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

Under Nital the compound layer is a featureless white band, which is why "white layer" became the shop-floor name. Under Murakami's it tells you what it is. ε etches darker, often brown-gray, because the higher nitrogen content opens it to the reagent; γ' etches lighter. A pure-γ' compound layer photographs as a uniform light band; a pure-ε layer as a darker, sometimes mottled band; a mixed layer shows a darker outer ε over a lighter γ' sublayer, with a visible boundary between them. A sharp dark/light interface inside the compound layer is the failure-prone case from Lesson 2: the two phases have different crystal structures and thermal expansion coefficients, and cracks run along the boundary under load.

Porous compound layers are the most-misread feature. The outer 20-50% of an ε-rich layer routinely forms a network of fine pores, the porous zone or Porensaum. It shows up in a photo as the outer edge of the white band full of small dark dots or short channels running roughly perpendicular to the surface. At 200× they look like fuzz; at 1000× they are unambiguous. Porosity is not automatically a defect: in a salt-bath FNC or QPQ part for sliding wear, the porous zone holds lubricant and is part of the design. In an impact-loaded part or a corrosion-critical part without QPQ, it is a crack-initiation site and a corrosion path. Specify the maximum acceptable porous-zone depth on the print if it matters; the common ask is "porous zone ≤ 30% of compound-layer thickness."

A compact γ'-dominant compound layer photographs as a thin uniform band, 4-8 µm, with no visible porosity. A porous ε-rich layer runs 10-20 µm with the outer third or half visibly channeled. The difference is what the Kn setpoint did. If the photo shows a thick porous layer and the print called for γ'-dominant, the cycle was not run to the spec.