ISO 643:2019

ISO 643:2019 pdf free.Steels – Micrographic determination of the apparent grain size.
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 specimens 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 equlaxial.
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 % HNO3.
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 may vary considerably from one method to the other.
6.3.2 “Bechet-Beaujard” method by etching with aqueous saturated picric acid solution Field of application
This method reveals austenitic grains formed during heat treatment of the specimen. It is applicable to specimens which have a martens itic or bainitic structure. For this etch to work, there shall be at least 0,005 % P. 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.ISO 643 pdf download.

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