ISO 643:2019 pdf download – Steels — Micrographic determination of the apparent grain size.
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 3 . 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 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.