Overview
ISO 643:2019 - "Steels - Micrographic determination of the apparent grain size" specifies a standardized micrographic method for determining the apparent ferritic or austenitic grain size in steels. The standard covers specimen selection and preparation, methods for revealing grain and prior‑austenitic boundaries, and quantitative characterization of grain size on metallographic cross‑sections. It recognizes that a two‑dimensional section produces a range of apparent grain sizes and defines procedures for estimating the mean grain size in specimens with a unimodal size distribution.
Key topics and requirements
- Scope and terminology: Definitions for grains, austenitic/ferritic grains, index and intercept concepts used in grain‑size metrics.
- Specimen selection & preparation: Polished metallographic sections, preferably taken longitudinally (parallel to principal deformation axis) and representative of bulk material; avoid heavily deformed areas.
- Revealing grain boundaries:
- Ferritic grains: typically revealed with nital (2–3% nitric in ethanol) or appropriate reagent.
- Austenitic/prior‑austenitic grains: chemical and electrolytic etches (e.g., glyceregia, Kalling’s reagent, Marble’s), electrolytic 60% HNO3 at 1.4 V d.c. for 60–120 s, or other methods including Bechet‑Beaujard (saturated picric acid), Kohn (controlled oxidation), McQuaid‑Ehn (carburization), sensitization and quench methods.
- Grain‑size characterization methods:
- Index method (comparison with standard charts or calculation of an index G derived from grain count per unit area).
- Planimetric method (counting grains per area).
- Intercept methods: linear and circular intercept techniques to obtain mean lineal intercept length.
- Assessment and reporting: Guidance on counting practice, averaging multiple sections, and content required in the test report. Annexes summarize revealing methods and provide evaluation procedures.
- Normative references: Includes related test methods such as ASTM E112 for average grain size determination.
Applications and practical value
ISO 643:2019 is used for:
- Quality control and process monitoring of steel production and heat treatments.
- Metallurgical characterization for product certification and specification compliance.
- Failure analysis where grain size influences mechanical properties (strength, toughness, creep).
- Laboratory standardization to ensure repeatable micrographic grain‑size measurement across suppliers and testing labs.
Who uses this standard
- Metallurgists and materials engineers
- QA/QC and inspection laboratories
- Steel manufacturers and heat‑treatment shops
- Research and academic laboratories performing metallography
Related standards
- ASTM E112 (Standard Test Methods for Determining Average Grain Size) - referenced for comparative methods and complementary procedures.
Keywords: ISO 643:2019, grain size, micrographic determination, steels, ferritic, austenitic, metallography, nital, Bechet‑Beaujard, McQuaid‑Ehn, intercept method, planimetric method.
