Reading a microsection — white layer, diffusion zone, and what the photo isn't telling you

A nitrided cross-section, properly prepared, shows three things from the surface inward. A bright white band at the top, 3-25 µm thick, is the compound layer, the iron-nitride zone of γ' and/or ε. Below it, a darker etched zone with no sharp boundary fades into the core; that is the diffusion zone. Below that, the base steel structure: tempered martensite for a quenched and tempered tool steel, banded or pearlitic for a low-alloy substrate.

The etchant decides what you can see. Nital 2-3% (nitric acid in ethanol) reveals the diffusion zone and base steel structure but flattens the contrast between ε and γ' inside the compound layer; both come out white. Alkaline sodium picrate (Murakami's reagent in the nitriding literature) darkens ε relative to γ' and is the standard reagent for compound-layer phase work. Klemm's reagent gives color contrast useful on stainless and high-alloy substrates. A certificate with a photo and no named etchant cannot be evaluated for phase, porosity, or layer thickness; the missing etchant name is itself the disqualifying gap.