Overview
ISO 23825:2020 defines a standardized micrographic method to evaluate the nodularity (degree of spheroidisation) of spheroidal carbides in steels intended for cold heading and cold extruding. The scope covers non‑alloy and low‑alloy steels with carbon content up to 1.20 % C, and it provides a reference image series and classification scheme for CHQ (Cold Heading Quality) wire, wire rod and bars after annealing. Annex A describes an automated machine‑vision / image‑processing approach for routine measurements.
Key topics and requirements
- Method basis: Visual comparison of etched micrographs against a series of standard reference images (500× and 1000× magnifications shown in the standard).
- Steel groups by carbon content:
- Group 1: ≤ 0.10 % C
- Group 2: > 0.10 ≤ 0.25 % C
- Group 3: > 0.25 ≤ 0.40 % C
- Group 4: > 0.40 ≤ 0.80 % C
- Group 5: > 0.80 ≤ 1.20 % C
- Spheroidisation levels (nodularity percentages): examples include S ≈ 0 % (lamellar), S ≥ 70 % (partial), S ≥ 80 % (satisfactory), and S ≥ 90 % (optimal). Results are reported as a code combining steel group and level (e.g., C45EC - 4.90).
- Sampling & preparation: Sampling follows ISO 4954; metallographic preparation (grinding, polishing, etching - commonly 2 % picric acid) must avoid altering the microstructure. The standard specifies observation field procedures and magnifications.
- Evaluation protocol:
- Pre-scan at ~100× to locate least‑spheroidised areas.
- Detailed analysis preferably at 500× (1000× images are provided as well).
- At least five homogeneously distributed fields per specimen; report mean and minimal levels.
- Where automated image analysis is used, report software name and parameters (Annex A provides guidelines).
- Test report content: material designation, heat‑treatment condition, sampling orientation, steel group/level/minimum spheroidisation, number of fields, magnification, analyst and laboratory details.
Applications and users
ISO 23825:2020 is practical for:
- Steel manufacturers and heat‑treatment shops producing CHQ wire for fasteners and cold‑formed parts.
- Quality control and inspection labs verifying annealing and spheroidising effectiveness.
- Metallurgists and R&D teams assessing process windows for cold heading / cold extrusion steels.
- OEMs (automotive, aerospace, industrial) specifying nodularity requirements for components produced by cold forming.
Keywords: ISO 23825:2020, nodularity, spheroidal carbides, spheroidisation, CHQ wire, cold heading, cold extruding, micrographic method, machine vision, image processing.
Related standards
- ISO 4954 - Steels for cold heading and cold extruding (referenced for sampling and material designations).
Open preview (PDF)Method for evaluating the nodularity of spheroidal carbides — Steels for cold heading and cold extruding
