Step 4 of 5
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Reading a microsection — white layer, diffusion zone, and what the photo isn't telling you
How to look at a vendor's etched cross-section and microhardness traverse, what a porous compound layer means, and which case-depth definition is on the certificate.
Step 4 of 5Three case-depth definitions, three different numbers
The number on the certificate depends on which standard the lab applied.
DIN 50190-3 / ISO 18203 NHD (nitriding hardness depth) is the depth at which the traverse hardness equals core plus 50 HV. It is the European default and the most commonly cited definition on nitriding certificates worldwide. On the H13 curve above (core HV 500), NHD sits where the traverse reads HV 550.
SAE / AMS effective case depth (ECD) is the depth to a specified absolute hardness, often HV 400 or a value called out in the relevant AMS spec (AMS 2759/10 for Kn-controlled gas nitriding, AMS 2759/12 for gaseous FNC). It does not reference core hardness. On a part with a steep gradient and a high-hardness core, NHD and ECD can fall in very different places.
Vendor eyeball is the depth that "looks dark" on the etched section. It is an estimate, not a measurement. On Nital-etched H13 the visible boundary can sit 20-40% off the NHD value depending on etch time and operator. Low-end shops report this number when no traverse was run.
On the same part, NHD and ECD can differ by a factor of two in either direction. A spec that says "case depth 0.30 mm minimum" without naming a standard is incomplete. A spec that says "NHD ≥ 0.30 mm per DIN 50190-3 with the full HV 0.5 traverse on file" is a measurement contract.
Quick check
A part has core hardness HV 600. One certificate reports NHD = 0.55 mm per DIN 50190-3. A second certificate on the same part reports ECD = 0.25 mm to HV 400 per AMS 2759/10. Are these consistent, and which one should the buyer accept against a print spec of "0.30 mm minimum case depth, no standard cited"?