Overview
ISO 18203:2016 - Steel: Determination of the thickness of surface-hardened layers specifies standardized methods to measure the depth and thickness of hardened surface layers in steel produced by thermal (flame, induction, electron/laser beam) or thermochemical (carburizing, carbonitriding, nitriding) treatments. The standard defines how to determine case hardening depth (CHD), surface hardening depth (SHD), nitriding hardness depth (NHD), total thickness of surface hardening depth (THD) and compound layer thickness (CLT) using hardness profiling and micrographic examination.
Key topics and technical requirements
- Definitions and limits: Clear definitions for CHD, SHD, NHD, THD and CLT, including recommended hardness-limit tables and rules for selecting limiting hardness (e.g., CHD based on 550 HV or agreed alternative).
- Principle: Depths derived from the hardness gradient on a cross-section (Vickers or optionally Knoop), and THD/CLT from microstructural observation.
- Apparatus and calibration: Use Vickers or Knoop hardness testers verified per ISO 6507‑2 and ISO 4545‑2. Microscopes suitable for THD display are recommended (see ISO 1463).
- Specimen preparation: Cross-sections perpendicular to the surface/axis; careful cutting, mounting, grinding and polishing to preserve edge retention; etching for microstructure when required.
- Hardness measurement geometry: Hardness impressions in one or more parallel lines normal to the surface within a 1.5 mm band. Minimum spacing rules (Δd ≥ 3× indent diagonal), center distance of first impression ≥ 2.5× diagonal, cumulative distance accuracy ±25 µm. Test forces typically in the range 0.9807 N to 9.807 N (commonly 0.9807 N to 2.942 N for fine spacing).
- Evaluation & reporting: Graphical derivation of depths from hardness vs. distance curves, micrographic assessment for THD/CLT, and requirements for test reports. Annex A gives an interpolation method for CHD; Annex B shows CLT examples.
Applications and users
- Practical for heat-treatment process control, acceptance testing and quality assurance of hardened components such as gears, shafts, bearings and tooling.
- Used in process development, contract specifications, inspection and failure analysis where accurate measurement of hardened-case depth and compound layers is required.
- Primary users: metallurgists, heat‑treatment laboratories, materials testing labs, OEM quality engineers, inspection bodies and standards specialists.
Related standards
Keywords: ISO 18203:2016, case hardening depth, surface hardening depth, nitriding hardness depth, Vickers hardness, Knoop hardness, microhardness profile, THD, CLT, steel surface hardening measurement.
