Overview
EN ISO 643:2024 - Steels: Micrographic determination of the apparent grain size (ISO 643:2024) specifies standardized micrographic methods to determine the apparent ferritic or austenitic grain size in steels. The standard describes how to reveal grain boundaries, how sectioning produces a range of apparent two‑dimensional grain sizes, and how to estimate the mean grain size for specimens with a unimodal size distribution. EN ISO 643:2024 supersedes the 2020 edition and includes updated annexes and methods (e.g., McQuaid‑Ehn test temperature for case‑hardening steels modified to 950 °C).
Key Topics
- Scope and definitions: Terms for grains (ferritic, austenitic), index G, intercepts and intersections as used for grain-size metrics.
- Specimen selection & preparation: Guidance on test location and metallographic preparation to reveal grain boundaries.
- Methods to reveal grain boundaries: Procedures for ferritic, austenitic and prior‑austenitic grain boundary etching (informative Annex A).
- Grain‑size characterization methods:
- Comparison method (using standard comparison charts; normative Annex B reinstated with new ISO grain size charts).
- Planimetric (counting) method (direct counting in a defined area).
- Intercept methods - linear and circular intercept procedures and assessment of results.
- Other specialized methods and evaluation procedures (Annexes C–E cover evaluation, calculations and grain size examples).
- Accuracy and reporting: Formulae for indices, guidance on accuracy, confidence intervals and information required in the test report.
Applications
EN ISO 643:2024 is used where reproducible, comparable measurement of steel grain size is needed, including:
- Quality control and production acceptance in steelmaking and heat‑treatment facilities.
- Metallurgical characterization in materials laboratories and certification testing.
- Failure analysis and forensic metallurgy to correlate grain size with mechanical properties (toughness, yield, fatigue).
- R&D and process optimization (grain refinement studies, annealing/normalizing effect).
Typical users: metallurgists, materials engineers, metallography labs, inspection bodies, and standards/quality managers.
Related standards
- ISO 4885 (grain definition and related terminology) is referenced for grain definitions.
- EN ISO 643:2024 was prepared by ISO/TC 17 (Steel) in collaboration with CEN/TC 459 and replaces EN ISO 643:2020 / ISO 643:2019.
Keywords: EN ISO 643:2024, ISO 643:2024, grain size, micrographic determination, steels, metallography, planimetric method, intercept method, comparison charts, ferritic grain, austenitic grain.
