Steel — Micrographic determination of the apparent grain size

Acier — Détermination micrographique de la grosseur de grain apparente

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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
---------------------- Page: 1 ----------------------
ISO 643:2012(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2014

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Published in Switzerland
ii © ISO 2012 – All rights reserved
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ISO 643:2012(E)
© ISO 2012 – All rights reserved iii
---------------------- Page: 3 ----------------------
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

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Violators may be prosecuted.
iv © ISO 2012 – All rights reserved
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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

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.

© ISO 2012 – All rights reserved 1
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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

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

m82
3.3
intercept
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,

in order to obtain the number of grains intercepted per unit length, N .
3.4
intersection

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

in millimetres, in order to obtain the number of grain boundary intersections per unit length, P .

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
2 © ISO 2012 – All rights reserved
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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
a a 
Mean area of grain in square millimetres
A Apparent area of the test figure in square millimetres —
d 
Mean grain diameter in millimetres
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
(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 —
Conversion factor from linear magnification × g to linear K
100
magnification × 100
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ISO 643:2012(E)
lN1/ 1/P
l Mean lineal intercept length, generally expressed in millimetres
True length of the test line divided by the magnification, in
L —
millimetres
m = 2 n
100
(magnification × 100)
Number of grains per square millimetre of test piece surface in the
area examined
m = 2 K n
(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 —
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 —
Total equivalent number of grains examined on the image of diameter
nn
100 1
100
D (with magnification × 100)
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
N Number of intercepts per millimetre in the longitudinal direction —
N Number of intercepts per millimetre in the transverse direction —
N Number of intercepts per millimetre in the perpendicular direction —
Mean number of counts of the number of grain boundaries intersected
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
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.

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 %.

© ISO 2012 – All rights reserved 5
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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.
© ISO 2012 – All rights reserved 7
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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

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;

8 © ISO 2012 – All rights reserved
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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

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):
m82 (1)
This formula maycan be stated as
lg m
G (2a)
lg2
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

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.
© ISO 2012 – All rights reserved 9
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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
...

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
---------------------- Page: 1 ----------------------
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 copyright@iso.org
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

iv © ISO 2015 – All rights reserved
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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.
© ISO 2015 – All rights reserved v
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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

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

m=×82 .
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ISO/FDIS 643:2015(E)
3.3
intercept
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

to obtain the number of grains intercepted per unit length, N .
3.4
intersection

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

millimetres, in order to obtain the number of grain boundary intersections per unit length, P .

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.
2 © ISO 2015 – All rights reserved
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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
a Mean area of grain in square millimetres a =
A Apparent area of the test figure in square millimetres —
d =
Mean grain diameter in millimetres
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
(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 —
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
True length of the test line divided by the magnification, in millime­
L —
tres
m = 2 n
100

Number of grains per square millimetre of test piece surface in the (magnification × 100)

area examined m = 2 K n (magnifica­
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 —
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 —
Total equivalent number of grains examined on the image of diameter 2
n nn=+
100 100 1
D (with magnification × 100)
Mean number of grains intercepted per unit length L —
N NN= /L
Mean number of grains intercepted per unit length of the line
L L T
N Number of intercepts per millimetre in the longitudinal direction —
N Number of intercepts per millimetre in the transverse direction —
N Number of intercepts per millimetre in the perpendicular direction —
Mean number of counts of the number of grain boundaries inter­
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
line
The method for designating the direction conforms to ISO 3785.
4 © ISO 2015 – All rights reserved
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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 .

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.
© ISO 2015 – All rights reserved 7
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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.
8 © ISO 2015 – All rights reserved
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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

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
...

PROJET
NORME ISO/FDIS
FINAL
INTERNATIONALE 643
ISO/TC 17/SC 7
Acier — Détermination
Secrétariat: AFNOR
micrographique de la grosseur de
Début de vote:
2015-04-02 grain apparente
Vote clos le:
Steel — Micrographic determination of the apparent grain size
2015-06-02
LES DESTINATAIRES DU PRÉSENT PROJET SONT
INVITÉS À PRÉSENTER, AVEC LEURS OBSER-
Veuillez consulter les notes administratives en page iii
VATIONS, NOTIFICATION DES DROITS DE PRO-
PRIÉTÉ DONT ILS AURAIENT ÉVENTUELLEMENT
CONNAISSANCE ET À FOURNIR UNE DOCUMEN-
TATION EXPLICATIVE.
OUTRE LE FAIT D’ÊTRE EXAMINÉS POUR
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ISO/FDIS 643:2015(F)
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TION NATIONALE. ISO 2015
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ISO/FDIS 643:2015(F)
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© 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

de ne pas avoir identifié de tels droits de propriété et averti de leur existence. Les détails concernant les

références aux droits de propriété intellectuelle ou autres droits analogues identifiés lors de l’élaboration

du document sont indiqués dans l’Introduction et/ou dans la liste des déclarations de brevets reçues par

l’ISO (voir www.iso.org/brevets).

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.
© ISO 2015 – Tous droits réservés v
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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

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

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)
m = 8 × 2
3.3
interception
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 ,

habituellement en millimètres, pour obtenir le nombre de grains interceptés par unité de longueur, N .

3.4
intersection

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

millimètres, pour obtenir le nombre de joints de grains traversés par unité de longueur, P .

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
a Aire moyenne du grain, en millimètres carrés
A Aire apparente de la figure d’essai, en millimètres carrés —
Diamètre moyen du grain, en millimètres
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-
(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 —
Facteur de conversion du rapport de grossissement linéaire × g au gros-
sissement linéaire × 100
100
Longueur linéaire moyenne d’interception, généralement exprimée en
lN==11//P
millimètres
Longueur réelle de la ligne de mesure divisée par le grossissement, en
L —
millimètres
m = 2 n
100

Nombre de grains par millimètre carré de surface de l’éprouvette dans la (grossissement × 100)

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 —
(avec grossissement × g)
n Nombre de grains complètement à l’intérieur du cercle de diamètre D —
n Nombre de grains coupés par le cercle de diamètre D —
Nombre équivalent total des grains examinés sur l’image de diamètre D
100 nn=+
(avec grossissement × 100) 100 1
Nombre moyen de grains interceptés par unité de longueur L —
Nombre moyen de grains interceptés par unité de longueur de la ligne NN= /L
N Nombre d’interceptions par millimètre dans la direction longitudinale —
N Nombre d’interceptions par millimètre dans la direction transversale —
Nombre d’interceptions par millimètre dans la direction perpendiculai-
N —
Nombre moyen de joints de grains traversés par la ligne de mesure appli-
quée de façon aléatoire à divers emplacements
Nombre moyen d’intersections de joints de grains par unité de longueur
PP= /L
de ligne de mesure
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 %.

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

6 © ISO 2015 – Tous droits réservés
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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
...

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