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

The case-depth number on the certificate comes from a microhardness traverse on the same polished cross-section. Vickers or Knoop indents are made in a line from just below the compound layer toward the core, with spacing set by ASTM E384: typically 50-100 µm, never closer than 2.5× the indent diagonal. Loads run 50-500 gf. HV 0.5 is the common diffusion-zone load; HV 0.05 is used inside the compound layer when it is thick enough to fit the indent. Too high a load on a thin layer runs the indent into the substrate underneath and reads soft.

The output is a curve: HV on the vertical axis, depth on the horizontal. A healthy nitrided H13 traverse starts near HV 1000-1200 at or just below the compound layer, drops moderately through the diffusion zone, and asymptotes to core hardness (HV 450-500 on properly tempered H13) somewhere between 0.3 and 0.6 mm. A short cycle hits core within 0.10-0.15 mm. A wrong substrate shows core around HV 350-400 and a shallow case no matter the cycle time. Three data points is not a traverse; ten to fifteen across the case is the working minimum.