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

2020-11-26: WI cancelled. CEN/BT C218/2020
2020-11-16: Request to BT to cancel this WI circulated on 2020-10-28
2019-09-16: TC will launch CIB to withdraw this WI, then a BT decision is needed to drop positive voting results
2016-02-09: WI ON HOLD - ISO WI cancelled at ISO level - ECISS/TC to advise on the future of this project at CEN level.

Stahl - Mikrophotographische Bestimmung der erkennbaren Korngröße (ISO/FDIS 643:2015)

Acier - Détermination micrographique de la grosseur de grain apparente (ISO/FDIS 643:2015)

Jekla - Mikrografsko ugotavljanje navidezne velikosti kristalnih zrn (ISO/FDIS 643:2015)

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SLOVENSKI STANDARD
kSIST FprEN ISO 643:2015
01-junij-2015
Jekla - Mikrografsko ugotavljanje navidezne velikosti kristalnih zrn (ISO/FDIS
643:2015)

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

Acier - Détermination micrographique de la grosseur de grain apparente (ISO/FDIS
643:2015)
Ta slovenski standard je istoveten z: FprEN ISO 643
ICS:
77.040.99 Druge metode za Other methods of testing of
preskušanje kovin metals
77.080.20 Jekla Steels
kSIST FprEN ISO 643:2015 en

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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kSIST FprEN ISO 643:2015
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kSIST FprEN ISO 643:2015
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:
2015­04 ­02
Acier — Détermination micrographique de la grosseur de grain
apparente
Voting terminates on:
2015­06 ­02
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 SUPPORTING
DOCUMENTATION.
IN ADDITION TO THEIR EVALUATION AS
Reference number
BEING ACCEPTABLE FOR INDUSTRIAL, TECHNO­
ISO/FDIS 643:2015(E)
LOGICAL, COMMERCIAL AND USER PURPOSES,
DRAFT INTERNATIONAL STANDARDS MAY ON
OCCASION HAVE TO BE CONSIDERED IN THE
LIGHT OF THEIR POTENTIAL TO BECOME STAN­
DARDS TO WHICH REFERENCE MAY BE MADE IN
NATIONAL REGULATIONS. ISO 2015
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kSIST FprEN ISO 643:2015
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

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Published in Switzerland
ii © ISO 2015 – All rights reserved
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kSIST FprEN ISO 643:2015
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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kSIST FprEN ISO 643:2015
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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kSIST FprEN ISO 643:2015
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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kSIST FprEN ISO 643:2015
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kSIST FprEN ISO 643:2015
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 a nd definiti ons
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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kSIST FprEN ISO 643:2015
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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kSIST FprEN ISO 643:2015
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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kSIST FprEN ISO 643:2015
ISO/FDIS 643:2015(E)
Table 1 — Symbols
Symbols D e f i n i t ion 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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kSIST FprEN ISO 643:2015
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 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 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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kSIST FprEN ISO 643:2015
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 10s 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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kSIST FprEN ISO 643:2015
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 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., can have an effect on the shape of the grain obtained; the

product specification shall state the treatments to be carried out before determination in cases where it

is advisable to take into account these considerations.

After carburizing, the specimen shall be cooled at a rate slow enough to precipitate cementite at the

grain boundaries in the hypereutectoid surface region of the carburized specimen.

Carburization shall be achieved by maintaining the specimen at (925 ± 10) °C for 6 h. This is generally

done by keeping the carburizing chamber at (925 ± 10) °C for 8 h, including a pre-heating period. In most

cases, a carburized layer of approximately 1 mm is obtained. After carburizing, cool the specimen at a rate

slow enough to ensure that the cementite is precipitated at the grain boundaries of the hypereutectoid

zone of the carburized layer.
Fresh carburizing compound shall be used each time.
6.3.4.3 Specimen preparation

The carburized specimen shall be sectioned normally to its surface. One of the sections shall be prepared

for micrographic examination and etched using either a) or b).
a) “Le Chatelier and Igewski” reagent (alkaline sodium picrate):
— picric acid 2 g;
— sodium hydroxide 25 g;
— water 100 ml.
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kSIST FprEN ISO 643:2015
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 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 ar

...

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