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IS0
I N T ERN AT I O N AL O RG A N I Z AT I ON F O R STAND AR D I2 AT I O N
IS0 RECOMMENDATION
R 643
MICROGRAPHIC DETERMINATION
OF THE AUSTENITIC GRAIN SIZE OF STEELS
1st EDITION
December 1967
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IS0 Recommendations and IS0 Standards
The copyright of
belongs to IS0 Member Bodies. Reproduction of these
documents, in any country, may be authorized therefore only
by the national standards organization of that country, being
a member of ISO.
For each individual country the only valid standard is the national standard of that country.
Printed in Switzerland
Also issued in French and Russian. Copies to be obtained through the national standards organizations.
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BRIEF HISTORY
The IS0 Recommendation R 643, Micrographic determination of the austenitic grain
size of steels, was drawn up by Technical Committee ISO/TC 17, Steel, the Secretariat of
which is held by the British Standards Institution (BSI).
Work on this question by the Technical Committee began in 1958 and led, in 1965,
to the adoption of a Draft IS0 Recommendation.
In January 1966, this Draft IS0 Recommendation (No. 920) was circulated to all the
IS0 Member Bodies for enquiry. It was approved, subject to a few modifications of an
editorial nature, by the following Member Bodies :
Argentina Germany Poland
Romania
Australia Hungary
Austria India Sweden
Belgium Israel Switzerland
Brazil Italy Turkey
Canada Japan U.A.R.
Chile Korea, Rep. of United Kingdom
Czechoslovakia Netherlands U.S.A.
Denmark New Zealand Yugoslavia
France Norway
No Member Body opposed the approval of the Draft.
The Draft IS0 Recommendation was then submitted by correspondence to the IS0
Council, which decided, in December 1967, to accept it as an IS0 RECOMMENDATION.
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lSO/ R 843 - lM7 (E)
IS0 Recommendation R 643 Decem ber 1967
MICROGRAPHIC DETERMINATION
OF THE AUSTENLTIC GRAIN SIZE OF STEELS
1. SCOPE
1.1 This IS0 Recommendation defines the methods of obtaining an estimate of the austenitic
grain size of unalloyed and low alloyed * steels and gives the rules which will characterize
the grain size observed.
NOTE. - The position at which the samples are taken from the product should be agreed between the pur-
chaser and the supplier.
1.2 The average grain size is shown by micrographic examination of a polished cross-section of
the sample, prepared by a method appropriate to the information required.
1.3 The index is obtained
- usually, by comparison with the standard grain size charts, or
- by count.
2. DEFINITIONS
2.1 Grain. A closed polygonal shape, with more or less curved sides, which can be revealed
within the network of the flat cross-section of the sample, polished and prepared for micro-
I
graphic examination.
2.2 Index. The whole positive, zero or possibly negative number G which is estimated from the
mean number m of grains defined above, counted in an area of 1 mm2 of the section of the
sample. By definition, G = 1 where m = 16; the other indices are obtained by the formula
m=8xP (1)
* This refers only to steels in which any alloying element does not exceed 5 % (m/rn).
I
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3. REVEAJJNG OF THE AUSTENITIC GRAIN
Depending on the information required, one of the appropriate methods described below should
be used:
- McQuaid Ehn Method by carburization, see clause 3.1,
- Kohn Method by light oxidation, see clause 3.2,
- Béchet Beaujard Method by concentrated picric acid etching, see clause 3.3,
- or, if need be, other methods stated either in the product specification or in the
order for the specification, see clause 3.4.
3.1 McQuaid Ehn Method by carburization
This method, generally applied to carburizing steels, shows the austenitic grain boundaries
formed during the carburization of the steel. In general, it is not suitable for the determination
of grains actually formed during other heat treatments.
3.1.1 Preparation. The samples should be free from all traces of decarburization and surface
oxidation. All previous treatments, whether cold, hot, mechanical or any other, can
have an influence on the shape of the grain obtained; the product specification should
state the treatments to be carried out before making the determination.
3.1.1.1
In general, the samples should be conveniently spaced in a carburizing chamber
having a lid and filled with dry and new carburizing compound.* Usually the
carburizing compound should consist principally of 60 % of charcoal in grains and
40% (m/m) of barium carbonate (BaC03). The volume of carburizing compound
used should be at least 30 times the volume of the samples to be carburized.
3.1.1.2
Carburization is achieved by maintaining the carburizing chamber at 925 "C for
8 hours. The carburizing chamber is then cooled to 600 "C at a rate depending on
the type of samples, but in every case between 30 and 100 "C per hour, the rate of
subsequent cooling below 600 "C having no influence on the test result.
3.1.2 Poiishing and etching. The carburized sample should be sectioned. One of the section
faces should be polished for micrographic examination. It should be etched
- either by means of a boiling alkaline solution of sodium picrate:
picric acid 2 g
caustic soda 25 g
water 100 ml
- or by nitric acid (nitai) :
"O3 3 to 5 ml
ethanol sufficient quantity per 100 ml
* The re-use of a carburizing compound already used should be forbidden.
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lSO/ R 643 - 19gl (E)
The use of other reagents is permitted, provided the same results are obtained.
3.1.3 Result. The grain boundaries of the carburized layer, which is about 1 mm thick, should
consist of proeutectoid cementite.
3.2 Kohn Method by light oxidation
This method reveals the austenitic grain formed during austenitization, at the temperature
of the desired heat treatment.
The application of this method on a number of samples from the same source and in various
conditions makes possible the study of the grain size variation as a function of temperature
and of time of austenitization.
3.2.1 Preparation. One face of the sample should be polished. The rest of its surface should
not have traces of oxide. The sample should be placed in a laboratory tube furnace in
which inert gas is circulating. It should be austenitized under the conditions of tem-
perature and time agreed or, in the absence of such, under the conditions stated in the
product specification for the treatment of test pieces for mechanical testing.
At the end of the heating time stated, air is introduced into the tube for 30 to 60 seconds.
The sample is then quenched as stated above.
3.2.2 Polishing and etching. The oxide which has formed and is adhering to the already
polished face should be removed by light polishing with a fine abrasive, and the polishing
completed in the usual manner, by etching,
- either by Villela’s reagent:
picric acid 1g
hydrochloric acid 5ml
ethanol 100 ml
- or by Benedicks reagent:
metanitrobenzene
sulphonic acid 5 ml
ethanol 100 ml
3.2.3 Result. The preferential oxidation of the boundaries will reveal the grain size.
Correct preparation should eliminate the appearance of oxide globules at the grain
boundaries.
It may be necessary, in certain cases, for improved grain boundary delineation by the
effect of relief, to use oblique illumination.
3.3 Béchet Beaujard Method by concentrated picric acid etching
This method is applicable to samples taken either from sample product treated so as to
produce martensitic or bainitic structures or on portions of test pieces already used to
determine the mechanical properties. It shows up the austenitic grain formed during heat
treatment of this sample product or this test piece.
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MO/ R 843 - 1967 (E)
3.3.1 Preparation. No special preparations or treatments are required. If a new austenitization
is carried out, this treatment should be carried out under the conditions of temperature
and time stated on the order, or, where no such requirement exists, in accordance with
the requirements of the product specification for the test pieces used to determine the
mechanical properties (see clause 3.2.1).
3.3.2 Polishing and etching. One flat face of the sample should be polished for micrographic
examination. It should be etched for a sufficient period by a saturated aqueous solution
of picric acid with 0.5 % of sodium alkylsulphonate.
NOTE. - The time of oxidation can vary from a few minutes to more than an hour. If necessary, the
procedure given in the Note to clause 3.3.3. should be followed.
3.3.3 Result. The boundaries of prior austenite grains appear directly in the micrographic
examination.
NOTE. - Successive polishings and etching are sometimes necessary to ensure a sufficient contrast
between the grain boundaries and the general form of the sample.
3.4 Other methods of preparation applicable to certain steels
For certain s
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