ISO 643-2012 pdf free download.Steels – Micrographic determination of the apparent grain size.
ISO 643 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 referenced documents are indispensable for the 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, Steel — Designation of test piece axes
ISO 14250, Steel — Metallographic characterization of duplex grain size and distributions
ASTM El 12, Standard Test Methods for Determining Average Grain Size
3 Terms and definitions
3.1.2 femtic grain
crystal with a body-centered cubic crystal structure which never contains annealing twins1
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.For the purposes of this document, the following terms and definitions apply.
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: austenitic grain crystal with a face-centered cubic crystal structure which may, or may not, contain annealing twins 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 %
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.ISO 643 pdf free download.