Overview
ISO 14250:2000 - Steel - Metallographic characterization of duplex grain size and distributions - defines a micrographic method for identifying and quantifying duplex grain size in rolled or forged steel products. The standard describes how to recognize mixed (duplex) grain-size populations, classify their topology, and estimate area fractions of distinct grain-size constituents using visual comparison charts or statistical point-counting. ISO 14250 is intended for metallography and quality control of steel where grain-size heterogeneity affects performance.
Key topics and requirements
- Scope: Applies to rolled or forged steels containing two or more distinct grain sizes distributed randomly or topologically across the section.
- Classification of duplex types:
- Random types: As Large As (ALA / occasional coarse grains ≤5% area), Wide Range (WR), Bimodal (BM - two sizes differing by >4 ISO grain-size numbers and comprising ≥75% area).
- Topological types: Cross-section variations (CS), Necklace (NL), Banding (BD) - differences defined as ≥3 ISO grain-size numbers between areas.
- Measurement methods:
- Comparison procedure using the chart in Annex A to visually estimate area fractions (minimum five fields for random duplex; whole specimen for topological duplex).
- Point count procedure using a grid/reticle per ISO 9042 for statistical area-fraction estimation.
- Direct measurement for layered topological specimens (depth measurements to compute area fraction).
- Grain-size determination: Use the devices and methods specified in ISO 643 to determine individual grain sizes (ISO grain-size numbers).
- Sampling & preparation:
- Evaluate the entire cross section; examine a longitudinal surface parallel to maximum deformation (except bars/tubes: transverse surface).
- Polish and etch to reveal grain boundaries clearly.
- Reporting: Required content includes steel grade, sample orientation, duplex type (symbol), method used, percentage area, grain-size numbers, and standardized report formats (examples provided in clause 9).
Practical applications and users
- Who uses it: Metallurgists, quality-control laboratories, steel producers, inspection agencies, and failure-analysis teams.
- Uses:
- Characterizing microstructural heterogeneity after rolling, forging, heat treatment or critical strain events.
- Quality acceptance and product certification where duplex grain distributions affect mechanical behavior.
- Supporting process control, microstructural documentation for product standards, and investigation of non-uniform grain growth or banding.
Related standards
- ISO 643 - Steels: micrographic determination of ferritic/austenitic grain size (used for grain-size measurement).
- ISO 9042 - Manual point counting method (used for area-fraction estimation).
- ASTM E1181 - referenced illustrative micrographs (informative source cited in ISO 14250).
Keywords: ISO 14250, duplex grain size, steel metallography, grain size distributions, point count, comparison chart, ISO 643, ISO 9042.