ISO 643:2024

Overview - What ISO 643:2024 covers

ISO 643:2024, "Steels - Micrographic determination of the apparent grain size", is the fifth edition international standard that specifies micrographic methods for determining the apparent ferritic or austenitic grain size in steels. The standard covers how to reveal grain boundaries, select and prepare specimens, and estimate mean grain size for materials with a unimodal size distribution. It clarifies that two‑dimensional metallographic sections produce a range of apparent grain sizes because a plane can cut grains at different positions.

Key technical topics and requirements

• Scope and definitions: Terms for grains, austenitic and ferritic grains, and the grain size index (G) are defined.
• Principle and methods: Three principal methods are detailed and stated to give comparable results:
  • Comparison method using standard grain size charts (see Clause 7.2 and Annex B).
  • Planimetric (point‑counting) method to determine mean number of grains per unit area (Clause 7.3).
  • Intercept method (linear and circular) counting grains or grain boundaries along known line lengths (Clause 7.4).
• Specimen selection and preparation: Guidance on representative test locations, polishing, and orientation of the polished surface relative to deformation (Clause 6).
• Revealing grain boundaries: Etching and other methods for ferritic, austenitic and prior‑austenitic boundaries are provided (Clauses 6.2–6.3 and informative Annex A).
• Symbols, formulae and accuracy: Symbols and conversion formulae are given (Clause 4, 7.1.2); the standard addresses accuracy, result assessment and reporting (Clauses 7.1.3, 7.4.4, 8).
• Supporting annexes: Annex B (normative) supplies standard comparison charts; Annex C covers evaluation methods; Annex D gives calculation of grain size and confidence intervals; Annex E illustrates grains of different size indices.
• Editorial changes: This edition updates methods (e.g., McQuaid‑Ehn test temperature for case‑hardening steels) and replaces earlier charts with new ISO grain size charts.

Practical applications and users

ISO 643:2024 is used for:

• Quality control and acceptance testing in steel production and heat treatment operations.
• Materials testing laboratories performing metallographic grain size determination.
• Failure analysis, process validation and R&D where grain size affects mechanical properties (strength, toughness, fatigue).
• Manufacturers and specifiers requiring certified grain size for product standards and contracts.

Typical users: metallurgists, materials engineers, QC/QA engineers, laboratory technicians, product standards writers, and researchers.

Related keywords (for search)

ISO 643:2024, grain size, micrographic determination, steels, ferritic grain, austenitic grain, metallography, intercept method, planimetric method, grain size charts, specimen preparation, grain size index.