requirements (ISO 5817 level C).

• Applicable Materials and Thicknesses
  The criteria apply to full penetration welds in ferritic steels with thicknesses from 8 mm to 100 mm. The standard may be applied to other welded joints, materials, and thicknesses, provided appropriate ultrasonic technique adjustments are made.

• Testing Techniques
  Probes typically use nominal frequencies between 2 MHz and 5 MHz, with justification required if frequencies outside this range are used due to material attenuation or resolution needs.

• Discontinuity Evaluation
  The document outlines methods for determining the length and nature of weld discontinuities and provides guidance on the grouping and cumulation of these discontinuities when assessing overall weld integrity.

Applications

oSIST prEN ISO 11666:2026 is crucial for:

• Quality Assurance in Fabrication:
  Ensures that welds in critical structures comply with required integrity and safety levels, reducing the risk of failure during service.

• Inspection and Testing Procedures:
  Guides ultrasonic testing personnel in setting up and calibrating equipment, performing inspections, and interpreting results based on defined acceptance levels.

• Compliance and Certification:
  Helps manufacturers and inspectors meet international regulations and client specifications for welded structures, particularly in sectors where weld integrity is paramount.

• Adaptability:
  The acceptance levels and techniques can be adapted to other weld types or materials, provided sensitivity and testing requirements outlined in the standard are observed.

Related Standards

For comprehensive non-destructive weld inspection programs, consider integrating the following standards:

• ISO 5817: Welding - Fusion-welded joints in steel, nickel, titanium, and their alloys - Quality levels for imperfections.
  Defines weld quality levels that oSIST prEN ISO 11666:2026 references for acceptance criteria.

• ISO 17640: Non-destructive testing of welds - Ultrasonic testing - Techniques, testing levels, and assessment.
  Establishes ultrasonic testing procedures to be used in conjunction with oSIST prEN ISO 11666:2026.

• ISO 23279: Non-destructive testing of welds - Ultrasonic testing - Characterization of discontinuities in welds.
  Provides methods for classifying the types and features of detected discontinuities.

• ISO 17635: Non-destructive testing of welds - General rules for metallic materials.
  Offers an overarching framework for NDT across different methods and materials.

Implementing oSIST prEN ISO 11666:2026 supports rigorous quality management, enhances structural safety, and promotes compliance with international best practices in weld testing using ultrasonic methods. The standard is a key resource for inspection agencies, fabricators, and regulatory bodies engaged in the assessment and certification of welded structures.

SLOVENSKI STANDARD
01-oktober-2026
Neporušitveno preskušanje zvarnih spojev - Ultrazvočno preskušanje - Stopnje
sprejemljivosti (ISO/DIS 11666:2026)
Non-destructive testing of welds - Ultrasonic testing - Acceptance levels (ISO/DIS
11666:2026)
Zerstörungsfreie Prüfung von Schweißverbindungen - Ultraschallprüfung -
Zulässigkeitsgrenzen (ISO/DIS 11666:2026)
Essais non destructifs des assemblages soudés - Contrôle par ultrasons - Niveaux
d'acceptation (ISO/DIS 11666:2026)
Ta slovenski standard je istoveten z: prEN ISO 11666
ICS:
25.160.40 Varjeni spoji in vari Welded joints and welds
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

DRAFT
International
Standard
ISO/DIS 11666
ISO/TC 44/SC 5
Non-destructive testing of welds —
Secretariat: AFNOR
Ultrasonic testing — Acceptance
Voting begins on:
levels
2026-09-07
Essais non destructifs des assemblages soudés — Contrôle par
Voting terminates on:
ultrasons — Niveaux d'acceptation
2026-11-30
ICS: 25.160.40
THIS DOCUMENT IS A DRAFT CIRCULATED
FOR COMMENTS AND APPROVAL. IT
IS THEREFORE SUBJECT TO CHANGE
AND MAY NOT BE REFERRED TO AS AN
INTERNATIONAL STANDARD UNTIL
PUBLISHED AS SUCH.
This document has not been edited by the ISO Central Secretariat.
IN ADDITION TO THEIR EVALUATION AS
BEING ACCEPTABLE FOR INDUSTRIAL,
TECHNOLOGICAL, COMMERCIAL AND
USER PURPOSES, DRAFT INTERNATIONAL
STANDARDS MAY ON OCCASION HAVE TO
ISO/CEN PARALLEL PROCESSING
BE CONSIDERED IN THE LIGHT OF THEIR
POTENTIAL TO BECOME STANDARDS TO
WHICH REFERENCE MAY BE MADE IN
NATIONAL REGULATIONS.
RECIPIENTS OF THIS DRAFT ARE INVITED
TO SUBMIT, WITH THEIR COMMENTS,
NOTIFICATION OF ANY RELEVANT PATENT
RIGHTS OF WHICH THEY ARE AWARE AND TO
PROVIDE SUPPORTING DOCUMENTATION.
Reference number
ISO/DIS 11666:2026(en)
DRAFT
ISO/DIS 11666:2026(en)
International
Standard
ISO/DIS 11666
ISO/TC 44/SC 5
Non-destructive testing of welds —
Secretariat: AFNOR
Ultrasonic testing — Acceptance
Voting begins on:
levels
Essais non destructifs des assemblages soudés — Contrôle par
Voting terminates on:
ultrasons — Niveaux d'acceptation
ICS: 25.160.40
THIS DOCUMENT IS A DRAFT CIRCULATED
FOR COMMENTS AND APPROVAL. IT
IS THEREFORE SUBJECT TO CHANGE
AND MAY NOT BE REFERRED TO AS AN
INTERNATIONAL STANDARD UNTIL
PUBLISHED AS SUCH.
This document has not been edited by the ISO Central Secretariat.
IN ADDITION TO THEIR EVALUATION AS
BEING ACCEPTABLE FOR INDUSTRIAL,
© ISO 2026
TECHNOLOGICAL, COMMERCIAL AND
USER PURPOSES, DRAFT INTERNATIONAL
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
STANDARDS MAY ON OCCASION HAVE TO
ISO/CEN PARALLEL PROCESSING
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
BE CONSIDERED IN THE LIGHT OF THEIR
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
POTENTIAL TO BECOME STANDARDS TO
WHICH REFERENCE MAY BE MADE IN
or ISO’s member body in the country of the requester.
NATIONAL REGULATIONS.
ISO copyright office
RECIPIENTS OF THIS DRAFT ARE INVITED
CP 401 • Ch. de Blandonnet 8
TO SUBMIT, WITH THEIR COMMENTS,
CH-1214 Vernier, Geneva
NOTIFICATION OF ANY RELEVANT PATENT
Phone: [phone]
RIGHTS OF WHICH THEY ARE AWARE AND TO
PROVIDE SUPPORTING DOCUMENTATION.
Email: [e-mail]
Website: www.iso.org
Published in Switzerland Reference number
ISO/DIS 11666:2026(en)
ii
ISO/DIS 11666:2026(en)
Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Determination of discontinuity length . 1
5 Sensitivity levels . 2
6 Acceptance levels. 2
6.1 General .2
6.2 Indications from longitudinal discontinuities.3
6.3 Indications from transverse discontinuities .3
6.4 Grouping of discontinuities .3
6.5 Cumulated length of acceptable discontinuities . .4
Annex A (normative) Levels . 6
Annex B (normative) Fixed-amplitude level technique .18
Bibliography .21

iii
ISO/DIS 11666:2026(en)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
ISO technical committees. Each member body interested in a subject for which a technical committee
has been established has the right to be represented on that committee. International organizations,
governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely
with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the different types
of ISO documents should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of any patent
rights identified during the development of the document will be in the Introduction and/or on the ISO list of
patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation on the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the World
Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see the following URL:
www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 44, Welding and allied processes, Subcommittee
SC 5, Testing and inspection of welds.
This third edition cancels and replaces the second edition (ISO 11666:2018), which has been technically
revised.
The main changes compared to the previous edition are as follows:
— this document has been editorially revised;
— the normative reference ISO 5817 has been dated;
— Clause 5 and 6.5 have been described in more detail.
Requests for official interpretations of any aspect of this document should be directed to the Secretariat
of ISO/TC 44/SC 5 via your national standards body. A complete listing of these bodies can be found at
www.iso.org.
iv
DRAFT International Standard ISO/DIS 11666:2026(en)
Non-destructive testing of welds — Ultrasonic testing —
Acceptance levels
1 Scope
This document specifies two ultrasonic acceptance levels known as acceptance level 2 (AL 2) and acceptance
level 3 (AL 3) for full penetration welded joints in ferritic steels, in accordance with the quality levels of
ISO 5817.
These acceptance levels are applicable to testing carried out in accordance with ISO 17640.
This document applies to the testing of fusion-welded joints in metallic materials with low sound attenuation
(especially that due to scatter) with thicknesses from 8 mm to 100 mm. It is primarily intended for use on
full penetration welded joints where both the welded and parent materials are ferritic. It can also be used
for other types of welds, materials and thicknesses, provided the tests have been performed with necessary
consideration of the geometry and acoustic properties of the component, and an adequate sensitivity can be
employed to enable the acceptance levels of this document to be applied.
The nominal frequency of probes used in this document is between 2 MHz and 5 MHz, unless attenuation
or requirements for higher resolution call for other frequencies. It is important to consider the use of these
acceptance levels in conjunction with frequencies outside this range carefully.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements of this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
ISO 5817, Welding — Fusion-welded joints in steel, nickel, titanium and their alloys (beam welding excluded) —
Quality levels for imperfections
ISO 17640, Non-destructive testing of welds — Ultrasonic testing — Techniques, testing levels, and assessment
ISO 23279, Non-destructive testing of welds — Ultrasonic testing — Characterization of discontinuities in welds
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 5577 apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
4 Determination of discontinuity length
The length of a discontinuity shall be determined by measuring the distance along the length over which the
echo amplitude is above the evaluation level, using the fixed-amplitude level technique specified in Annex B.

ISO/DIS 11666:2026(en)
5 Sensitivity levels
a) The sensitivity setting has been performed by one of the following techniques accordingly to ISO 17640.
Technique 1: the reference is a distance-amplitude curve (DAC) or a time-corrected gain (TCG) for side-
drilled holes of diameter 3 mm.
Technique 2: the references for transverse and longitudinal waves using the distance-gain-size (DGS) system
based on the diameter of disk-shaped reflectors (D ).
DSR
Technique 3: using a reference notch. The reference notch shall be rectangular, 1 mm wide with a depth of
1 mm. This
...