ISO/FDIS 643

Steel — Micrographic determination of the apparent grain size

Acier — Détermination micrographique de la grosseur de grain apparente

General Information

• Status: Withdrawn
• ICS: 77.040.99 - Other methods of testing of metals
• 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: 5098 - Project deleted
• Completion Date: 18-Jan-2016

Relations

• Revises: ISO 643:2012 - Steels — Micrographic determination of the apparent grain size
• Effective Date: 30-Nov-2013

Standards Content (Sample)

FINAL
INTERNATIONAL ISO/FDIS
DRAFT
STANDARD 643
ISO/TC 17/SC 7
Steel — Micrographic determination
Secretariat: AFNOR
of the apparent grain size
Voting begins on:
2015­04­02
Acier — Détermination micrographique de la grosseur de grain
apparente
Voting terminates on:
2015­06­02
Please see the administrative notes on page iii
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 SUPPOR TING
DOCUMENTATION.
IN ADDITION TO THEIR EVALUATION AS
Reference number
BEING ACCEPTABLE FOR INDUSTRIAL, TECHNO­
ISO/FDIS 643:2015(E)
LOGICAL, COMMERCIAL AND USER PURPOSES,
DRAFT INTERNATIONAL STANDARDS MAY ON
OCCASION HAVE TO BE CONSIDERED IN THE
LIGHT OF THEIR POTENTIAL TO BECOME STAN­
DARDS TO WHICH REFERENCE MAY BE MADE IN
©
NATIONAL REGULATIONS. ISO 2015

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ISO/FDIS 643:2015(E)

COPYRIGHT PROTECTED DOCUMENT
© ISO 2015
All rights reserved. Unless otherwise specified, 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
Case postale 56 • CH­1211 Geneva 20
Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail [e-mail]
Web www.iso.org
Published in Switzerland
ii © ISO 2015 – All rights reserved

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ISO/FDIS 643:2015(E)

ISO/CEN PARALLEL PROCESSING
This final draft has been developed within the International Organization for Standardization (ISO), and pro­
cessed under the ISO-lead mode of collaboration as defined in the Vienna Agreement. The final draft was
established on the basis of comments received during a parallel enquiry on the draft.
This final draft is hereby submitted to the ISO member bodies and to the CEN member bodies for a parallel
two­month approval vote in ISO and formal vote in CEN.
Positive votes shall not be accompanied by comments.
Negative votes shall be accompanied by the relevant technical reasons.
© ISO 2015 – All rights reserved iii

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ISO/FDIS 643:2015(E)

Contents Page
Foreword .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and abbreviated terms . 2
5 Principle . 2
6 Selection and preparation of the specimen . 5
6.1 Test location . 5
6.2 Revealing ferritic grain boundaries . 5
6.3 Revealing austenitic and prior­austenitic 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 gradient­quench method . 8
6.3.7 Sensitization of austenitic stainless and manganese steels . 9
6.3.8 Other methods for revealing prior­austenitic grain boundaries . 9
7 Characterization of grain size . 9
7.1 Characterization by an index . 9
7.1.1 Formulae . 9
7.1.2 Assessment by comparison with standard grain size charts .10
7.1.3 Planimetric method .10
7.1.4 Estimation of the index .10
7.2 Characterization by the intercept method .10
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) Determination of grain size — Standard charts taken from ASTM E112 .16
Annex C (normative) Evaluation method .31
Bibliography .37
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ISO/FDIS 643:2015(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 meaning of ISO specific terms and expressions related to conformity
assessment, as well as information about ISO’s adherence to the WTO principles in the Technical Barriers
to Trade (TBT) see the following URL: Foreword - Supplementary information
The committee responsible for this document is 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), of which it constitutes a minor
revision. A note was added after the first paragraph of 7.1.2.
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FINAL DRAFT INTERNATIONAL STANDARD ISO/FDIS 643:2015(E)
Steel — Micrographic determination of the apparent grain
size
1 Scope
This International Standard 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, in whole or in part, are normatively referenced in this document and are
indispensable for its application. For dated references, only the edition cited applies. For undated
references, the latest edition of the referenced document (including any amendments) applies.
ISO 3785, Metallic materials — Designation of test specimen axes in relation to product texture
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.
3.1
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
3.1.1
austenitic grain
crystal with a face-centered cubic crystal structure which may or may not contain annealing twins
3.1.2
ferritic grain
crystal with a body-centered cubic crystal structure which never contains annealing twins
Note 1 to entry: Ferritic grain size is generally estimated for non-alloy 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.
3.2
index
positive, zero, or possibly negative number G which is derived from the mean number m of grains counted
2
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 the formula
G
m=×82 .
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ISO/FDIS 643:2015(E)

3.3
intercept
N
number of grains 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
3.4
intersection
P
number of intersection points between grain boundaries and a test line, either straight or curved
Note 1 to entry: See Figure 1.
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 and abbreviated terms
The symbols used are given in Table 1.
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
— usually by comparison with standard charts for the measurement of grain size;
— or by counting to determine the average number of grains per unit area;
b) or by the mean value of the intercepted segment.
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ISO/FDIS 643:2015(E)

Interception, N, counts for a straight line on a single-phase grain structure where the arrows
point to six intercepts and two line segments ending within grain (2 × 1/2 = 1 N) and N = 7
Intersection, P, counts for a straight test line placed over a single-phase grain structure where
the arrows point to seven intersection points and P = 7
Figure 1 — Examples of intersection, P, and interception, N
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ISO/FDIS 643:2015(E)

Table 1 — Symbols
Symbols Definition Value
1
a Mean area of grain in square millimetres   a =
m
A Apparent area of the test figure in square millimetres —
F
1
   d =
Mean grain diameter in millimetres
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 surface
2
(area = 5 000 mm )
of the test piece
Linear magnification (to be noted as a reference) of the microscopic
g In principle 100
image
G Equivalent index of grain size —
g
Conversion factor from linear magnification × g to linear magnifica­
K   K=
tion × 100
100
l Mean lineal intercept length, generally expressed in millimetres
   lN11//P
LL
True length of the test line divided by the magnification, in millime­
L —
T
tres
m = 2 n
100
Number of grains per square millimetre of test piece surface in the (magnification × 100)
m
2
area examined m = 2 K n (magnifica­
g
tion × g)
M Number of the closest standard chart picture where g is not 100 —
Total equivalent number of grains examined on the image of diameter
n —
g
D (with a magnification × g)
n Number of grains completely inside the circle of diameter D —
1
n Number of grains intersected by the circle of diameter D —
2
n
Total equivalent number of grains examined on the image of diameter 2
n   nn=+
100 100 1
D (with magnification × 100)
2
Mean number of grains intercepted per unit length L —
N
N   NN= /L
Mean number of grains intercepted per unit length of the line
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 inter­
—
P
sected by the test line applied randomly at various locations
Mean number of grain boundary intersections per unit length of test
   PP= /L
P L
L
T
line
a
The method for designating the direction conforms to ISO 3785.
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==

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ISO/FDIS 643:2015(E)

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 specimen
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 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 single­phase or two­phase 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 .
3
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 can 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.
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ISO/FDIS 643:2015(E)

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.
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 carbon 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 can 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 specified by the customer, or as defined by the International Standard governing the product.
At the end of this specified heating period, air shall be introduced into the furnace for a period of 10 s to 15 s.
The specimen shall then be water-quenched. The specimen can usually be directly examined using
a microscope.
NOTE 1 The oxidation method can be done without the inert atmosphere.
NOTE 2 The oxide adhering to the previously polished surface should be removed by light polishing with a
fine abrasive, taking care that the oxide network which has formed on the grain boundaries is retained; then the
polishing should be completed by the usual methods. The specimen should then be etched using Vilella’s reagent:
6 © ISO 2015 – All rights reserved

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ISO/FDIS 643:2015(E)

— picric acid 1 g;
— hydrochloric acid 5 ml;
— ethanol 100 ml.
6.3.3.3 Result
The preferential oxidation of the boundaries shows up the pattern of austenitic grains.
If the preparation is effected correctly, no oxide globules should appear at the grain boundaries.
In certain cases, it might be necessary to use oblique illumination, or Differential Interference Contrast
(DIC) methods to show up the boundaries in better relief.
6.3.4 “McQuaid-Ehn” method by carburization at 925 °C
6.3.4.1 Field of application
This is a method specifically for carburizing steels and shows up austenitic grain boundaries formed
during carburization of these steels. It is not usually suitable for revealing grains actually formed during
other heat treatments.
NOTE The “mock carburizing” procedure can also be used. The specimen is subjected to the same thermal
treatment but without a carbon­rich atmosphere. It is then heat­treated as the product would be treated. The
Bechet­Beaujard reagent is used to reveal the grain boundaries (see 6.3.2).
6.3.4.2 Preparation
The specimens shall be free from any trace of decarburization or of surface oxidation. Any prior
treatment, either cold, hot, mechanical, etc., can have an effect on the shape of the grain obtained; the
product specification shall state the treatments to be carried out before determination in cases where it
is advisable to take into account these considerations.
After carburizing, the specimen shall be cooled at a rate slow enough to precipitate cementite at the
grain boundaries in the hypereutectoid surface region of the carburized specimen.
Carburization shall be achieved by maintaining the specimen at (925 ± 10) °C for 6 h. This is generally
done by keeping the carburizing chamber at (925 ± 10) °C for 8 h, including a pre-heating period. In most
cases, a carburized layer of approximately 1 mm is obtained. After carburizing, cool the specimen at a rate
slow enough to ensure that the cementite is precipitated at the grain boundaries of the hypereutectoid
zone of the carburized layer.
Fresh carburizing compound shall be used each time.
6.3.4.3 Specimen preparation
The carburized specimen shall be sectioned normally to its surface. One of the sections shall be prepared
for micrographic examination and etched using either a) or b).
a) “Le Chatelier and Igewski” reagent (alkaline sodium picrate):
— picric acid 2 g;
— sodium hydroxide 25 g;
— water  100 ml.
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ISO/FDIS 643:2015(E)

Use this reagent by immersion at 100 °C, for at least 1 min, or at room temperature by means of electrolytic
etching 6 V d.c. for 60 s.
b) Nital:
— nitric acid 2 ml to 5 ml
— ethanol to make up to 100 ml
Other reagents can be used as long as the same results are obtained.
6.3.4.4 Result
The prior-austenite grain boundaries in the hypereutectoid carburized surface layer will be delineated
by proeutectoid cementite.
6.3.5 Proeutectoid ferrite method
NOTE Guidelines for the use of this method depending on the microstructure of the steel product are
given in Annex A.
6.3.5.1 Principle
This method is suitable for carbon steel with about 0,25 % to 0,6 % carbon and for low-alloy steels such
as manganese-molybdenum, 1 % chromium, 1 % chromium-molybdenum, and 1,5 % nickel-chromium.
The prior­austenitic grain boundaries are revealed as a network of proeutectoid ferrite.
6.3.5.2 Preparation
Use the austenizing conditions as given in the product standard. In the case of carbon or other low-
hardenability steel, either air cool, furnace cool, or partially transform isothermally the test pieces in
such a manner as to outline the austenitic grain boundaries with ferrite.
In the case of alloy steels, after austenitizing, partially transform isothermally the test pieces at an
appropriate temperature within the range 650 °C to 720 °C and then water quench.
NOTE 1 The time required for transformation will vary according to the steel, but usually sufficient ferrite has
precipitated in 1 min to 5 min, although longer times, up to about 20 min, can sometimes be required.
NOTE 2 For alloy steels, a test piece 12 mm × 6 mm × 3 mm is suitable to obtain uniform transformation during
the isothermal treatment.
6.3.5.3 Polishing and etching
Section, polish, and etch the test pieces for micrographic examination. Etch the test pieces with a suitable
etchant such as hydrochloric acid and picric acid (Vilellas’ reagent).
6.3.6 Bainite or gradient-quench method
NOTE Guidelines for the use of this method depending on the microstructure of the steel product are
given in Annex A.
6.3.6.1 Principle
This method is suitable for steels of approximately eutectoid composition, i.e. having a carbon content of
0,7 % by mass or higher. The boundaries of the prior-austenitic grains are revealed by a network of fine
pearlite or bainite outlining the martensite grains.
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ISO/FDIS 643:2015(E)

6.3.6.2 Preparation
Heat the test piece to a temperature not more than 30 °C above A (i.e. the temperature at which ferrite
C3
completes its transformation to austenite during heating) to ensure full austenitization.
Cool the specimen at a controlled rate to produce a partially hardened structure of fine pearlite or
bainite outlining the martensite grains.
This structure can be produced in one of the following ways:
a) by completely quenching in water or oil, as appropriate, a bar of cross-sectional dimensions such
that it will fully harden at the surface but only partially harden in the centre;
b) by gradient quenching a length of bar, 12 mm to 25 mm diameter or square, by immersing it in water
for a part of the length only.
Then polish and etch.
6.3.7 Sensitization of austenitic
...

ISO 643
Fourth edition
2014
Steels — Micrographic
determination of the
apparent grain size
Aciers — Détermination
micrographique de la grosseur
de grain apparente



Reference number
ISO 643:2012(E)
©
ISO 2014

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ISO 643:2012(E)

COPYRIGHT PROTECTED DOCUMENT


©  ISO 2014
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means,
electronic or mechanical, including photocopying and microfilm, without permission in writing from either ISO at the address below or
ISO's member body in the country of the requester.
ISO copyright office
Case postale 56  CH-1211 Geneva 20
Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail [e-mail]
Web www.iso.org
Published in Switzerland

ii © ISO 2012 – All rights reserved

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ISO 643:2012(E)
© ISO 2012 – All rights reserved iii

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ISO 643:2012(E)
Copyright notice
This ISO document is a working draft or committee draft and is copyright‐protected by ISO. While the
reproduction of working drafts or committee drafts in any form for use by participants in the ISO
standards development process is permitted without prior permission from ISO, neither this
document nor any extract from it may be reproduced, stored or transmitted in any form for any other
purpose without prior written permission from ISO.
Requests for permission to reproduce this document for the purpose of selling it should be addressed
as shown below or to ISO's member body in the country of the requester:
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as appropriate, of the Copyright Manager of the ISO member body responsible for the secretariat of
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Reproduction for sales purposes may be subject to royalty payments or a licensing agreement.
Violators may be prosecuted.
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ISO 643:2012(E)
Contents Page
Foreword iv
1 Scope . 2
2 Normative references . 2
3 Terms and definitions . 2
4 Symbols and abbreviated terms . 3
5 Principle . 3
6 Selection and preparation of the specimen . 5
6.1 Test location . 5
6.2 Revealing ferritic grain boundaries . 6
6.3 Revealing austenitic and prior-austenitic grain boundaries . 6
7 Characterization of grain size . 11
7.1 Characterization by an index. 11
7.2 Characterization by the intercept method . 12
8 Test report . 15
Annex A (informative) Summary of methods for revealing ferritic, austenitic or prior-
austenitic grain boundaries in steels . 16
Annex B (normative) Determination of grain size — Standard charts taken from ASTM E112 . 17
Annex C (normative) Evaluation method 32

© ISO 2012 – All rights reserved v

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ISO 643:2012(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.
International StandardsThe 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 given inof the ISO/IEC Directives, Part 2
(see www.iso.org/directives.).
The main task of technical committees is to prepare International Standards. Draft International
Standards adopted by the technical committees are circulated to the member bodies for voting.
Publication as an International Standard requires approval by at least 75 % of the member bodies
casting a vote.
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 meaning of ISO specific terms and expressions related to conformity
assessment, as well as information about ISO's adherence to the WTO principles in the Technical
Barriers to Trade (TBT) see the following URL: Foreword ‐ Supplementary informationISO 643 was
prepared by Technical Committee
The committee responsible for this document is ISO/TC 17, Steel, Subcommittee SC 7, Methods of testing
(other than mechanical tests and chemical analysis).
This thirdfourth edition cancels and replaces the secondthird edition (ISO 643:20032012), of which it
constitutes a minor revision. A note was added after the first paragraph of 7.1.2.
vi © ISO 2012 – All rights reserved

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INTERNATIONAL STANDARD ISO 643:2012(E)

Steels — Micrographic determination of the apparent grain size
1 Scope
This International Standard 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, in whole or in part, are normatively referenced documentsin this document
and are indispensable for theits application 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 3785, SteelMetallic materials — Designation of test piecespecimen axes in relation to product texture
ISO 14250, Steel — Metallographic characterization of duplex grain size and distributions
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.
3.1
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
A distinction is made between:
3.1.1
austenitic grain
crystal with a face‐centered cubic crystal structure which may, or may not, contain annealing twins
3.1.2
ferritic grain
1)
crystal with a body‐centered cubic crystal structure which never contains annealing twins

1) Ferritic grain size is generally estimated for non-alloy 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.
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ISO 643:2012(E)
Note 1 to entry: Ferritic grain size is generally estimated for non‐alloy 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.
3.2
index
positive, zero, or possibly negative number G which is derived from the mean number m of grains
2
counted in an area of 1 mm of the section of the specimen
NOTE By definition, G = = 1 where m = = 16; the other indices are obtained by the formula
G
m82
.
3.3
intercept
N
number of grains intercepted by a test line, either straight or curved
Note 1 to entry: See Figure 1.
NOTE 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. Nis divided by the true line length, L , usually measured in millimetres,
T
in order to obtain the number of grains intercepted per unit length, N .
L
3.4
intersection
P
number of intersection points between grain boundaries and a test line, either straight or curved
Note 1 to entry: See Figure 1.
NOTE 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
T
in millimetres, in order to obtain the number of grain boundary intersections per unit length, P .
L
4 Symbols and abbreviated terms
The symbols used are given in Table 1.
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
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ISO 643:2012(E)
— usually by comparison with standard charts for the measurement of grain size;
— or by counting to determine the average number of grains per unit area;
b) or by the mean value of the intercepted segment.
643_ed4fig1a.EPS
Interception, N, counts for a straight line on a single-phase grain structure where the arrows point to six
intercepts and two line segments ending within grain (2 × 1/2 = 1 N) and N = 7
643_ed4fig1b.EPS
Intersection, P, counts for a straight test line placed over a single-phase grain structure where the arrows
point to seven intersection points and P = 7


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

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 1 — Examples of intersection, P, and interception, N
Table 1 — Symbols
Symbols Definition Value
1
a a 
Mean area of grain in square millimetres
m
A Apparent area of the test figure in square millimetres —
F
1
d 
d
Mean grain diameter in millimetres
m
Diameter of the circle on the ground glass screen of the microscope or
79,8 mm
D on a photomicrograph enclosing the image of the reference surface of
2
(area = 5 000 mm )
the test piece
Linear magnification (to be noted as a reference) of the microscopic
g In principle 100
image
G Equivalent index of grain size —
g
Conversion factor from linear magnification × g to linear K
K
100
magnification × 100
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ISO 643:2012(E)
lN1/ 1/P
LL
l Mean lineal intercept length, generally expressed in millimetres
True length of the test line divided by the magnification, in
L —
T
millimetres
m = 2 n
100
(magnification × 100)
Number of grains per square millimetre of test piece surface in the
m
2
area examined
m = 2 K n
g
(magnification × g)
M Number of the closest standard chart picture where g is not 100 —
Total equivalent number of grains examined on the image of diameter
n —
g
D (with a magnification × g)
n Number of grains completely inside the circle of diameter D —
1
n Number of grains intersected by the circle of diameter D —
2
n
2
Total equivalent number of grains examined on the image of diameter
nn
100 1
n
100
2
D (with magnification × 100)
N
Mean number of grains intercepted per unit length L —
N NN /L
L L T
Mean number of grains intercepted per unit length of the line
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 intersected
P
—
by the test line applied randomly at various locations
Mean number of grain boundary intersections per unit length of test PP /L
P L
L T
line
a
The method for designating the direction conforms to ISO 3785.
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
specimensspecimen 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 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.
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ISO 643:2012(E)
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.
3
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 maycan 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.
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 carbon 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 %.
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ISO 643:2012(E)
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 maycan vary from a few minutes to more than one hour. Heating of the solution
to 60 °C maycan 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 maycan 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 specified by the customer, or as defined by the International Standard governing the
product.
At the end of this specified heating period, air shall be introduced into the furnace for a period of 10 s to
15 s.
The specimen shall then be water‐quenched. The specimen can usually be directly examined using a
microscope.
NOTE 1 The oxidation method can be done without the inert atmosphere.
NOTE 2 The oxide adhering to the previously polished surface should be removed by light polishing with a fine
abrasive, taking care that the oxide network which has formed on the grain boundaries is retained; then the
polishing should be completed by the usual methods. The specimen should then be etched using Vilella's reagent:
— picric acid 1 g;
— hydrochloric acid 5 ml;
— ethanol 100 ml.
6.3.3.3 Result
The preferential oxidation of the boundaries shows up the pattern of austenitic grains.
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ISO 643:2012(E)
If the preparation is effected correctly, no oxide globules should appear at the grain boundaries.
In certain cases, it maymight be necessary to use oblique illumination, or DIC (Differential Interference
Contrast (DIC) methods, to show up the boundaries in better relief.
6.3.4 “McQuaid-Ehn” method by carburization at 925 °C
6.3.4.1 Field of application
This is a method specifically for carburizing steels and shows up austenitic grain boundaries formed
during carburization of these steels. It is not usually suitable for revealing grains actually formed during
other heat treatments.
NOTE The “mock carburizing” procedure maycan also be used. The specimen is subjected to the same
thermal treatment but without a carbon‐rich atmosphere. It is then heat‐treated as the product would be treated.
The Bechet‐Beaujard reagent is used to reveal the grain boundaries, (see 6.3.2.).
6.3.4.2 Preparation
The specimens shall be free from any trace of decarburization or of surface oxidation. Any prior
treatment, either cold, hot, mechanical, etc., maycan have an effect on the shape of the grain obtained;
the product specification shall state the treatments to be carried out before determination in cases
where it is advisable to take into account these considerations.
After carburizing, the specimen mustshall be cooled at a rate slow enough to precipitate cementite at
the grain boundaries in the hypereutectoid surface region of the carburized specimen.
Carburization shall be achieved by maintaining the specimen at (925 ± 10) °C for 6 h. This is generally
done by keeping the carburizing chamber at (925 ± 10) °C for 8 h, including a pre‐heating period. In
most cases, a carburized layer of approximately 1 mm is obtained. After carburizing, cool the specimen
at a rate slow enough to ensure that the cementite is precipitated at the grain boundaries of the
hypereutectoid zone of the carburized layer.
Fresh carburizing compound shall be used each time.
6.3.4.3 Specimen preparation
The carburized specimen shall be sectioned normally to its surface. One of the sections shall be
prepared for micrographic examination and etched using either a) or b).
a) “Le Chatelier and Igewski” reagent (alkaline sodium picrate):
— picric acid 2 g;
— sodium hydroxide 25 g;
— water 100 ml.
Use this reagent by immersion at 100 °C, for at least 1 min, or at room temperature by means of
electrolytic etching 6 V d.c. for 60 s.
b) Nital:
— nitric acid 2 ml to 5 ml
— ethanol to make up to 100 ml
Other reagents maycan be used as long as the same results are obtained.
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ISO 643:2012(E)
6.3.4.4 Result
The prior‐austenite grain boundaries in the hypereutectoid carburized surface layer will be delineated
by proeutectoid cementite.
6.3.5 Proeutectoid ferrite method
NOTE Guidelines for the use of this method depending on the microstructure of the steel product are given in
Annex A.
6.3.5.1 Principle
This method is suitable for carbon steel with about 0,25 % to 0,6 % carbon and for low‐alloy steels such
as manganese‐molybdenum, 1 % chromium, 1 % chromium‐molybdenum, and 1,5 % nickel‐chromium.
The prior‐austenitic grain boundaries are revealed as a network of proeutectoid ferrite.
6.3.5.2 Preparation
Use the austenizing conditions as given in the product standard. In the case of carbon or other low ‐
hardenability steel, either air cool, furnace cool, or partially transform isothermally the test pieces in
such a manner as to outline the austenitic grain boundaries with ferrite.
In the case of alloy steels, after austenitizing, partially transform isothermally the test pieces at an
appropriate temperature within the range 650 °C to 720 °C and then water quench.
NOTE 1 The time required for transformation will vary according to the steel, but usually sufficient ferrite has
precipitated in 1 min to 5 min, although longer times, up to about 20 min, can sometimes be required.
NOTE 2 For alloy steels, a test piece 12 mm  × 6 mm  × 3 mm is suitable to obtain uniform transformation
during the isothermal treatment.
6.3.5.3 Polishing and etching
Section, polish, and etch the test pieces for micrographic examination. Etch the test pieces with a
suitable etchant such as hydrochloric acid and picric acid (Vilellas' reagent).
6.3.6 Bainite or gradient-quench method
NOTE Guidelines for the use of this method depending on the microstructure of the steel product are given in
Annex A.
6.3.6.1 Principle
This method is suitable for steels of approximately eutectoid composition, i.e.,. having a carbon content
of 0,7 % by mass or higher. The boundaries of the prior‐austenitic grains are revealed by a network of
fine pearlite or bainite outlining the martensite grains.
6.3.6.2 Preparation
Heat the test piece to a temperature not more than 30 °C above A (i.e.,. the temperature at which
C3
ferrite completes its transformation to austenite during heating) to ensure full austenitization.
Cool the specimen at a controlled rate to produce a partially hardened structure of fine pearlite or
bainite outlining the martensite grains.
This structure maycan be produced in one of the following ways:
a) by completely quenching in water or oil, as appropriate, a bar of cross‐sectional dimensions such
that it will fully harden at the surface but only partially harden in the centre;
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ISO 643:2012(E)
b) by gradient quenching a length of bar, 12 mm to 25 mm diameter or square, by immersing it in
water for a part of the length only.
Then polish and etch.
6.3.7 Sensitization of austenitic stainless and manganese steels
The grain boundaries maycan be developed through precipitation of carbides by heating within the
sensitizing temperature range, 482 °C to 704 °C (900 °F to 1 300 °F). Any suitable carbide‐revealing
etchant can be used.
NOTE This method should not be used in case of very low carbon contents in austenitic grades.
6.3.8 Other methods for revealing prior-austenitic grain boundaries
For certain steels, after simple heat treatment (annealing or normalizing, quenching and tempering,
etc.), the pattern of the austenitic grains maycan appear in the following forms under micrographic
examination: a network of proeutectoid ferrite surrounding pearlite grains, a network of very fine
pearlite surrounding martensite grains, etc. The austenitic grain maycan also be revealed by thermal
etching under vacuum (not necessarily followed by oxidation). The product specification shall mention
2)
these simplified methods in these cases.
7 Characterization of grain size
7.1 Characterization by an index
7.1.1 Formulae
The index is defined in 3.2 by the formulaFormula (1):
G
m82 (1)
This formula maycan be stated as
lg m
G (2a)
3
lg2
or
lg m
G3 (2b)
0,301
7.1.2 Assessment by comparison with standard grain size charts
The image examined on the screen (or on a photomicrograph) is compared with a series of standard
3)
charts or overlays (eye‐piece graticules designed for grain size measurement can be used providing

2) Amongst these methods are the following:
 precipitation on the grain boundaries during cooling;
, gradient quenching method, etc.
3) These standard charts are defined in ASTM E112 [(plates IA and IB) (Annex B)]. The standard charts selected should
be adhered to throughout the whole of the examination.
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ISO 643:2012(E)
these are traceable to Nationalnational or International standardsStandards). The standard charts at a
magnification of  × 100 are numbered from 00 to 10 so that their number is equal to the index G.
NOTE All standard charts in Annex B are displa
...

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Steel — Micrographic determination of the apparent grain size
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ISO/FDIS 643:2015(F)

DOCUMENT PROTÉGÉ PAR COPYRIGHT
© ISO 2015
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ii © ISO 2015 – Tous droits réservés

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ISO/FDIS 643:2015(F)

TRAITEMENT PARALLÈLE ISO/CEN
Le présent projet final a été élaboré dans le cadre de l’Organisation internationale de normalisation (ISO) et
soumis selon le mode de collaboration sous la direction de l’ISO, tel que défini dans l’Accord de Vienne. Le
projet final a été établi sur la base des observations reçues lors de l’enquête parallèle sur le projet.
Le projet final est par conséquent soumis aux comités membres de l’ISO et aux comités membres du CEN en
parallèle à un vote d’approbation de deux mois au sein de l’ISO et à un vote formel au sein du CEN.
Les votes positifs ne doivent pas être accompagnés d’observations.
Les votes négatifs doivent être accompagnés des arguments techniques pertinents.
© ISO 2015 – Tous droits réservés iii

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ISO/FDIS 643:2015(F)

Sommaire Page
Avant-propos .v
1 Domaine d’application . 1
2 Références normatives . 1
3 Termes et définitions . 1
4 Symboles et termes abrégés . 2
5 Principe . 2
6 Sélection et préparation de l’échantillon . 5
6.1 Prélèvement . . 5
6.2 Mise en évidence des joints de grains ferritiques . 5
6.3 Mise en évidence des joints de grains austénitiques et préausténitiques . 5
6.3.1 Généralités . 5
6.3.2 Méthode de «Bechet-Beaujard» par attaque avec une solution aqueuse
saturée en acide picrique . 6
6.3.3 Méthode de «Kohn» par oxydation ménagée . 6
6.3.4 Méthode de «McQuaid-Ehn» par cémentation à 925 °C . 7
6.3.5 Méthode de la ferrite proeutectoïde . 8
6.3.6 Méthode de la bainite ou par gradient de trempe . . 9
6.3.7 Sensibilisation des aciers inoxydables austénitiques et des aciers au
manganèse austénitiques . 9
6.3.8 Autres méthodes de mise en évidence des joints de grains préausténitiques . 9
7 Caractérisation de la grosseur de grain.10
7.1 Caractérisation par un indice .10
7.1.1 Formules .10
7.1.2 Évaluation par comparaison à des images types .10
7.1.3 Méthode planimétrique .11
7.1.4 Estimation de l’indice .11
7.2 Caractérisation par la méthode du segment intercepté .11
7.2.1 Méthode du segment intercepté linéaire .12
7.2.2 Méthode du segment intercepté circulaire .13
7.2.3 Évaluation des résultats .13
8 Rapport d’essai .15
Annexe A (informative) Résumé des méthodes de mise en évidence des joints de grains
ferritiques, austénitiques et préausténitiques dans les aciers .16
Annexe B (normative) Détermination de la grosseur de grain — Images types extraites de
l’ASTM E112 .17
Annexe C (normative) Méthode d’évaluation .32
iv © ISO 2015 – Tous droits réservés

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ISO/FDIS 643:2015(F)

Avant-propos
L’ISO (Organisation internationale de normalisation) est une fédération mondiale d’organismes
nationaux de normalisation (comités membres de l’ISO). L’élaboration des Normes internationales est
en général confiée aux comités techniques de l’ISO. Chaque comité membre intéressé par une étude
a le droit de faire partie du comité technique créé à cet effet. Les organisations internationales,
gouvernementales et non gouvernementales, en liaison avec l’ISO participent également aux travaux.
L’ISO collabore étroitement avec la Commission électrotechnique internationale (IEC) en ce qui concerne
la normalisation électrotechnique.
Les procédures utilisées pour élaborer le présent document et celles destinées à sa mise à jour sont
décrites dans les Directives ISO/IEC, Partie 1. Il convient, en particulier de prendre note des différents
critères d’approbation requis pour les différents types de documents ISO. Le présent document a été
rédigé conformément aux règles de rédaction données dans les Directives ISO/IEC, Partie 2 (voir www.
iso.org/directives).
L’attention est appelée sur le fait que certains des éléments du présent document peuvent faire l’objet de
droits de propriété intellectuelle ou de droits analogues. L’ISO ne saurait être tenue pour responsable
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Les appellations commerciales éventuellement mentionnées dans le présent document sont données pour
information, par souci de commodité, à l’intention des utilisateurs et ne sauraient constituer un engagement.
Pour une explication de la signification des termes et expressions spécifiques de l’ISO liés à l’évaluation de
la conformité, ou pour toute information au sujet de l’adhésion de l’ISO aux principes de l’OMC concernant
les obstacles techniques au commerce (OTC), voir le lien suivant: Avant-propos — Informations
supplémentaires.
L’ISO 643 a été élaborée par le comité technique ISO/TC 17, Acier, sous-comité SC 7, Méthodes d’essais
(autres que les essais mécaniques et les analyses chimiques).
Cette quatrième édition annule et remplace la troisième édition (ISO 643:2012), qui a fait l’objet d’une
révision mineure. Une note a été ajoutée après le premier alinéa en 7.1.2.
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PROJET FINAL DE NORME INTERNATIONALE ISO/FDIS 643:2015(F)
Acier — Détermination micrographique de la grosseur de
grain apparente
1 Domaine d’application
La présente Norme internationale 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.
2 Références normatives
Les documents de référence suivants sont indispensables pour l’application du présent document. Pour
les références datées, seule l’édition citée s’applique. Pour les références non datées, la dernière édition
du document de référence s’applique (y compris les éventuels amendements).
ISO 3785, Matériaux métalliques — Désignation des axes des éprouvettes en relation avec la texture du produit
ISO 14250, Aciers — Caractérisation métallographique de la grosseur et de la distribution de grain duplex
ASTM E112, Standard Test Methods for Determining Average Grain Size
3 Termes et définitions
Pour les besoins du présent document, les termes et définitions suivants s’appliquent.
3.1
grain
forme polygonale fermée à côtés plus ou moins courbes, qui peuvent être révélés sur une coupe plane de
l’échantillon, polie et préparée pour l’examen micrographique
Une distinction est faite entre:
3.1.1
grain austénitique
cristal avec une structure cubique à face centrée qui peut, ou peut ne pas, contenir des macles de recuit
3.1.2
grain ferritique
1)
cristal avec une structure cubique centrée qui ne contient jamais de macles de recuit
3.2
indice
nombre G positif, nul ou éventuellement négatif, qui est déterminé à partir du nombre moyen m des
2
grains dénombrés sur une aire de 1 mm de la coupe de l’échantillon.
Note 1 à l’article: Par définition, G = 1 pour m = 16; les autres indices sont obtenus par la formule
1) L’estimation du grain ferritique se fait généralement pour les aciers non alliés dont la teneur en carbone est
inférieure ou égale à 0,25 %. En présence d’îlots perlitiques de dimensions identiques à celles des grains de ferrite,
les îlots sont alors comptés comme des grains de ferrite.
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ISO/FDIS 643:2015(F)

G
m = 8 × 2
3.3
interception
N
nombre de grains traversés par une ligne de mesure droite ou courbe
Note 1 à l’article: Voir Figure 1.
Note 2 à l’article: Les lignes droites de mesure se termineront normalement dans un grain. Ces segments terminaux
sont comptés comme une 1/2 interception. N est le nombre moyen de grains traversés par la ligne de mesure
appliquée de façon aléatoire à divers emplacements. N est divisé par la longueur réelle de la ligne de mesure, L ,
T
habituellement en millimètres, pour obtenir le nombre de grains interceptés par unité de longueur, N .
L
3.4
intersection
P
nombre de points d’intersection entre les joints de grains et une ligne de mesure droite ou courbe
Note 1 à l’article: Voir Figure 1.
Note 2 à l’article: P est le nombre moyen de joints de grains traversés par la ligne de mesure appliquée de façon
aléatoire à divers emplacements. P est divisé par la longueur réelle de la ligne de mesure, L , habituellement en
T
millimètres, pour obtenir le nombre de joints de grains traversés par unité de longueur, P .
L
4 Symboles et termes abrégés
Les symboles utilisés sont donnés dans le Tableau 1.
5 Principe
La grosseur de grain est mise en évidence par l’examen micrographique d’une section polie de l’échantillon
préparée par une méthode appropriée au type d’acier et à l’information recherchée.
NOTE Si la commande ou la Norme internationale définissant le produit ne stipule pas la méthode de mise en
évidence du grain, le choix de cette méthode est laissé à l’initiative du producteur.
Cette grosseur moyenne est caractérisée
a) par un indice obtenu
— habituellement par comparaison avec des images types pour le mesurage de la grosseur de grain, ou
— par comptage pour déterminer le nombre moyen de grains par unité de surface;
b) ou par la valeur moyenne du segment intercepté.
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ISO/FDIS 643:2015(F)

Interception (N) comptée pour une ligne droite sur une structure de grain monophasée où les
flèches indiquent six interceptions et deux segments de ligne finissant dans un grain (2 × 1/2 =
1 N) et N = 7
Intersection (P) comptée pour une ligne de mesure droite placée sur une structure de grain
monophasée où les flèches indiquent sept points d’intersection et P = 7
Figure 1 — Exemples d’intersection, P, et d’interception, N
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ISO/FDIS 643:2015(F)

Tableau 1 — Symboles
Symboles Définition Valeur
1
a Aire moyenne du grain, en millimètres carrés
a=
m
A Aire apparente de la figure d’essai, en millimètres carrés —
F
1
d=
Diamètre moyen du grain, en millimètres
d
m
Diamètre du cercle limitant sur le verre dépoli du microscope ou sur une
79,8 mm
D épreuve photographique l’image de la surface de référence de l’éprou-
2
(surface = 5 000 mm )
vette
g Grossissement linéaire (à noter en référence) de l’image microscopique En principe 100
G Indice équivalent de grosseur du grain —
g
Facteur de conversion du rapport de grossissement linéaire × g au gros-
K
K=
sissement linéaire × 100
100
Longueur linéaire moyenne d’interception, généralement exprimée en
l
lN==11//P
LL
millimètres
Longueur réelle de la ligne de mesure divisée par le grossissement, en
L —
T
millimètres
m = 2 n
100
Nombre de grains par millimètre carré de surface de l’éprouvette dans la (grossissement × 100)
m
2
région examinée m = 2 K ng
(grossissement × g)
Numéro de la planche d’images types la plus proche quand g n’est pas
M —
égal à 100
Nombre équivalent total des grains examinés sur l’image de diamètre D
n —
g
(avec grossissement × g)
n Nombre de grains complètement à l’intérieur du cercle de diamètre D —
1
n Nombre de grains coupés par le cercle de diamètre D —
2
n
Nombre équivalent total des grains examinés sur l’image de diamètre D
2
n
100 nn=+
(avec grossissement × 100) 100 1
2
Nombre moyen de grains interceptés par unité de longueur L —
N
Nombre moyen de grains interceptés par unité de longueur de la ligne NN= /L
L
N
L
T
a
N Nombre d’interceptions par millimètre dans la direction longitudinale —
x
a
N Nombre d’interceptions par millimètre dans la direction transversale —
y
Nombre d’interceptions par millimètre dans la direction perpendiculai-
N —
z
a
re
Nombre moyen de joints de grains traversés par la ligne de mesure appli-
—
P
quée de façon aléatoire à divers emplacements
Nombre moyen d’intersections de joints de grains par unité de longueur
PP= /L
L
P
L
T
de ligne de mesure
a
  La méthode pour désigner la direction doit être conforme à l’ISO 3785.
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ISO/FDIS 643:2015(F)

6 Sélection et préparation de l’échantillon
6.1 Prélèvement
Si la commande ou la Norme internationale définissant le produit ne spécifie pas le nombre d’échantillons
et l’emplacement auquel ils doivent être prélevés sur le produit, ceux-ci sont laissés à l’initiative du
producteur, bien qu’il ait été montré que la précision de la détermination de la grosseur de grain est
améliorée si davantage d’échantillons sont évalués. Par conséquent, il est recommandé d’évaluer deux
sections ou plus. On prendra soin de s’assurer que les échantillons sont représentatifs de la totalité du
produit (c’est-à-dire, éviter le matériel fortement déformé, comme celui qui se trouve à l’extrémité de
certains produits ou là où le cisaillage a été utilisé pour prélever l’échantillon, etc.). Les échantillons
doivent être polis conformément aux techniques habituelles.
Sauf indication contraire dans la norme de produit ou par accord avec le client, la face polie de l’échantillon
doit être longitudinale, c’est-à-dire parallèle à l’axe principal de la déformation des produits corroyés.
Les mesurages de la grosseur de grain sur un plan transversal seront biaisés si le grain n’est pas équiaxe.
6.2 Mise en évidence des joints de grains ferritiques
Les grains ferritiques doivent être mis en évidence par attaque au nital (solution de 2 % à 3 % d’acide
nitrique dans de l’éthanol), ou à l’aide d’un réactif approprié.
6.3 Mise en évidence des joints de grains austénitiques et préausténitiques
6.3.1 Généralités
Dans le cas des aciers présentant une structure austénitique monophasée ou biphasée (grains de ferrite
delta dans une matrice austénitique) à la température ambiante, le grain doit être mis en évidence par
une solution d’attaque. Pour les aciers inoxydables austénitiques monophasés, les réactifs chimiques les
plus couramment utilisés sont le réactif glyceregia, le réactif de Kalling (n° 2) et le réactif de Marble. La
meilleure attaque électrolytique pour les aciers inoxydables monophasés ou biphasés est l’acide nitrique
aqueux à 60 % à 1,4 V c.c. pendant 60 s à 120 s, car il met en évidence les joints de grains mais pas les
macles. L’acide oxalique à 10 %, 6 V c.c., jusqu’à 60 s, est couramment utilisé mais est moins efficace
qu’une solution de HNO à 60 %.
3
Dans le cas d’autres aciers, l’une ou l’autre des méthodes décrites ci-après doit être utilisée, compte tenu
de l’information recherchée, à savoir:
— méthode de «Bechet-Beaujard» par attaque avec une solution aqueuse saturée en acide picrique
(voir 6.3.2);
— méthode de «Kohn» par oxydation ménagée (voir 6.3.3);
— méthode de «McQuaid-Ehn» par cémentation (voir 6.3.4);
— méthode de sensibilisation des joints de grains (voir 6.3.7);
— d’autres méthodes prévues par accord particulier à la commande.
NOTE Les trois premières méthodes s’appliquent aux joints de grains préausténitiques, les autres aux aciers
au manganèse austénitiques ou aux aciers inoxydables austénitiques; voir l’Annexe A.
Si des essais comparatifs sont effectués pour les différentes méthodes, il est indispensable d’utiliser
les mêmes conditions de traitement thermique. Les résultats peuvent sensiblement diverger d’une
méthode à l’autre.
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ISO/FDIS 643:2015(F)

6.3.2 Méthode de «Bechet-Beaujard» par attaque avec une solution aqueuse saturée en
acide picrique
6.3.2.1 Domaine d’application
Cette méthode met en évidence le grain austénitique formé au cours du traitement thermique de
l’échantillon. Elle est applicable aux échantillons qui ont une structure martensitique ou bainitique.
Pour que cette attaque soit efficace, la teneur en P doit être ≥ 0,005 %.
6.3.2.2 Préparation
Le réactif de Bechet-Beaujard est normalement utilisé sur un échantillon en acier soumis à un traitement
thermique. Normalement, aucun traitement thermique ultérieur n’est nécessaire si l’échantillon présente
une structure martensitique ou bainitique. Dans le cas contraire, un traitement thermique est nécessaire.
Si les conditions de traitement de l’éprouvette ne sont pas prévues par la Norme internationale définissant
le produit et sauf spécification contraire, les conditions suivantes doivent être appliquées dans le cas des
aciers de construction au carbone pour traitement thermique et des aciers faiblement alliés:
— 1,5 h à (850 ± 10) °C pour les aciers dont la teneur en carbone est supérieure à 0,35 %;
— 1,5 h à (880 ± 10) °C pour les aciers dont la teneur en carbone est inférieure ou égale à 0,35 %.
Après ce traitement, l’éprouvette est généralement trempée dans l’eau ou dans l’huile.
6.3.2.3 Polissage et attaque
Une surface plane de l’échantillon doit être polie pour l’examen micrographique. Elle doit être attaquée
pendant un temps suffisant au moyen d’une solution aqueuse saturée en acide picrique additionnée d’au
moins 0,5 % d’alkylsulfonate de sodium ou d’un autre agent mouillant approprié.
NOTE La durée d’attaque peut varier de quelques minutes à plus d’une heure. Chauffer la solution à 60 °C peut
améliorer l’action d’attaque et réduire la durée d’attaque.
Plusieurs attaques et polissages successifs sont parfois nécessaires pour assurer un contraste suffisant
entre les joints de grains et le fond général de l’échantillon. Dans le cas de l’acier trempé à cœur, un
revenu peut être effectué avant prélèvement de l’échantillon.
AVERTISSEMENT — En chauffant des solutions contenant de l’acide picrique, des précautions
doivent être prise pour éviter l’ébullition des solutions car l’extrait sec d’acide picrique peut
devenir explosif.
6.3.2.4 Résultat
Les joints de grains préausténitiques doivent immédiatement être mis en évidence à l’examen
microscopique.
6.3.3 Méthode de «Kohn» par oxydation ménagée
6.3.3.1 Domaine d’application
Cette méthode met en évidence la configuration des grains austénitiques formée par oxydation préférentielle
des joints au cours de l’austénitisation à la température d’un traitement thermique déterminé.
6.3.3.2 Préparation
Une face de l’échantillon doit être polie. Le reste de sa surface ne doit pas présenter de traces d’oxyde.
L’échantillon doit être placé dans un four de laboratoire dans lequel soit un vide de 1 Pa est obtenu, soit un
gaz inerte y circule (par exemple de l’argon purifié). Traiter thermiquement l’échantillon conformément
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ISO/FDIS 643:2015(F)

à la procédure d’austénitisation spécifiée par le client ou comme définie par la Norme internationale
définissant le produit.
À la fin de la période de chauffage indiquée, de l’air doit être introduit dans le four pendant une durée
de 10 s à 15 s.
L’échantillon est ensuite trempé à l’eau. En général, l’échantillon peut être directement observé au microscope.
NOTE 1 La méthode d’oxydation peut être appliquée sans atmosphère inerte.
NOTE 2 Il convient d’éliminer l’oxyde adhérant à la surface précédemment polie par un polissage léger à l’aide
d’un abrasif fin, en veillant à ce que le réseau d’oxyde qui s’est formé sur les joints de grains soit conservé, puis
il convient de terminer le polissage par les méthodes habituelles. Il convient alors d’attaquer l’échantillon en
utilisant le réactif de Vilella:
—  acide picrique 1 g
—  acide chlorhydrique 5 ml
—  éthanol 100 ml
6.3.3.3 Résultat
L’oxydation préférentielle des joints révèle la configuration des grains austénitiques.
Si la préparation est bien conduite, l’apparition d’oxyde aux joints de grains n’a généralement pas lieu.
Dans certains cas, il peut être nécessaire de recourir à un éclairage oblique, ou aux méthodes de contraste
d’interférence différentielle (DIC, Differential Interference Contrast), pour mieux distinguer les joints
par effet de relief.
6.3.4 Méthode de «McQuaid-Ehn» par cémentation à 925 °C
6.3.4.1 Domaine d’application
Cette méthode spécifique aux aciers de cémentation met en évidence les joints de grains austénitiques
formés pendant la cémentation de ces aciers. Elle ne convient généralement pas pour mettre en évidence
les grains formés au cours d’un autre traitement thermique.
NOTE Le procédé de «cémentation simulée» (mock carburizing) peut également être utilisé. L’échantillon est
soumis au même traitement thermique mais sans atmosphère riche en carbone. Il est alors soumis au traitement
thermique auquel le produit serait traité. Le réactif de Bechet-Beaujard est employé pour révéler les joints de
grains, voir 6.3.2.
6.3.4.2 Préparation
Les échantillons doivent être exempts de toute trace de décarburation ou d’oxydation superficielle. Tout
traitement antérieur froid, chaud, mécanique, etc., peut voir un effet sur la forme du grain obtenu; la
spécification du produit doit énoncer les traitements à effectuer avant détermination dans les cas où il
est recommandé de tenir compte de ces considérations.
Après cémentation, l’échantillon doit être refroidi à une vitesse suffisamment lente pour assurer la
précipitation de la cémentite aux joints de grains de la zone hypereutectoïde de la couche cémentée.
La cémentation doit être effectuée par maintien de l’échantillon à (925 ± 10) °C pendant 6 h. Ceci est
généralement obtenu par maintien de la caissette de cémentation à (925 ± 10) °C pendant 8 h, ce qui
comprend la durée de préchauffage. Dans la plupart des cas, on obtient une couche cémentée d’environ
1 mm. Après la cémentation, refroidir l’échantillon à une vitesse suffisamment lente pour assurer la
précipitation de la cémentite aux joints de grains de la zone hypereutectoïde de la couche cémentée.
Un cément neuf doit être employé à chaque fois.
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ISO/FDIS 643:2015(F)

6.3.4.3 Préparation des échantillons
L’échantillon cémenté doit être découpé normalement à sa surface. Une des coupes doit être préparée
pour examen micrographique et attaquée selon a) ou b).
a) Réactif de «Le Chatelier et Igewski» (picrate alcalin de sodium):
—  acide picrique 2 g
—  hydroxyde de sodium 25 ml
—  eau 100 ml
Utiliser ce réactif par immersion à 100 °C, pendant au moins 1 min, ou à température ambiante sous
forme d’attaque électrolytique à 6 V c.c pendant 60 s.
b) Nital:
—  acide nitriqu 2 ml à 5 ml
—  éthanol pour compléter à 100 ml
D’autres réactifs peuvent être utilisés à condition de conduire aux mêmes résultats.
6.3.4.4 Résultat
Les joints de grains préausténitiques dans la couche superficielle cémentée hypereutectoïde doivent
être révélés par un réseau de cémentite proeutectoïde.
6.3.5 Méthode de la ferrite proeutectoïde
NOTE Des lignes directrices pour l’usage de cette méthode en fonction de la microstructure du produit en
acier sont données dans l’Annexe A.
6.3.5.1 Principe
Cette méthode convient à l’acier au carbone ayant une teneur en carbone d’environ 0,25 % à 0,6 % et
aux aciers faiblement alliés tels que manganèse-molybdène, 1 % de chrome, 1 % de chrome-molybdène
et 1,5 % de nickel-chrome. Les joints de grains préausténitiques sont révélés sous forme d’un réseau de
ferrite proeutectoïde.
6.3.5.2 Préparation
Utiliser les conditions d’austénitisation spécifiées dan
...