🔩 Grain size in steels — micrographic determination of the apparent ferritic and austenitic grain size by the comparison method (charts), the planimetric method and the intercept method, index G; PN-EN ISO 643:2025-02

Materials and NDT PN-EN ISO 643

Overview

WHAT THE STANDARD COVERS. Scope from the PKN card of PN-EN ISO 643:2025-02, in full (our translation): "This document specifies a micrographic method of determining the ferritic and austenitic grain size in steels. It describes methods of revealing grain boundaries and of estimating the mean grain size in test specimens with a unimodal statistical distribution of the sizes of these grains. Although the shape of grains is three-dimensional, the metallographic cross-sectional plane can cut a grain at any of its points, from a grain corner up to its maximum diameter, thus producing a range of sizes of the grains visible on the two-dimensional plane, even in a specimen with a perfectly uniform grain size." We read the content of the method in the sample extract of ISO 643:2024 (fifth edition, 2024-08) from the cover to clause 7.3.4 with the table of contents; we do not have clauses 7.3.5–7.5, clause 8 (test report) or Annexes A–E (including the charts of Annex B).

ACCORDING TO THE TEXT OF ISO 643:2024 (foreword, clauses 1–7.3.4). The standard was prepared by ISO/TC 17 "Steel", SC 7 "Methods of testing (other than mechanical tests and chemical analysis)", in collaboration with CEN/TC 459. Changes compared with the 2019 edition: the temperature of the McQuaid-Ehn method for case-hardening steels changed to 950 °C (A.4); Annex B restored with new ISO charts instead of the ASTM charts; part of the former Annex B moved to 7.3; new Annexes D (confidence interval) and E (grains of different sizes). The standard has no normative references. The index G is related to the mean number of grains m per 1 mm² by the formula m = 8 × 2^G, i.e. G = log₂ m − 3 (G = 1 for m = 16). Principle (5): the apparent grain size is determined on a suitably prepared section by the comparison method with charts (7.2), the planimetric method (7.3) or the intercept method (7.4); "all methods give comparable results". Specimens (6.1): the number and location of sampling are specified by the order or the product standard, and failing that — by the manufacturer; heavily deformed material should be avoided (e.g. ends of products, places cut with shears); with deformed grains the section should be parallel to the main axis of deformation, because measurement on a cross-section with non-equiaxed grains is biased. Revealing the boundaries: ferrite — nital (2–3 % by volume of nitric acid in ethanol) or another suitable reagent (6.2); single-phase austenitic steels — glyceregia, Kalling's reagent No. 2 or Marble's reagent, and best of all electrolytically with 60 % HNO₃ at 1.4 V for 60–120 s (reveals the grain boundaries without twin boundaries); for other steels (prior austenite grain boundaries) — the methods of Béchet-Beaujard (saturated solution of picric acid), Kohn (controlled oxidation), McQuaid-Ehn (carburizing) or sensitization of the boundaries; "results can differ considerably depending on the method", so for comparisons the same heat treatment conditions must be kept (6.3.1). Accuracy (7.1.3): comparison method — 0.5, planimetric and intercept methods — 0.1; for comparisons the indices are rounded to multiples of 0.5; with a bimodal distribution the mean may be meaningless — then ISO 14250. Comparison method (7.2): charts at a magnification of 100:1 numbered from −1 to 10; at another magnification g the index G = M + 2·log₂(g/100) = M + 6.64·lg(g/100) (formula 3, Table 2); at least three randomly selected fields per specimen; when the difference between the largest and the smallest index is less than 3 — the result is the arithmetic mean, otherwise at least six additional determinations, and if even that is not enough — a description of the scatter in the report; the mean is rounded to 0.5. The standard states that the ASTM E112 charts give essentially the same results as the charts of Annex B. Planimetric method (7.3): at least 50 grains in the circle; number of grains nt = ne + ni/2 (ne — entirely inside, ni — intersected by the circle), m = nt/AC; the bias of this assumption with ≥ 50 grains is about 2 %; in a rectangle nt = ne + ni/2 + 1 (corners).

STATUS (status of PKN cards as of 08.10.2026). The card of PN-EN ISO 643:2025-02 (English version) has no withdrawal label: title "Steel -- Micrographic determination of grain size" (our translation), publication 26-02-2025, 58 pages, price group U, Metallurgy Sector, KT 123 Testing of Properties of Metals, "Introduces: EN ISO 643:2024 [IDT], ISO 643:2024 [IDT]", "Replaces: PN-EN ISO 643:2020-07 – English version, PN-EN ISO 643:2020-07 – Polish version", ICS 77.040.99. Card of PN-EN ISO 643:2020-07 (Polish version): "Withdrawn and replaced by PN-EN ISO 643:2025-02 – English version", publication 23-11-2023, withdrawal 26-02-2025, 28 pages, it introduced EN ISO 643:2020 and ISO 643:2019. Card of PN-EN ISO 643:2013-06 (English version): withdrawn on 02-07-2020, it introduced ISO 643:2012. The current edition thus has only an English version; the Polish version of the previous edition was valid for 15 months.

HOW MANY LABORATORIES AND IN WHAT FORM (copy of the accreditation scope database and current PCA documents, read on 08.10.2026). Number 643 appears in 18 records at 13 laboratories; in the current PCA documents (issues from 23.04.2025 to 22.09.2026) — in 15 entries at the same laboratories (AB 001 and AB 1629 have two each, AB 053 one entry recorded in the database copy in five records); no entry is suspended. Notations: "PN-EN ISO 643:2025-02" — 10 laboratories (AB 053 alongside "ISO 643:2024"); "PN-EN ISO 643" without a year — AB 019, AB 128 and AB 608. Alongside this standard the laboratories enter ASTM E112 (AB 019, AB 053, AB 099, AB 128, AB 490, AB 608, AB 792, AB 1629) and the national standard PN-H-04507-01 (AB 001, AB 193, AB 490, AB 699). The "Laboratories" tab ("PN-EN ISO 643") shows all 13 laboratories.

WHAT THE LABORATORIES TEST (PCA documents). Grain size — optical microscopy; methods: comparison against a scale of charts (most of them), at AB 128 and AB 1629 also the intercept method and the planimetric method (counting grains). Subject: steels, steel metallurgical and structural products, rolled products and open-die forgings (AB 019), metallurgical products and castings (AB 099), metals and alloys (AB 001, AB 699, AB 1629).

WHERE A SEEMINGLY CORRECT RESULT IS EASY TO GET. First — charts: the 2024 edition restored Annex B with new ISO charts instead of the ASTM ones; a laboratory that assesses according to the charts of the previous edition or ASTM E112 should state this in the report — the standard speaks of "essentially the same" results, not identical ones. Second — bimodal distribution: the mean index of a specimen with two grain populations describes neither of them (7.1.3) — an assessment according to ISO 14250 or a description of the scatter is needed. Third — the section: with elongated grains the measurement on a cross-section is biased (6.1). Fourth — revealing prior austenite grain boundaries: the results of different methods (Béchet-Beaujard, Kohn, McQuaid-Ehn) can differ considerably — the method and the heat treatment must be stated in the report. Fifth — magnification: at a magnification other than 100:1 the chart number must be converted according to Table 2; without the conversion the result at 200:1 is understated by two index numbers.

WHAT WE DO NOT GIVE. We do not have clauses 7.3.5–7.5 (the remaining part of the planimetric method, the linear and circular intercept methods, other methods), clause 8 or the annexes — we do not give the charts, the conversions of the intercept method, the confidence interval or the content of the report. We have not checked the status of PN-H-04507-01 and of ASTM E112 — we list them only as notations from the scopes.

Method principle

On a metallographic section the grain boundaries of ferrite (e.g. with nital) or of austenite or prior austenite (reagents or methods from Annex A) are revealed and the grain size index G is determined under the microscope: by comparing the image with charts at a magnification of 100:1, by counting the grains in a field of known area (planimetric method) or by counting the intersections of a test line with the grains (intercept method); m = 8 × 2^G grains per 1 mm² (ISO 643:2024). The methods apply to specimens with a unimodal grain size distribution.

Applications

• Steels and steel products (rolled, forged, cast) — ferritic and austenitic grain size; 13 laboratories in PCA scopes
• Acceptance testing according to steel product standards which require a specified index G
• Assessment of the effect of heat treatment on the grain (prior austenite grain boundaries by the methods of Annex A)

Key parameters

Standard

Standard number

PN-EN ISO 643:2025-02 (wersja angielska; wprowadza EN ISO 643:2024 [IDT] i ISO 643:2024 [IDT])

Title (PL)

Stal — Mikrograficzne określanie wielkości ziarna

Title (EN)

Steels — Micrographic determination of the apparent grain size

Step-by-step procedure

1. Specimen

   Take a representative specimen, away from heavily deformed material; with deformed grains the section parallel to the main axis of deformation (6.1). PN-EN ISO 643:2025-02 current (status of PKN cards as of 08.10.2026).

2. Revealing the boundaries

   Ferrite — nital (6.2); austenite — chemical or electrolytic etching; prior austenite grain boundaries — a method from Annex A chosen for the information needed, under the same heat treatment conditions (6.3).

3. Comparison method

   Compare at least 3 random fields with the charts at 100:1 (or convert the index at another magnification); with a scatter smaller than 3 — the mean rounded to 0.5 (7.2).

4. Planimetric method

   Count the whole grains (ne) and the intersected grains (ni) in a circle with at least 50 grains: nt = ne + ni/2, m = nt/AC, G from m = 8 × 2^G (7.3).

5. Result

   Index G (comparison method with an accuracy of 0.5, planimetric and intercept methods 0.1); the intercept method and the content of the report (7.4, clause 8) — WE DO NOT GIVE.

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Etching reagents (nitric acid, picric acid, glyceregia) — work in a fume cupboard, goggles and gloves; the standard warns that it calls for substances and procedures that are injurious to health without adequate safety precautions
• TECHNICAL NOTE: the 2024 edition has new charts (Annex B) — when assessing according to other charts, state them in the report; with a bimodal distribution the mean index does not describe the specimen

Frequently asked questions