ISO 643:2019 pdf free download.Steels-Micrographic determination of the apparent grain size.
I Scope
ISO 643 specifies a micrugraphic method of determining apparent lerritic or austenitic grain size in steels. It describes the methods of revealing grain boundaries and ol estimating the mean grain size of specimens with unimodal size distribution. Although grains are three-dimensional in shape, the metallographic sectioning plane can cut through a grain at any point from a grain corner, to the maximum diameter of the grain. thus producing a range of apparent grain sizes on the two-dimensional plane, even In a sample with a perfectly consistent grain size.
2 NormatIve references
The following documents are referred to In the text In such a way that some or all of their content constitutes requirements of this document. For dated references, only the edition cited applies. For undated reterences. the latest edition of the referenced document (including any amendments) applies.
ASTM El 12. Standard Test Methadsior Determining Average Grain Size
3.2 General
3.2.1
index
positive, zero or possibly negative number C which is derived from the mean number m of grains (3.JJ) counted in an area of 1 mm2 of the section of the specimen
Note Ito entry: By definition. G 1 where m 16; the other indices are obtained by Esismuia.1lj. m.8×2
3.2.2
intercept
N
number of grams (ILl) intercepted by a test line, either straight or curved
Note ito 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 or grains intercepted by the test line applied randomly at various locations. N is divided by the true line length, £t. usually measured In millimetres. in order to obtain the number of grains intercepted per unit length, Nt,
3.2.3
intersection
number of intersection points between grain (lid) boundaries and a test line, either straight or curved Note Ito entry: See Floure 2.
Note 2 to entry: P Is the average of a number of counts of the number ofgra,n boundaries intersected by the test line applied randomly at various locations. P is divided by the true line length. L1. usually measured In millimetres. In order to obtain the number of grain boundary intersections per unit length. Pt.
be taken to ensure that the spec3mens 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 prioraustenltic 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 graiI shall be revealed by an etching solution. For single phase austenitic stainless steels, the most commonly used chemical etchants are giyceregia, 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 605 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 % HNO3.
For other steels, one or other of the methods specified below shall be used depending on the Information required.
— iiechet-Beaujard method by etching with aqueous saturated plcric acid solution (see 6.32).
— Kohn method by controlled oxidation (see 13).
— McQuaid.Ehn method by carburizatlon (see 6.14).
— grain boundary sensitization method (see 6.31).
— other methods specially agreed upon when ordering.
NOTE The first three methods are br pricw-austenltlc grain boundaries while the others are for austenkic
Mn or austenitic stainless. see AnnczA.
IF comparative tests are carried out for the different methods, it Is essential to use the same heat
treatment conditions. Results may vary considerably from one method to the other.
6.3.2 Bechet-Beaujarcr 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 lithe specimen has a martensltic or bainltic structure. If this is not the case, heat treatment Is necessary.
6.3.6 H.ainite or gradient-quench method
NOTE Gwdehnes 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 ci 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 4:4 (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 balnite outlining the martensite grains.
This structure may 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 olaustenitic stainless and manganese steels
The grain boundaries may be developed through precipitation of carbides by heating within the sensitizing temperature range, 4112 C to 704 C (900 F to 1 300 eF). Any suitable carbide-revealing etchant can be used.
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 may appear in the following forms under micrographic examination: a network of proeutectoid ferrite surrounding pearilte grains, a network of very fine pearlite surrounding martensite grains, etc. The austenitic grain may also be revealed by thermal etching under vacuum (not necessarily followed by oxidation). The product specification shall mention these simplified methods In these cast’s.
NOTE Amongst these methods are the following:
— precipitation on the grain boundaries during cooling.
— gradient quenching method. etc.
8 Test report
The test report shall contain the following in format Ion:
a) reference to this document, i.e. ISO 643;
b) grade of the steel examined;
c) type of grain determined;
d) method used, operating conditions, method of evaluation (i.e. manual or automatic image analyses); e) grain size index or the value of the mean segment;
fl any deviations from the procedure;
g) any unusual features observed;
h) the date of the test.