EN ISO 643:2020

Steels - Micrographic determination of the apparent grain size (ISO 643:2019, Corrected version 2023-11)

This document specifies a micrographic method of determining apparent ferritic or austenitic grain size in steels. It describes the methods of revealing grain boundaries and of estimating the mean grain size of specimens with unimodal size distribution. Although grains are three-dimensional in shape, the metallographic sectioning plane can cut through a grain at any point from a grain corner, to the maximum diameter of the grain, thus producing a range of apparent grain sizes on the two-dimensional plane, even in a sample with a perfectly consistent grain size.

Stahl - Mikrophotographische Bestimmung der erkennbaren Korngröße (ISO 643:2019, korrigierte Fassung 2023-11)

Dieses Dokument legt mikrophotographische Verfahren zur Bestimmung der erkennbaren Ferrit  oder Austenitkorngröße von Stählen fest. Es werden Verfahren zum Sichtbarmachen der Korngrenzen und zur Schätzung der mittleren Korngröße von Proben mit einer gleichmäßigen Größenverteilung beschrieben. Obwohl die Körner dreidimensional sind, können sie durch eine metallographische Schnittebene an einem beliebigen Punkt am Rand eines Korns bis einschließlich durch seinen größten Durchmesser so durchschnitten werden, dass auch in einer Probe mit völlig konstanter Korngröße in der zweidimensionalen Ebene eine erkennbare Bandbreite von Korngrößen erzeugt wird.

Aciers - Détermination micrographique de la grosseur de grain apparente (ISO 643:2019, Version corrigée 2023-11)

La présente document spécifie une méthode de détermination micrographique de la grosseur apparente du grain ferritique ou austénitique des aciers. Elle décrit les méthodes de mise en évidence des joints de grains et d'estimation de la grosseur moyenne de grain d'un échantillon ayant une distribution granulométrique unimodale. Bien que les grains soient de forme tridimensionnelle, le plan de la préparation métallographique peut couper un grain en tout point, passant par un coin du grain ou au travers du diamètre maximal du grain ou entre les deux, produisant de ce fait une gamme de grosseurs de grain apparentes sur le plan bidimensionnel, même dans le cas d'un échantillon présentant une grosseur de grain parfaitement cohérente.

Jekla - Mikrografsko določevanje navidezne velikosti kristalnih zrn (ISO 643:2019, popravljena različica 2020-03)

General Information

• Status: Withdrawn
• Publication Date: 14-Jan-2020
• Withdrawal Date: 13-Apr-2025
• ICS: 77.040.99 - Other methods of testing of metals
• Technical Committee: ECISS/TC 101 - Test methods for steel (other than chemical analysis)
• Drafting Committee: ECISS/TC 101 - Test methods for steel (other than chemical analysis)
• Current Stage: 9960 - Withdrawal effective - Withdrawal
• Start Date: 04-Sep-2024
• Completion Date: 14-Apr-2025
• Ref Project: SIST EN ISO 643:2020 - Steels - Micrographic determination of the apparent grain size (ISO 643:2019, Corrected version 2020-03)

Relations

• Replaces: EN ISO 643:2012 - Steels - Micrographic determination of the apparent grain size (ISO 643:2012)
• Effective Date: 21-Jan-2020
• Replaced By: EN ISO 643:2024 - Steels - Micrographic determination of the apparent grain size (ISO 643:2024)
• Effective Date: 19-Jan-2023

Standards Content (Sample)

SLOVENSKI STANDARD
01-april-2020
Jekla - Mikrografsko določevanje navidezne velikosti kristalnih zrn (ISO 643:2019,
popravljena različica 2020-03)
Steels - Micrographic determination of the apparent grain size (ISO 643:2019, Corrected
version 2020-03)
Stahl - Mikrophotographische Bestimmung der erkennbaren Korngröße (ISO 643:2019,
korrigierte Fassung 2020-03)
Aciers - Détermination micrographique de la grosseur de grain apparente (ISO
643:2019, Version corrigée 2020-03)
Ta slovenski standard je istoveten z: EN ISO 643:2020
ICS:
77.040.99 Druge metode za Other methods of testing of
preskušanje kovin metals
77.080.20 Jekla Steels
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN ISO 643
EUROPEAN STANDARD
NORME EUROPÉENNE
January 2020
EUROPÄISCHE NORM
ICS 77.040.99 Supersedes EN ISO 643:2012
English Version
Steels - Micrographic determination of the apparent grain
size (ISO 643:2019, Corrected version 2020-03)
Aciers - Détermination micrographique de la grosseur Stahl - Mikrophotographische Bestimmung der
de grain apparente (ISO 643:2019, Version corrigée erkennbaren Korngröße (ISO 643:2019, korrigierte
2020-03) Fassung 2020-03)
This European Standard was approved by CEN on 26 October 2018.

This European Standard was corrected and reissued by the CEN-CENELEC Management Centre on 08 April 2020.

CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this
European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references
concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN
member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by
translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management
Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and
United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATIO N

EUROPÄISCHES KOMITEE FÜR NORMUN G

CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2020 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 643:2020 E
worldwide for CEN national Members.

Contents Page
European foreword . 3

European foreword
This document (EN ISO 643:2020) has been prepared by Technical Committee ISO/TC 17 "Steel" in
collaboration with Technical Committee CEN/TC 459/SC 1 “Test methods for steel (other than chemical
analysis)” the secretariat of which is held by AFNOR.
This European Standard shall be given the status of a national standard, either by publication of an
identical text or by endorsement, at the latest by July 2020, and conflicting national standards shall be
withdrawn at the latest by July 2020.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
This document supersedes EN ISO 643:2012.
According to the CEN-CENELEC Internal Regulations, the national standards organizations of the
following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria,
Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland,
Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Republic of
North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and the
United Kingdom.
Endorsement notice
The text of ISO 643:2019, Corrected version 2020-03 has been approved by CEN as EN ISO 643:2020
without any modification.
INTERNATIONAL ISO
STANDARD 643
Fourth edition
2019-12
Corrected version
2020-03
Steels — Micrographic determination
of the apparent grain size
Aciers — Détermination micrographique de la grosseur de grain
apparente
Reference number
ISO 643:2019(E)
©
ISO 2019
ISO 643:2019(E)
© ISO 2019
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: [phone]
Fax: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland
ii © ISO 2019 – All rights reserved

ISO 643:2019(E)
Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
3.1 Grains . 1
3.2 General . 2
4 Symbols . 2
5 Principle . 3
6 Selection and preparation of the specimen . 4
6.1 Test location . 4
6.2 Revealing ferritic grain boundaries . 5
6.3 Revealing austenitic and prior-austenitic grain boundaries . 5
6.3.1 General. 5
6.3.2 “Bechet-Beaujard” method by etching with aqueous saturated picric acid
solution . 5
6.3.3 “Kohn” method by controlled oxidation . 6
6.3.4 “McQuaid-Ehn” method by carburization at 925 °C . 7
6.3.5 Proeutectoid ferrite method . 8
6.3.6 Bainite or gradient-quench method . 9
6.3.7 Sensitization of austenitic stainless and manganese steels . 9
6.3.8 Other methods for revealing prior-austenitic grain boundaries . 9
7 Characterization of grain size .10
7.1 Characterization by an index .10
7.1.1 Formulae .10
7.1.2 Assessment by comparison with standard grain size charts .10
7.1.3 Planimetric method .11
7.1.4 Estimation of the index .11
7.2 Characterization by the intercept method .11
7.2.1 Linear intercept segment method .11
7.2.2 Circular intercept segment method .12
7.2.3 Assessment of results .13
8 Test report .14
Annex A (informative) Summary of methods for revealing ferritic, austenitic or prior-
austenitic grain boundaries in steels .15
Annex B (normative) Evaluation method .16
Bibliography .21
ISO 643:2019(E)
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 17, Steel, Subcommittee SC 7, Methods of
testing (other than mechanical tests and chemical analysis).
This fourth edition cancels and replaces the third edition (ISO 643:2012), which has been technically
revised. The main changes compared to the previous edition are as follows:
— 7.1.2 has been modified;
— the original Annex B has been deleted and the original Annex C has been renumbered as Annex B.
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.
This corrected version of ISO 643:2019 incorporates the following corrections:
— minus sign replaced with plus sign between the values in Formula B.9.
iv © ISO 2019 – All rights reserved

INTERNATIONAL STANDARD ISO 643:2019(E)
Steels — Micrographic determination of the apparent
grain size
1 Scope
This document specifies a micrographic method of determining apparent ferritic or austenitic grain
size in steels. It describes the methods of revealing grain boundaries and of estimating the mean grain
size of specimens with unimodal size distribution. Although grains are three-dimensional in shape,
the metallographic sectioning plane can cut through a grain at any point from a grain corner, to the
maximum diameter of the grain, thus producing a range of apparent grain sizes on the two-dimensional
plane, even in a sample with a perfectly consistent grain size.
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.
ASTM E112, Standard Test Methods for Determining Average Grain Size
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— IEC Electropedia: available at http:// www .electropedia .org/
— ISO Online browsing platform: available at http:// www .iso .org/ obp
3.1 Grains
3.1.1
grain
closed polygonal shape with more or less curved sides, which can be revealed on a flat cross-section
through the sample, polished and prepared for micrographic examination
3.1.2
austenitic grain
crystal with a face-centred cubic crystal structure which may, or may not, contain annealing twins
3.1.3
ferritic grain
crystal with a body-centred cubic crystal structure which never contains annealing twins
Note 1 to entry: Ferritic grain size is generally estimated for unalloyed steels with a carbon content of 0,25 %
or less. If pearlite islands of identical dimensions to those of the ferrite grains are present, the islands are then
counted as ferrite grains.
ISO 643:2019(E)
3.2 General
3.2.1
index
positive, zero or possibly negative number G which is derived from the mean number m of grains (3.1.1)
counted in an area of 1 mm of the section of the specimen
Note 1 to entry: By definition, G = 1 where m = 16; the other indices are obtained by Formula (1).
G
m=×82 (1)
3.2.2
intercept
N
number of grains (3.1.1) intercepted by a test line, either straight or curved
Note 1 to entry: See Figure 1.
Note 2 to entry: Straight test lines will normally end within a grain. These end segments are counted as 1/2 an
interception. N is the average of a number of counts of the number of grains intercepted by the test line applied
randomly at various locations. N is divided by the true line length, L usually measured in millimetres, in order
T
to obtain the number of grains intercepted per unit length, N .
L
3.2.3
intersection
P
number of intersection points between grain (3.1.1) boundaries and a test line, either straight or curved
Note 1 to entry: See Figure 2.
Note 2 to entry: P is the average of a number of counts of the number of grain boundaries intersected by the test
line applied randomly at various locations. P is divided by the true line length, L usually measured in
T
millimetres, in order to obtain the number of grain boundary intersections per unit length, P .
L
4 Symbols
The symbols used are given in Table 1.
Table 1 — Symbols
Symbols Definition Value
a Mean area of grain in square millimetres
a =
m
A Apparent area of the test figure in square millimetres —
F
Mean grain diameter in millimetres d =
d
m
Diameter of the circle on the ground glass screen of the microscope
79,8 mm
D or on a photomicrograph enclosing the image of the reference
(area = 5 000 mm )
surface of the test piece
Linear magnification (to be noted as a reference) of the
g In principle 100
microscopic image
G Equivalent index of grain size —
a
The method for designating the direction conforms to ISO 3785.
2 © ISO 2019 – All rights reserved

ISO 643:2019(E)
Table 1 (continued)
Symbols Definition Value
g
Conversion factor from linear magnification × g to linear
K
K=
magnification ×100
Mean lineal intercept length, generally expressed in
l
lN==11//P
LL
millimetres
True length of the test line divided by the magnification, in
L —
T
millimetres
m = 2 n
Number of grains per square millimetre of test piece surface in the
m (magnification × 100)
area examined
m = 2 K n (magnification × g)
g
M Number of the closest standard chart picture where g is not 100. —
Total equivalent number of grains examined on the image of
n —
g
diameter D (with a magnification × g)
n Number of grains completely inside the circle of diameter D —
n Number of grains intersected by the circle of diameter D —
n
Total equivalent number of grains examined on the image of
n
nn=+
diameter D (with magnification × 100) 100 1
Mean number of grains intercepted per unit length L —
N
N Mean number of grains intercepted per unit length of the line NN= /L
L L T
a
N Number of intercepts per millimetre in the longitudinal direction —
x
a
N Number of intercepts per millimetre in the transverse direction —
y
a
N Number of intercepts per millimetre in the perpendicular direction —
z
Mean number of counts of the number of grain boundaries
—
P
intersected by the test line applied randomly at various locations
Mean number of grain boundary intersections per unit length of
PP= /L
P L
L
T
test line
a
The method for designating the direction conforms to ISO 3785.
5 Principle
The grain size is revealed by micrographic examination of a polished section of the specimen prepared
by an appropriate method for the type of steel and for the information sought.
NOTE If the order or the International Standard defining the product does not stipulate the method of
revealing the grain, the choice of this method is left to the manufacturer.
This average size is characterized either
a) by an index obtained
1) usually by comparison with standard charts for the measurement of grain size;
2) or by counting to determine the average number of grains per unit area;
b) or by the mean value of the intercepted segment.
ISO 643:2019(E)
NOTE Interception, N, counts for a straight line on a single-phase grain structure where the arrows point to
6 intercepts and two line segments ending within grain (2 × 1/2 = 1 N) and N = 7.
Figure 1 — Example of interception, N
NOTE Intersection, P, counts for a straight test line placed over a single-phase grain structure where the
arrows point to 7 intersection points and P = 7.
Figure 2 — Example of intersection, P
6 Selection and preparation of the specimen
6.1 Test location
If the order, or the International Standard defining the product, does not specify the number of specimens
and the point at which they are to be taken from the product, these are left to the manufacturer,
although it has been shown that precision of grain size determination increases the higher the number
of specimens assessed. Therefore, it is recommended that two or more sections be assessed. Care shall
4 © ISO 2019 – All rights reserved

ISO 643:2019(E)
be taken to ensure that the specimens are representative of the bulk of the product (i.e. avoid heavily
deformed material such as that found at the extreme end of certain products or where shearing has
been used to remove the specimen, etc.). The specimens shall be polished in accordance with the usual
methods.
Unless otherwise stated by the product standard or by agreement with the customer, the polished
face of the specimen shall be longitudinal, i.e. parallel to the principal axis of deformation in wrought
products. Measurements of the grain size on a transverse plane will be biased if the grain shape is not
equiaxial.
6.2 Revealing ferritic grain boundaries
The ferritic grains shall be revealed by etching with nital (ethanolic 2 % to 3 % nitric acid solution), or
with an appropriate reagent.
6.3 Revealing austenitic and prior-austenitic grain boundaries
6.3.1 General
In the case of steels having a single-phase or two-phase austenitic structure (delta ferrite grains in an
austenitic matrix) at ambient temperature, the grain shall be revealed by an etching solution. For single
phase austenitic stainless steels, the most commonly used chemical etchants are glyceregia, Kalling’s
reagent (No. 2) and Marble's reagent. The best electrolytic etch for single or two-phase stainless steels
is aqueous 60 % nitric acid at 1,4 V d.c. for 60 s to 120 s, as it reveals the grain boundaries but not the
twin boundaries. Aqueous 10 % oxalic acid, 6 V d.c., up to 60 s, is commonly used but is less effective
than electrolytic 60 % HNO .
For other steels, one or other of the methods specified below shall be used depending on the information
required.
— “Bechet-Beaujard” method by etching with aqueous saturated picric acid solution (see 6.3.2).
— “Kohn” method by controlled oxidation (see 6.3.3).
— “McQuaid-Ehn” method by carburization (see 6.3.4).
— grain boundary sensitization method (see 6.3.7).
— other methods specially agreed upon when ordering.
NOTE The first three methods are for prior-austenitic grain boundaries while the others are for austenitic
Mn or austenitic stainless, see Annex A.
If comparative tests are carried out for the different methods, it is essential to use the same heat
treatment conditions. Results may vary considerably from one method to the other.
6.3.2 “Bechet-Beaujard” method by etching with aqueous saturated picric acid solution
6.3.2.1 Field of application
This method reveals austenitic grains formed during heat treatment of the specimen. It is applicable to
specimens which have a martensitic or bainitic structure. For this etch to work, there shall be at least
0,005 % P.
6.3.2.2 Preparation
The Bechet-Beaujard etchant is normally used on a heat-treated steel specimen. Normally, no
subsequent heat treatment is necessary if the specimen has a martensitic or bainitic structure. If this is
not the case, heat treatment is necessary.
ISO 643:2019(E)
If the conditions for treating the test piece are not provided for by the International Standard defining
the product and there is no specification to the contrary, the following conditions shall be applied in the
case of heat-treated structural unalloyed steels and low-alloy steels:
— 1,5 h at (850 ± 10) °C for steels whose carbon content is greater than 0,35 %;
— 1,5 h at (880 ± 10) °C for steels whose carbon content is less than or equal to 0,35 %.
After this treatment, the test piece shall be quenched into water or oil.
6.3.2.3 Polishing and etching
A flat specimen surface shall be polished for micrographic examination. It shall be etched for an
adequate period of time by means of an aqueous solution saturated with picric acid together with at
least 0,5 % sodium alkylsulfonate or another appropriate wetting agent.
NOTE The period of etching can vary from a few minutes to more than one hour. Heating of the solution to
60 °C can improve the etching action and reduce etching time.
Several successive etching and polishing operations are sometimes necessary to ensure a sufficient
contrast between the grain boundaries and the general base of the specimen. In the case of through-
hardened steel, tempering may be carried out before selecting the specimen.
WARNING — When heating solutions containing picric acid, caution shall be taken to avoid the
solution boiling dry as picric acid can become explosive.
6.3.2.4 Result
The prior-austenite grain boundaries shall be immediately apparent on microscopic examination.
6.3.3 “Kohn” method by controlled oxidation
6.3.3.1 Field of application
This method shows up the austenitic grain pattern formed by preferential oxidation of the boundaries
during austenization at the temperature of a given heat treatment.
6.3.3.2 Preparation
One surface of the specimen shall be polished. The rest of its surface s
...

SLOVENSKI STANDARD
01-april-2020
Jekla - Mikrografsko določevanje navidezne velikosti kristalnih zrn (ISO 643:2019)
Steels - Micrographic determination of the apparent grain size (ISO 643:2019)
Stahl - Mikrophotographische Bestimmung der erkennbaren Korngröße (ISO 643:2019)
Aciers - Détermination micrographique de la grosseur de grain apparente (ISO
643:2019)
Ta slovenski standard je istoveten z: EN ISO 643:2020
ICS:
77.040.99 Druge metode za Other methods of testing of
preskušanje kovin metals
77.080.20 Jekla Steels
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN ISO 643
EUROPEAN STANDARD
NORME EUROPÉENNE
January 2020
EUROPÄISCHE NORM
ICS 77.040.99 Supersedes EN ISO 643:2012
English Version
Steels - Micrographic determination of the apparent grain
size (ISO 643:2019)
Aciers - Détermination micrographique de la grosseur Stahl - Mikrophotographische Bestimmung der
de grain apparente (ISO 643:2019) erkennbaren Korngröße (ISO 643:2019)
This European Standard was approved by CEN on 26 October 2018.

CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this
European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references
concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN
member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by
translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management
Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and
United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2020 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 643:2020 E
worldwide for CEN national Members.

Contents Page
European foreword . 3

European foreword
This document (EN ISO 643:2020) has been prepared by Technical Committee ISO/TC 17 "Steel" in
collaboration with Technical Committee CEN/TC 459/SC 1 “Test methods for steel (other than chemical
analysis)” the secretariat of which is held by AFNOR.
This European Standard shall be given the status of a national standard, either by publication of an
identical text or by endorsement, at the latest by July 2020, and conflicting national standards shall be
withdrawn at the latest by July 2020.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
This document supersedes EN ISO 643:2012.
According to the CEN-CENELEC Internal Regulations, the national standards organizations of the
following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria,
Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland,
Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Republic of
North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and the
United Kingdom.
Endorsement notice
The text of ISO 643:2019 has been approved by CEN as EN ISO 643:2020 without any modification.

INTERNATIONAL ISO
STANDARD 643
Fourth edition
2019-12
Steels — Micrographic determination
of the apparent grain size
Aciers — Détermination micrographique de la grosseur de grain
apparente
Reference number
ISO 643:2019(E)
©
ISO 2019
ISO 643:2019(E)
© ISO 2019
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: [phone]
Fax: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland
ii © ISO 2019 – All rights reserved

ISO 643:2019(E)
Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
3.1 Grains . 1
3.2 General . 2
4 Symbols . 2
5 Principle . 3
6 Selection and preparation of the specimen . 4
6.1 Test location . 4
6.2 Revealing ferritic grain boundaries . 5
6.3 Revealing austenitic and prior-austenitic grain boundaries . 5
6.3.1 General. 5
6.3.2 “Bechet-Beaujard” method by etching with aqueous saturated picric acid
solution . 5
6.3.3 “Kohn” method by controlled oxidation . 6
6.3.4 “McQuaid-Ehn” method by carburization at 925 °C . 7
6.3.5 Proeutectoid ferrite method . 8
6.3.6 Bainite or gradient-quench method . 9
6.3.7 Sensitization of austenitic stainless and manganese steels . 9
6.3.8 Other methods for revealing prior-austenitic grain boundaries . 9
7 Characterization of grain size .10
7.1 Characterization by an index .10
7.1.1 Formulae .10
7.1.2 Assessment by comparison with standard grain size charts .10
7.1.3 Planimetric method .11
7.1.4 Estimation of the index .11
7.2 Characterization by the intercept method .11
7.2.1 Linear intercept segment method .11
7.2.2 Circular intercept segment method .12
7.2.3 Assessment of results .13
8 Test report .14
Annex A (informative) Summary of methods for revealing ferritic, austenitic or prior-
austenitic grain boundaries in steels .15
Annex B (normative) Evaluation method .16
Bibliography .21
ISO 643:2019(E)
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 17, Steel, Subcommittee SC 7, Methods of
testing (other than mechanical tests and chemical analysis).
This fourth edition cancels and replaces the third edition (ISO 643:2012), which has been technically
revised. The main changes compared to the previous edition are as follows:
— 7.1.2 has been modified;
— the original Annex B has been deleted and the original Annex C has been renumbered as Annex B.
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 © ISO 2019 – All rights reserved

INTERNATIONAL STANDARD ISO 643:2019(E)
Steels — Micrographic determination of the apparent
grain size
1 Scope
This document specifies a micrographic method of determining apparent ferritic or austenitic grain
size in steels. It describes the methods of revealing grain boundaries and of estimating the mean grain
size of specimens with unimodal size distribution. Although grains are three-dimensional in shape,
the metallographic sectioning plane can cut through a grain at any point from a grain corner, to the
maximum diameter of the grain, thus producing a range of apparent grain sizes on the two-dimensional
plane, even in a sample with a perfectly consistent grain size.
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.
ASTM E112, Standard Test Methods for Determining Average Grain Size
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— IEC Electropedia: available at http:// www .electropedia .org/
— ISO Online browsing platform: available at http:// www .iso .org/ obp
3.1 Grains
3.1.1
grain
closed polygonal shape with more or less curved sides, which can be revealed on a flat cross-section
through the sample, polished and prepared for micrographic examination
3.1.2
austenitic grain
crystal with a face-centred cubic crystal structure which may, or may not, contain annealing twins
3.1.3
ferritic grain
crystal with a body-cantered cubic crystal structure which never contains annealing twins
Note 1 to entry: Ferritic grain size is generally estimated for unalloyed steels with a carbon content of 0,25 %
or less. If pearlite islands of identical dimensions to those of the ferrite grains are present, the islands are then
counted as ferrite grains.
ISO 643:2019(E)
3.2 General
3.2.1
index
positive, zero or possibly negative number G which is derived from the mean number m of grains (3.1.1)
counted in an area of 1 mm of the section of the specimen
Note 1 to entry: By definition, G = 1 where m = 16; the other indices are obtained by Formula (1).
G
m=×82 (1)
3.2.2
intercept
N
number of grains (3.1.1) intercepted by a test line, either straight or curved
Note 1 to entry: See Figure 1.
Note 2 to entry: Straight test lines will normally end within a grain. These end segments are counted as 1/2 an
interception. N is the average of a number of counts of the number of grains intercepted by the test line applied
randomly at various locations. N is divided by the true line length, L usually measured in millimetres, in order
T
to obtain the number of grains intercepted per unit length, N .
L
3.2.3
intersection
P
number of intersection points between grain (3.1.1) boundaries and a test line, either straight or curved
Note 1 to entry: See Figure 2.
Note 2 to entry: P is the average of a number of counts of the number of grain boundaries intersected by the test
line applied randomly at various locations. P is divided by the true line length, L usually measured in
T
millimetres, in order to obtain the number of grain boundary intersections per unit length, P .
L
4 Symbols
The symbols used are given in Table 1.
Table 1 — Symbols
Symbols Definition Value
a Mean area of grain in square millimetres
a =
m
A Apparent area of the test figure in square millimetres —
F
Mean grain diameter in millimetres d =
d
m
Diameter of the circle on the ground glass screen of the microscope
79,8 mm
D or on a photomicrograph enclosing the image of the reference
(area = 5 000 mm )
surface of the test piece
Linear magnification (to be noted as a reference) of the
g In principle 100
microscopic image
G Equivalent index of grain size —
a
The method for designating the direction conforms to ISO 3785.
2 © ISO 2019 – All rights reserved

ISO 643:2019(E)
Table 1 (continued)
Symbols Definition Value
g
Conversion factor from linear magnification × g to linear
K
K=
magnification ×100
Mean lineal intercept length, generally expressed in
l
lN==11//P
LL
millimetres
True length of the test line divided by the magnification, in
L —
T
millimetres
m = 2 n
Number of grains per square millimetre of test piece surface in the
m (magnification × 100)
area examined
m = 2 K n (magnification × g)
g
M Number of the closest standard chart picture where g is not 100. —
Total equivalent number of grains examined on the image of
n —
g
diameter D (with a magnification × g)
n Number of grains completely inside the circle of diameter D —
n Number of grains intersected by the circle of diameter D —
n
Total equivalent number of grains examined on the image of
n
nn=+
diameter D (with magnification × 100) 100 1
Mean number of grains intercepted per unit length L —
N
N Mean number of grains intercepted per unit length of the line NN= /L
L L T
a
N Number of intercepts per millimetre in the longitudinal direction —
x
a
N Number of intercepts per millimetre in the transverse direction —
y
a
N Number of intercepts per millimetre in the perpendicular direction —
z
Mean number of counts of the number of grain boundaries
—
P
intersected by the test line applied randomly at various locations
Mean number of grain boundary intersections per unit length of
PP= /L
P L
L
T
test line
a
The method for designating the direction conforms to ISO 3785.
5 Principle
The grain size is revealed by micrographic examination of a polished section of the specimen prepared
by an appropriate method for the type of steel and for the information sought.
NOTE If the order or the International Standard defining the product does not stipulate the method of
revealing the grain, the choice of this method is left to the manufacturer.
This average size is characterized either
a) by an index obtained
1) usually by comparison with standard charts for the measurement of grain size;
2) or by counting to determine the average number of grains per unit area;
b) or by the mean value of the intercepted segment.
ISO 643:2019(E)
NOTE Interception, N, counts for a straight line on a single-phase grain structure where the arrows point to
6 intercepts and two line segments ending within grain (2 × 1/2 = 1 N) and N = 7.
Figure 1 — Example of interception, N
NOTE Intersection, P, counts for a straight test line placed over a single-phase grain structure where the
arrows point to 7 intersection points and P = 7.
Figure 2 — Example of intersection, P
6 Selection and preparation of the specimen
6.1 Test location
If the order, or the International Standard defining the product, does not specify the number of specimens
and the point at which they are to be taken from the product, these are left to the manufacturer,
although it has been shown that precision of grain size determination increases the higher the number
of specimens assessed. Therefore, it is recommended that two or more sections be assessed. Care shall
4 © ISO 2019 – All rights reserved

ISO 643:2019(E)
be taken to ensure that the specimens are representative of the bulk of the product (i.e. avoid heavily
deformed material such as that found at the extreme end of certain products or where shearing has
been used to remove the specimen, etc.). The specimens shall be polished in accordance with the usual
methods.
Unless otherwise stated by the product standard or by agreement with the customer, the polished
face of the specimen shall be longitudinal, i.e. parallel to the principal axis of deformation in wrought
products. Measurements of the grain size on a transverse plane will be biased if the grain shape is not
equiaxial.
6.2 Revealing ferritic grain boundaries
The ferritic grains shall be revealed by etching with nital (ethanolic 2 % to 3 % nitric acid solution), or
with an appropriate reagent.
6.3 Revealing austenitic and prior-austenitic grain boundaries
6.3.1 General
In the case of steels having a single-phase or two-phase austenitic structure (delta ferrite grains in an
austenitic matrix) at ambient temperature, the grain shall be revealed by an etching solution. For single
phase austenitic stainless steels, the most commonly used chemical etchants are glyceregia, Kalling’s
reagent (No. 2) and Marble's reagent. The best electrolytic etch for single or two-phase stainless steels
is aqueous 60 % nitric acid at 1,4 V d.c. for 60 s to 120 s, as it reveals the grain boundaries but not the
twin boundaries. Aqueous 10 % oxalic acid, 6 V d.c., up to 60 s, is commonly used but is less effective
than electrolytic 60 % HNO .
For other steels, one or other of the methods specified below shall be used depending on the information
required.
— “Bechet-Beaujard” method by etching with aqueous saturated picric acid solution (see 6.3.2).
— “Kohn” method by controlled oxidation (see 6.3.3).
— “McQuaid-Ehn” method by carburization (see 6.3.4).
— grain boundary sensitization method (see 6.3.7).
— other methods specially agreed upon when ordering.
NOTE The first three methods are for prior-austenitic grain boundaries while the others are for austenitic
Mn or austenitic stainless, see Annex A.
If comparative tests are carried out for the different methods, it is essential to use the same heat
treatment conditions. Results may vary considerably from one method to the other.
6.3.2 “Bechet-Beaujard” method by etching with aqueous saturated picric acid solution
6.3.2.1 Field of application
This method reveals austenitic grains formed during heat treatment of the specimen. It is applicable to
specimens which have a martensitic or bainitic structure. For this etch to work, there shall be at least
0,005 % P.
6.3.2.2 Preparation
The Bechet-Beaujard etchant is normally used on a heat-treated steel specimen. Normally, no
subsequent heat treatment is necessary if the specimen has a martensitic or bainitic structure. If this is
not the case, heat treatment is necessary.
ISO 643:2019(E)
If the conditions for treating the test piece are not provided for by the International Standard defining
the product and there is no specification to the contrary, the following conditions shall be applied in the
case of heat-treated structural unalloyed steels and low-alloy steels:
— 1,5 h at (850 ± 10) °C for steels whose carbon content is greater than 0,35 %;
— 1,5 h at (880 ± 10) °C for steels whose carbon content is less than or equal to 0,35 %.
After this treatment, the test piece shall be quenched into water or oil.
6.3.2.3 Polishing and etching
A flat specimen surface shall be polished for micrographic examination. It shall be etched for an
adequate period of time by means of an aqueous solution saturated with picric acid together with at
least 0,5 % sodium alkylsulfonate or another appropriate wetting agent.
NOTE The period of etching can vary from a few minutes to more than one hour. Heating of the solution to
60 °C can improve the etching action and reduce etching time.
Several successive etching and polishing operations are sometimes necessary to ensure a sufficient
contrast between the grain boundaries and the general base of the specimen. In the case of through-
hardened steel, tempering may be carried out before selecting the specimen.
WARNING — When heating solutions containing picric acid, caution shall be taken to avoid the
solution boiling dry as picric acid can become explosive.
6.3.2.4 Result
The prior-austenite grain boundaries shall be immediately apparent on microscopic examination.
6.3.3 “Kohn” method by controlled oxidation
6.3.3.1 Field of application
This method shows up the austenitic grain pattern formed by preferential oxidation of the boundaries
during austenization at the temperature of a given heat treatment.
6.3.3.2 Preparation
One surface of the specimen shall be polished. The rest of its surface shall not show any traces of oxide.
The specimen shall be placed in a laboratory furnace in which either a vacuum of 1 Pa is attained or an
inert gas is circulated (e.g. purified argon). Heat treat the specimen in accordance with the austenitizing
procedure spec
...