ISO 24497-1:2020

INTERNATIONAL ISO
STANDARD 24497-1
Second edition
2020-03
Non-destructive testing — Metal
magnetic memory —
Part 1:
Vocabulary and general requirements
Essais non destructifs — Mémoire magnétique des métaux —
Partie 1: Vocabulaire et exigences générales
Reference number
©
ISO 2020
© ISO 2020
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting
on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address
below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Fax: +41 22 749 09 47
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2020 – All rights reserved

Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 General requirements . 3
5 Requirements for the inspected object . 4
6 Requirements for the test equipment . 5
7 Preparation for testing . 6
8 Test procedure . 6
9 Test report . 8
10 Safety requirements and personnel qualification . 9
Annex A (informative) Example of stray field distribution of an indication .10
Bibliography .12
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 of 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 www .iso .org/
iso/ foreword .html.
This document was prepared by IIW, International Institute for Welding, Commission V, NDT and Quality
Assurance of Welded Products.
This second edition cancels and replaces the first edition (ISO 24497-1:2007) and ISO 24497-2:2007,
which have been technically revised and merged.
The main changes compared to the previous edition are as follows:
— the scope has revised and extended;
— new normative references have been added;
— Clause 3 has been revised;
— details on the test procedure have been added;
— details of the required test report have been added;
— a test example has been added in Annex A.
A list of all parts in the ISO 24497 series can be found on the ISO website.
Any feedback, question or request for official interpretation related to any aspect of this document
should be directed to IIW via your national standards body. A complete listing of these bodies can be
found at www .iso .org/ members .html.
iv © ISO 2020 – All rights reserved

INTERNATIONAL STANDARD ISO 24497-1:2020(E)
Non-destructive testing — Metal magnetic memory —
Part 1:
Vocabulary and general requirements
1 Scope
This document specifies terms and definitions for non-destructive testing (NDT) by the technique of
metal magnetic memory (MMM) as well as general requirements for application of this technique of the
magnetic testing method.
The terms specified in this document are mandatory for application in all types of documentation and
literature of non-destructive testing, using the metal magnetic memory technique.
This NDT technique has the following objectives:
— determination of the heterogeneity of the magneto-mechanical state of ferromagnetic objects,
detection of defect concentration and boundaries of metal microstructure heterogeneity;
— determination of locations with magnetic stray field aberrations for further microstructural
analysis and/or non-destructive testing and evaluation;
— early diagnostics of fatigue damage of the inspected object and evaluation of its structural life time;
— quick sorting of new and used inspection objects by their magnetic heterogeneity for further testing;
— efficiency improvement of non-destructive testing by combining metal magnetic memory testing
with other NDT methods or techniques (ultrasonic testing, x-ray, etc.) by fast detection of the most
probable defect locations;
— quality control of welded joints of various types and their embodiment (including contact and spot
welding). See ISO 24497-2 for details of this application.
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 9712, Non-destructive testing — Qualification and certification of NDT personnel
ISO/TS 18173, Non-destructive testing — General terms and definitions
ISO 24497-2, Non-destructive testing°— Metal magnetic memory — Part 2: Testing of welded joints
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO/TS 18173 and the
following 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/
3.1
metal magnetic memory
MMM
magnetic state of a ferromagnetic object, depending on how the field has changed in the past and a
consequence of the magneto-mechanical hysteresis of the material
Note 1 to entry: For a given magnetic field (e.g. the magnetic field of the earth), a ferromagnetic object formed
in the course of its fabrication or in operation changes its residual magnetization due to diverse environmental
[35] [6][10][17]
factors which influence the magnetic domain distribution (e.g. temperature, mechanical loads or
microstructural changes of the material).
3.2
magnetic stray field
SF
magnetic field that leaves or enters the surface of a part without intentional magnetization of that part
Note 1 to entry: A ferromagnetic material produces magnetic fields both within its own volume and in the space
around it. The field generated by the magnetization distribution of the material itself is known as the stray field
outside the body or as the demagnetizing field within it. Demagnetizing fields and stray fields are geometry
dependent and arise whenever the magnetization is non-uniform or has a component normal to external
[46]
or internal surfaces . High local changes of the stray field – similar to magnetic flux leakage – can indicate
heterogeneity of material properties.
Note 2 to entry: Other terms that have been used in literature are, for example, self-magnetic leakage field,
residual magnetic field, surface magnetic field, magnetic leakage field, magnetic field density or surface field.
Stray field is the recommended term for passive magnetic field measurements when used for non-destructive
testing purposes, whereas magnetic flux leakage defines a magnetic flux intentionally amplified due to external
sources before or during testing.
3.3
metal magnetic memory testing
MMM testing
technique of the magnetic testing method in NDT based on the measurement and analysis of the
magnetic stray field (3.2) distribution on the surface of inspected objects (IOs) without intentional
(active) magnetization
Note 1 to entry: Magnetic field sensitive probes are used to measure the stray field distribution
3.4
stray field vector
H
SF,i
magnitude in direction i (i= x, y, z) of the magnetic field of the inspected object surface determined by
passive magnetic field sensing
3.5
stray field indication
SFI
any deviation from SF (stray field) uniformity caused by high mechanical stress/strain gradients as
[6][10][17][47]
sources of local stray fields
Note 1 to entry: An SFI is also formed at positions with local magnetic permeability changes, which can be
caused by defect concentrations (e.g. cracks, pitting corrosion), boundaries of strong heterogeneities in the metal
[24][25][57][60] [46]
microstructure, impurities, abrupt geometry changes , internal and external surfaces , separation
of the inspection objects body, irreversible deformations (with high dislocation densities) and changes of the
chemical compositions (e.g. depositing or leaching).
Note 2 to entry: An SFI is not necessary a defect indication and requires interpretation to determine its relevance;
see also Annex A. SFI replaces the term stress concentration zone (SCZ) as used before this revision. It is
recommended to use SCZ only for locations where mechanical stress is concentrated (e.g. sharp corners, crack tips).
2 © ISO 2020 – All rights reserved

3.6
stray field gradient
K
SF
change in stray field magnitude with respect to change of sensor position and/or change of time, t, for
the same sensor position
j
Note 1 to entry: The stray field gradient, K , is calculated according to Formulae (2) and/or (3).
SF,i
3.7
median stray field gradient
K
med
median slope of SF along and/or between measuring line(s) calculated according to Formula (4)
Note 1 to entry: It is related to the shape anisotropy of the IO and its magnetic polarization. If the magnetization
state of initial operating state of the IO is unknown, the median gradient provides an estimation of the proper
state of the IO. In particular, the normal SF component shows frequently a characteristic curve between positive
and negative values.
Note 2 to entry: Changes of the median gradient between periodic (Δt, time-dependent) measurements and/
or changes between working conditions of the IO, e.g. the in-service state and without operation loads can be
related to magneto-mechanical effects.
3.8
magnetic index
j
m
i
ration of the local SFI gradient to the median SFI gradient for evaluation of the SFI, according to
Formula (6)
3.9
distance between neighbouring scanning lines
∆y
distance between the centre points of the sensors in the head and/or distance between two adjacent
measurement lines
j
Note 1 to entry: This distance affects the stray field gradient (3.6), K .
SF,i
3.10
discrete sampling distance in the scanning line
∆x
distance between two adjacent measuring points of the magnitude or components of the stray field
j
Note 1 to entry: This sampling distance affects the stray field gradient (3.6), K .
SF,i
3.11
magnetic stray field diagram
graph displaying the stray field distribution and/or stray field gradient (3.6) and/or median stray field
gradient (3.7) versus the scanning path
3.12
lift-off
distance between surface of IO and centre of the magnetic probe’s sensing area/volume
Note 1 to entry: A small lift-off is essential for the reliability of SFI evaluation.
4 General requirements
4.1 The MMM technique is based on measurement and analysis of the SF distribution of ferromagnetic
objects. The magnetization can reflect the microstructural and technological past and load history of
ferromagnetic metallic components, including welded joints. SFs generated by the residual magnetization
formed in magnetic fields during the process of the fabrication and the service life time of the IO shall be
used during testing.
4.2 The ММM technique enables the detection of SFIs and gives recommendations for additional non-
destructive testing of vessels, pipelines, equipment (e.g. steam generators, turbines, heat exchangers,
rails), and construction welded joints. ISO 24497-2 shall be applied for testing of welded joints.
NOTE SFIs of IOs are conditioned by the fabrication technology (fusion, forging, rolling, turning, press
forming, thermal treatment, etc.).
4.3 Under certain conditions the MMM technique can be used on non-magnetic IOs, particularly if a
ferromagnetic phase is present (e.g. metastable austenitic steels, mill scale, coatings).
NOTE Metastable austenitic steels can be inspected if their microstructure is sensitive to γ - α phase
[18]
transformation . The evaluation of SFs is restricted to the ferromagnetic phase.
4.4 The temperature range during MMM testing shall be within the normal and safe working range for
the operator (NDT inspector).
5 Requirements for the inspected object
5.1 Equipment and structures (IOs) should be inspected by MMM in in-service state (under load)
as well as in the maintenance state (after removal of operating loads). If possible, the initial magnetic
service state of the IO should be determined.
5.2 Surface dressing and preparation are not required. It is recommended to remove insulation to
reduce sensor to surface lift-off to gain reliability and avoid SFI from the insulation. In particular cases,
non-magnetic insulation can be allowed during inspection. Any permissible insulation layer shall be
verified experimentally. The results shall be attached to the test report.
5.3 Limiting factors for the application of MMM testing are the following:
— de-magnetization and intentional magnetization of the IO;
— foreign external (electro-)magnetic fields near to the inspected object, near the inspected region of
interest;
— temperature changes can influence the test results (e.g. at Curie temperature);
— Sensor to IO surface distance (lift-off) and its changes during the measurement.
5.4 Strong temperature changes in the IO cause changes of the thermoremanent magnetization and
should be taken into account during processing of the inspection results.
5.5 Sources of SFI along the IO are the following:
— shape and geometry of the IO (geometry changes and the edges of the IO) are sources of SF and have
to be considered, because surface geometry are sources of strong local stray and demagnetizing
[24][25][46][60]
fields ;
— high mechanical stress gradients;
— boundaries of heterogeneous plastic deformation;
— changes in the microstructure;
— external magnetic fields, e.g. (welding) electric current flow at the inspected object, strong and
heterogeneous magnetic fields close to the tested area;
4 © ISO 2020 – All rights reserved

— foreign ferromagnetic material on the inspected object and near the region of interest;
— local “artificial” magnetization, induced due to former magnetic fields;
— second phase particles with different magnetic properties;
— temperature changes.
All the above sources can influence the evaluation of an SFI and should be taken into account for SFI
assessment.
6 Requirements for the test equipment
6.1 The operation principle shall be based on sensitive magnetic sensors detecting the SF of the
near-surface area of the IO. Magnetic-sensitive probes (e.g. fluxgate transducers) in magnetometer or
gradiometer configuration can be used.
6.2 MMM instruments shall have a display of the testing parameters, a microprocessor based digital
data acquisition and storage and a position encoded movement of the sensors. An external computer
interface shall enable external data storage, retrieving and display of results. External evaluation software
should be provided together with the instrument.
6.3 The sensor type and sensing size is determined by the specific inspection tasks. The equipment
should have at least two measurement channels, one for the SF measurement at the IO and the other for
compensation of influences of external magnetic fields, H . The sensor type and setup (e.g. gradiometer/
e
magnetometer) shall be documented in the test report.
6.4 The sensor shall be manipulated by a scanner and a position encoder shall determine the actual
sensor position during the scanning path. On an IO, where it is difficult to use a scanner, it is allowed to
acquire real-time data.
6.5 The following factors influence the SF measurements:
— sensor lift-off from the IO surface;
— sensor sampling rate along the IO surface;
— sensor sensitivity;
— sensor size;
— alignment of the sensor sensitive direction in relation to IO;
— rotation of the sensors in relation to external field sources (e.g. magnetic field of the earth).
6.6 MMM instruments shall fulfil at least the following minimum requirements:
— the relative error of the measured magnitude of the magnetic SF for each sensor shall be less
than ±5 %;
— the range of sensor sensitivity should be in the order of 1 nT/√(Hz) to 100 µT/√(Hz);
— the relative error of the length measurement shall be less than ±5 %;
— the measurement range of the sensors shall not be less than ±1 000 A/m at a resolution of at
least 1 A/m;
— the sampling distance (distance between the two adjacent measurement points) shall be in the
order of the sensor size and according to the test procedure;
NOTE The sampling size affects SF gradients and SFI detection and evaluation (see 7.2).
— the overall electronic noise level generated by sensors and system shall be less than ±5 A/m;
— inspection tools shall be operable at temperatures from -20 °C to +60 °C.
7 Preparation for testing
7.1 The preparation procedure shall contain the following basic stages:
— analysis of the technical documentation of the IO and preparation of the IO chart (inspection plan,
preparation of IO logfile);
— selection of sensors and equipment;
— preparation of a written procedure for this testing;
— setting and calibration of instruments and sensors according to the written instruction;
— segmentation of the IO into individual inspection areas and inspection units and their indication in
the IO logfile.
7.2 The analysis of technical documentation of the inspected object includes the following:
— information about the steel grades and the dimensions and positions of the selected inspection areas;
— analysis of the IO operation modes and reasons of possible failures (damages);
— surface condition of the IO (e.g. mill scale, polished, corrosion, paint);
— geometry of the IO, design and locations of welded joints.
8 Test procedure
8.1 The magnetization of the inspection object is generally unknown. The three Cartesian components
of the SF shall be measured along the IO surface by continuous or discrete scanning with the instrument
sensors. If possible, the sensor alignment shall coincide Cartesian with the scanning direction. Otherwise,
this shall be documented in the test report.
The IO position in relation to external magnetic field shall not be changed during the measurement. The
surface of the test object shall be covered with a dense network of measurement lines. The positions with
extreme H changes on the IO surface shall be determined and registered by the measurement system.
SF,i
The modulus of the resulting field, ||H || in A/m, is cal
...