ISO 5490:2025

Steel - Rating and classifying nonmetallic inclusions using the scanning electron microscope

This document specifies procedures for the rating and statistical analysis of non-metallic inclusions (referred to as inclusions hereafter) using a scanning electron microscope (SEM) with an energy dispersive X-ray spectrometer (EDS), a backscattered detector (BSD) and automatic image analysis capabilities.

Acier — Classement et classification des inclusions non métalliques à l'aide du microscope électronique à balayage

General Information

• Status: Published
• Publication Date: 25-Sep-2025
• ICS: 77.080.20 - Steels
• Technical Committee: ISO/TC 17/SC 7 - Methods of testing (other than mechanical tests and chemical analysis)
• Drafting Committee: ISO/TC 17/SC 7 - Methods of testing (other than mechanical tests and chemical analysis)
• Current Stage: 6060 - International Standard published
• Start Date: 26-Sep-2025
• Due Date: 15-Aug-2026
• Completion Date: 26-Sep-2025

Overview

ISO 5490:2025 - "Steel - Rating and classifying nonmetallic inclusions using the scanning electron microscope" - specifies standardized procedures for detecting, rating and statistically analysing non‑metallic inclusions in steel using an automated scanning electron microscope (SEM) equipped with an energy dispersive X‑ray spectrometer (EDS), a backscattered electron detector (BSD) and automatic image‑analysis software. The standard defines two main approaches: morphology (shape/size) or chemistry‑based rating and a statistical analysis method for stereological measurements.

Key topics and requirements

• Equipment & software: automated, computer‑controlled SEM with EDS and BSD; light‑element detector required if analysing C, B or N. EDS performance must meet ISO 22309.
• Calibration & checks: magnification calibration per ISO 16700; periodic EDS performance checks (e.g. ISO 15632 / ISO 22309); beam current adjusted to keep dead time < 40%.
• Analysis modes:
  • Morphology rating: automated image analysis classifies inclusions by shape/size (AR threshold of 3 suggested for elongated particles) and chemistry as auxiliary input to reproduce ISO 4967 ratings.
  • Chemistry rating: classifies primarily by elemental composition (may yield different ratings from ISO 4967 if chemistry is primary).
  • Statistical analysis: reports area fraction, number per mm2, size distributions and chemical classes without producing ISO‑style ratings.
• Imaging & measurement parameters: magnification and resolution chosen so minimum particle (e.g., 2 µm) has adequate pixels (pixel size ≤ 1 µm, preferably < 0.6 µm). Suggested accelerating voltage for EDS ~15–20 kV. Spectrum acquisition counts typically set high enough (example: > 3 000 counts) for reliable classification.
• Automated rules: grey‑level thresholds for separating inclusions from matrix, rules for connecting stringers across field boundaries, and criteria for peak significance in EDS spectra (document provides example peak/background criteria).

Applications and users

ISO 5490:2025 is intended for:

• Metallurgical laboratories and quality control teams in steel mills for inclusion rating and process control.
• Failure analysis and R&D teams requiring objective, automated inclusion quantification.
• Suppliers and OEMs specifying inclusion limits for critical components (bearing steels, structural steels, forgings).
• Accreditation and inspection bodies seeking standardized SEM‑based inclusion data.

Practical benefits include reproducible automated ratings, faster large‑area analysis, and flexible statistical reporting tailored to application needs.

Related standards

• ISO 4967 - Micrographic method using standard diagrams (sampling & rating)
• ISO 15632 - EDS performance parameters
• ISO 16700 - SEM magnification calibration
• ISO 22309 - Quantitative EDS analysis and detector requirements

Keywords: ISO 5490:2025, steel inclusions, SEM analysis, EDS, BSD, automatic image analysis, morphology rating, chemistry rating, statistical analysis.

Standards Content (Sample)

International
Standard
ISO 5490
First edition
Steel — Rating and classifying
2025-09
nonmetallic inclusions using the
scanning electron microscope
Acier — Classement et classification des inclusions non
métalliques à l'aide du microscope électronique à balayage
Reference number
© ISO 2025
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
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or ISO’s member body in the country of the requester.
ISO copyright office
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Website: www.iso.org
Published in Switzerland
ii
Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principles . 2
5 Equipment and software . 4
6 Calibration and inspection of equipment . 5
7 Sampling . 5
8 Preparation of specimens . 5
9 Procedure . 5
10 Rating and statistical analysis of inclusions . 7
10.1 Rating method . . .7
10.1.1 Morphology rating method .7
10.1.2 Chemistry rating method .8
10.2 Statistical analysis method .9
11 Test report . 10
Annex A (informative) Typical acquisition analysis rules to be set for SEM analysis.11
Annex B (informative) Example of test results using morphology rating method .12
Annex C (informative) Examples of test results using statistical analysis method . 14
Bibliography . 19

iii
Foreword
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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 Technical Committee ISO/TC 17, Steel, Subcommittee SC 7, Methods of testing
(other than mechanical tests and chemical analysis).
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

iv
International Standard ISO 5490:2025(en)
Steel — Rating and classifying nonmetallic inclusions using
the scanning electron microscope
1 Scope
This document specifies procedures for the rating and statistical analysis of non-metallic inclusions
(referred to as inclusions hereafter) using a scanning electron microscope (SEM) with an energy dispersive
X-ray spectrometer (EDS), a backscattered detector (BSD) and automatic image analysis capabilities.
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 4967, Steel — Determination of content of non-metallic inclusions — Micrographic method using standard
diagrams
ISO 15632, Microbeam analysis — Selected instrumental performance parameters for the specification and
checking of energy-dispersive X-ray spectrometers (EDS) for use with a scanning electron microscope (SEM) or
an electron probe microanalyser (EPMA)
ISO 16700, Microbeam analysis — Scanning electron microscopy — Guidelines for calibrating image
magnification
ISO 22309, Microbeam analysis — Quantitative analysis using energy-dispersive spectrometry (EDS) for
elements with an atomic number of 11 (Na) or above
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 16700, ISO 22309 and the
following apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
maximum Feret’s diameter
maximum distance between the two parallel lines tangent to outer boundary of the particle measured in all
directions, as shown in Figure 1
Note 1 to entry: It is a parameter used to describe the size of irregular particles.

Figure 1 — Schematic of maximum Feret's diameter
3.2
equivalent circle diameter
ECD
diameter of the circle with an area equivalent to the particle
3.3
length
l
dimension of a particle or an inclusion in the main deformation direction, usually larger than the width
3.4
width
w
largest local dimension of a particle or an inclusion perpendicular to the main deformation direction
(“calliper width”)
3.5
aspect ratio
AR
ratio of length to width
3.6
analysis area
certain region of the specimen used to detect inclusions
3.7
rating area
square field of 0,5 mm
Note 1 to entry: It is the specified area of rating inclusions in ISO 4967.
4 Principles
4.1 This document specifies two methods: rating method and statistical analysis method.
4.2 Rating method can use morphology or chemistry as the primary basis for sorting particles into types.
When morphology is mainly used to sort inclusions with chemistry as auxiliary means, it produces the same
ratings as ISO 4967, as illustrated in Figure 2. However, the rating is performed using a fully automated SEM
image analysis system. When agreed by mutual parties, it is also allowed to sort inclusions mainly according
to chemistry, but use morphology as auxiliary means, as illustrated in Figure 3. In this case, it might produce
different ratings from ISO 4967. The two rating methods are called morphology rating method and chemistry
rating method, respectively. The rating method is intended for inclusions with a width larger than 2 μm in

rolled or forged steel products with a reduction ratio of at least 3. The rating method used shall be listed in
the result.
4.3 Statistical analysis method is used for statistics of inclusions and does not produce ratings as shown
in Figure 4. This method defines procedures to analyse and report inclusions by arbitrary size distribution
and chemical classifications. It may be applied to all sizes of inclusions in a variety of billets or rolled steels
by appropriate choice of these classifications. Statistical analysis method determines and reports basic
stereological measurements (for example, area fraction of sulfides or oxides, the number of sulfides or oxides
per square millimetre, and so forth).
Figure 2 — Illustration of morphology rating method

Figure 3 — Illustration of chemistry rating method
Figure 4 — Illustration of statistical analysis method
5 Equipment and software
5.1 An automated computer-controlled scanning electron microscope (SEM) shall be equipped, and the
SEM shall have accessories of EDS and backscattered electron (BSE) detector. The resolution of EDS detector
shall conform to ISO 22309. If analysis of carbon, boron, or nitrogen is requested, a light element detector
shall be equipped with a sufficiently thin window to effectively transmit the low energy X rays characteristic
of the elements below sodium.
5.2 The SEM shall have an automated image analysis software. The software shall meet the following
requirements.
a) It shall allow controlling the beam and stage and collecting of images and spectra according to user-
specified parameters.
b) It shall allow setting analysis rules where chemical classifications can be made and features sorted
according to chemistry as well as size and morphology.
c) It shall be capable of distinguishing between elongated and globular particles based on:
— aspect ratio; separating the stringer according to the difference in morphology;
— classifying inclusions according to morphology or chemistry or both;
— rating inclusions based on the length or number or diameter.
d) It shall be capable of connecting stringers which cross field boundaries and treat inclusions which cross
field boundaries.
e) It shall be able to set one or more grey thresholds to enable discrimination between inclusions and matrix.
6 Calibration and inspection of equipment
6.1 The magnification of images generated by SEM shall be calibrated periodically in accordance with
ISO 16700.
NOTE The accuracy of image magnification of SEM is needed for accurate ratings and to minimize the analysis
time. The number of particles larger than a given size usually increases strongly as the size threshold is lowered. If the
particles smaller than the size threshold are included due to magnification bias, the number of spectra collected, and
therefore the total analysis time, increases significantly.
6.2 The periodical check of EDS performance, especially the energy resolution, shall be carried out using a
reference material (e.g. Co, Ni, Mn) that is appropriate for the type of analysis to be conducted in accordance
with ISO 15632 or ISO 22309.
7 Sampling
7.1 For rating method, sampling shall be carried out in accordance with ISO 4967.
7.2 For statistical analysis method, sampling may be agreed upon by mutual parties.
8 Preparation of specimens
8.1 The preparation of specimens shall be performed in accordance with ISO 4967.
8.2 For irregular specimens, a flat test surface can be obtained by using a special specimen holder to fix
the specimen.
8.3 If mounted, the specimens shall have a good conductivity in accordance with ISO 22309.
9 Procedure
9.1 Put the specimen with a reference material into the SEM at a working distance that is suitable for both
BSE and EDS. In general, aluminium foil may be used as a reference material.

9.2 Set appropriate beam accelerating voltage according to the elements of interest. Set SEM parameters
to optimize the beam current stability and image quality such as saturating the filament, aligning the
column, etc. Accelerating voltage of 15 kV to 20 kV may be used during EDS analysis to detect major elements
although lightly lower or higher voltages may be appropriate depending on the particular application.
9.3 Move the reference material under the beam and record X-ray counts. Adjust the beam current to
obtain sufficient counting rate with less than 40 % dead time to perform an accurate classification of the
inclusion. This optimum beam current may be recorded to use in the next time when similar analyses are
conducted.
9.4 Select the BSE imaging mode. Move the reference material and the specimen into the same field. Adjust
the brightness and contrast to form distinct contrast of the grey level between the reference material and
the specimen. In general, if the grey level falls between 0 to 255 and aluminium foil is used as the reference
material, the grey level of aluminium foil may be set as about 40, and the specimen as around 200. Then
under this setting of grey level move inclusion area of the specimen into the field to set the appropriate
grey threshold interval to discriminate inclusions from the matrix. In general, one threshold interval may
be enough for discriminate the traditional inclusions, such as oxides and sulfides, from the steel matrix.
Multiple grey threshold intervals are needed to include both inclusions containing heavy elements, such
as rare earth or lead and traditional inclusions containing light elements, such as silicon or aluminium.
For a tungsten filament electron microscope, image grey compensation may be used during the testing to
ensure the consistence of discriminating inclusions. The image grey compensation may be set to perform
automatically in every five minutes.
NOTE 1 The steel matrix, which consists primarily of iron, is brighter than some inclusions (for example, MnS) and
darker than other inclusions (for example, Pb).
NOTE 2 With the same grey contrast setting even different operators would obtain the same or similar test results
on the same specimen.
NOTE 3 Multiple grey threshold can be used to separate different inclusions when it is proper.
9.5 Set the relevant imaging parameters, such as the magnification(s) to be used, the minimum and
maximum particle sizes to be recorded. Annex A provides a more complete list of analysis rules in Table A.1.
9.5.1 According to the minimum inclusion size of interest and the number of pixels required for the
minimum size, magnification and image resolution can be chosen such that there are an adequate number of
pixels in each inclusion for the computer program to accurately make measurements. In order to detect a 2 µm
particle, the pixel size shall be at most 1 µm and better less than 0,6 µm. Depending upon the inclusion analysis
software, magnification can be calculated automatically based on the minimum inclusion size of interest, the
number of pixels required for the minimum inclusion size, and the image resolution input by the user.
9.5.2 A critical parameter in the morphological characterization of a particle is the AR. In rating method, a
threshold value of 3 for AR, at or above which a particle is considered elongated, is suggested for consistency
with ISO 4967.
9.6 Set the relevant analysis parameters of EDS, such as X-ray acquisition mode, spectrum acquisition
time, etc. If possible, the minimum number of counts in a peak necessary for peak identification shall be
entered. One accepted criterion for a peak to be considered significant may be that the number of net counts
in the peak (P) shall exceed the background counts (B) by three times the square root of B, or P > 3 B .
Spectrum acquisition time shall be set properly to collect sufficient spectrum counts (e.g. above 3 000) to
perform an accurate classification of the inclusion. Annex A provides a more complete list of analysis rules
in Table A.1.
9.7 Define the relevant chemical classes and their analysis rules. In morphology rating method, at least
two chemical classes are defined: sulfides and oxides. In chemistry rating method, at least three chemical
classes are defined: sulfides, aluminates, and silicates. Additional classes may be defined, depending on the
application. For example, a "calcium silicate" class may be defined and included as Type B, as such inclusions

appear similar to and have the same detrimental effects as traditional Type B inclusions. Each chemical class
and the main inclusion type to which it is assigned should be reported.
9.8 Select and store the analysis area of the specimen to be examined. For specimens with irregular
shape, if there is no special specimen holder to ensure an even test surface, multiple z-axis positions may
be recorded to locate the analysis area. For specimens containing a batch of multiple sections of thin steel
plate, the procedure shall be set to perform the analysis continuously on under the same working conditions
in order to obtain consistent results. The superposed boundary between multiple specimens shall not be
analysed.
9.9 As the beam rasters the analysis area, the software recognizes features that fall within the previously
defined grey threshold intervals. Morphological and chemical parameters are immediately calculated and
stored or, alternatively, raw data is stored for off-line processing.
9.10 In statistical method, the analysis automatically terminates when a minimum number of inclusions
has been classified (e.g. 1 000) or when a specified area of the specimen has been examined (e.g. 200 mm ),
whichever occurs firs
...