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Lesson 07·QC and inspection

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.

9 min readLesson 7 of 12

Tying it together

What this means on the shop floor

The receiving inspection package on a production nitrided part should include, at minimum: a cross-section photograph at 200× and 500× with the etchant named; a microhardness traverse plot with depth, load, and indent spacing stated; the case-depth definition and standard cited on the reported number; the compound-layer thickness measured optically on the etched section; and a witness coupon of the same substrate run in the same cycle so the inspection can be repeated independently. A forge die certificate without a traverse plot has not measured the result it is reporting. A plastic mold certificate with a thick visible white layer and no porous-zone callout is buying a corrosion problem in storage.

Pushback questions for the metallurgy lab

What etchant was used on this section, at what concentration and exposure?

Can you send the full microhardness traverse data (depth, HV, load, spacing), not just the case-depth number?

What case-depth definition is on the certificate: DIN 50190-3 NHD, AMS effective case depth to a stated hardness, or something else?

Was the section cold-mounted, and was a sacrificial electroplated layer applied to preserve the compound-layer edge before sectioning?

Common confusions

"Case depth 0.020 inch" with no definition can correspond to an NHD of 0.51 mm and an ECD to HV 400 of anywhere from 0.25 to 0.55 mm on the same part. The number is meaningless without the standard.

HRC measurements on a 10 µm compound layer are the most common bad number on a nitriding certificate. The Rockwell C penetrator pushes 150-200 µm into the surface, deeper than the compound layer and most of the diffusion zone. The reading is a weighted average that has nothing to do with the case hardness. Insist on HV 0.5 or HV 0.05 with depth specified.

A photo with no scale bar is not a measurement. A 14 µm compound layer at 500× looks identical to a 28 µm compound layer at 1000×. Every metallographic photo on a certificate should carry a calibrated scale bar.

Surface hardness alone does not constrain a nitrided case. A part can read HV 1100 at the surface, hit core at 0.05 mm, and fail in two weeks. The traverse curve is what the spec stands on.

Up next: pre-nitride heat treatment.

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