ISO 2639
Steels — Determination and verification of the depth of carburized and hardened cases
Method for determining the case hardening depth (CHD) of carburized or carbonitrided and hardened steel parts by a Vickers hardness traverse, and a simplified method for verifying a specified CHD in production.
What ISO 2639 specifies
ISO 2639 defines the case hardening depth CHD (formerly Eht) as the perpendicular distance from the surface to the point where the hardness falls to a limit value — 550 HV by default, measured with a test force of 9.807 N (HV 1), with other limits permitted in steps of 25 HV when agreed. The determination method is a hardness traverse on a polished cross-section perpendicular to the surface, with indentations placed within a 1.5 mm wide band at least 2.5 diagonals apart, the depth being read from the interpolated hardness–distance curve. A verification method for series production places two indentations at depths bracketing the specified CHD to confirm that hardness is above the limit at the shallower point and below it at the deeper one. The content was consolidated into ISO 18203, but "CHD 550 HV1" remains the standard drawing notation in ISO-based supply chains.
Tests and the drawing call-out
Tests the document imposes
- Vickers hardness traverse per ISO 6507-1 at HV 1 (9.807 N) on a perpendicular, polished cross-section
- Interpolation of the hardness–distance curve to the limit hardness (550 HV default)
- Two-point production verification at depths bracketing the specified CHD
- Specimen preparation with edge retention (mounting, grinding, polishing)
How to cite it
Drawing call-out
CASE HARDEN, CHD 0.8 +0.4 MM (550 HV 1) PER ISO 2639, SURFACE HARDNESS 60 +4 HRCAngle brackets mark the choices the drawing must make; every finish page shows a completed example.
Heat treatments specified to ISO 2639
| Treatment | Role | Cost |
|---|---|---|
Carbonitriding diffuses both carbon and nitrogen into low-carbon steel in the austenitic range, then quenches and tempers the surface to martensite. | Alternative | $$ |
Carburizing diffuses carbon into low-carbon steel at austenitizing temperature, then quenches and tempers the enriched surface into a hard martensitic case. | Alternative | $$$ |
ISO 2639 — frequently asked
How does CHD compare with SAE J423 effective case depth?
Both are depth-to-a-hardness definitions, but ISO uses 550 HV 1 (about 52.5 HRC) while J423 uses 50 HRC (513 HV), so an ISO CHD reads slightly shallower than the J423 effective depth on the same part — typically by 0.05–0.15 mm on a normal gradient. State which standard governs when a drawing crosses supply chains.
Can I specify a different limit hardness?
Yes. ISO 2639 allows the limit to be agreed in 25 HV steps (for example CHD 525 HV1 or CHD 600 HV1) and the notation must then carry it explicitly. Lower limits suit carbonitrided low-alloy parts that do not reach 550 HV in the transition zone.
Why HV 1 rather than HV 0.5 or HRC?
HV 1 (1 kgf) gives an indentation small enough — about 55 µm diagonal at 600 HV — to resolve the gradient at 0.1 mm steps yet large enough to average the microstructure; Rockwell C cannot be used on a traverse, and HV 0.5 gives slightly different numbers on the same section. Use the force the drawing states.
Is ISO 2639 still current?
Its content lives on in ISO 18203:2016, which adds surface hardening depth (SHD) for induction and flame hardening and nitriding hardness depth (NHD). Many drawings and supplier certificates still cite ISO 2639, and the CHD 550 HV1 definition is unchanged, so heat treaters understand either citation.
ISO 2639 is copyrighted by International Organization for Standardization. This page explains the document so a drawing can be read; it does not reproduce the standard. Purchase the current edition from the publisher. Official source ↗