Overview
ISO 16574:2015 specifies how to determine the percentage of resolvable pearlite in high carbon steel wire rod (control cooling steels with > 0.65 % C). The standard defines resolvable pearlite (ferrite and carbide lamellae visible at 500x magnification, numerical aperture ≥ 0.8) and prescribes two measurement methods - a manual point count and an automatic image analysis - to estimate the fraction of pearlitic microstructure that is resolvable by optical metallography.
Key topics and requirements
- Scope: Applies to wire rod produced from control cooling steel with carbon content > 0.65 % C.
- Definition: Resolvable pearlite = pearlite whose ferrite and carbide lamellae are observable at 500x with NA ≥ 0.8.
- Sample preparation:
- Transverse polished sections recommended.
- Careful sectioning, mounting, grinding and final polish to at least 1 µm finish.
- Clean, dry specimens after preparation.
- Etching:
- Recommended etchants: picral (4 g picric acid / 100 ml ethanol) or nital (2 ml HNO3 / 100 ml ethanol).
- Picral often gives more uniform pearlite contrast for high carbon rod; nital is orientation sensitive.
- Microscopy parameters:
- Magnification: 500x
- Numerical aperture: ≥ 0.8
- Light source: white or equivalent
- Measurement locations and sampling:
- Four measuring locations at mid-radius (D/4 from center) designated O1–O4.
- At each Oi, acquire three continuous fields of view (Si1, Si2, Si3) → total 12 measurements; report the average and standard deviation.
- Measurement methods:
- Manual point count (ISO 9042 referenced) - grid/reticle point counting; used as referee method in disputes.
- Automatic image analysis - grey-level discrimination to separate constituents; brightness must cover all phases.
- Reporting:
- Include steel grade, heat number, rod diameter, test locations/number, magnification/NA/light/etchant, measuring method, average percentage and standard deviation.
Applications and users
- Ensures quality control for high-carbon wire rod destined for drawing applications (e.g., wire drawing, spring wire).
- Used to assess drawability: higher percentage of resolvable pearlite generally correlates with reduced drawability.
- Typical users:
- Steel producers and rolling mills
- Metallography and QA laboratories
- Wire manufacturers and drawing shops
- Materials engineers and metallurgists validating microstructure against product specs
Related standards
- ISO 9042 - manual point counting for volume fraction estimation (normative reference)
- ISO 16120-1 - general requirements for non-alloy steel wire rod for conversion to wire (normative reference)
Keywords: ISO 16574:2015, resolvable pearlite, high carbon steel wire rod, manual point count, automatic image analysis, picral, nital, 500x magnification, metallography, drawability.
