Step 6 of 6
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FNC vs nitriding — stop confusing them
How carbon in the atmosphere changes the compound layer, why FNC owns stamping and powder-metal tooling, and how to tell the two processes apart on a quote and a microsection.
Step 6 of 6Reading a microsection
On an etched cross-section (Nital 2-3% or Marble's reagent) under 500-1000× optical magnification, the compound layer is the bright white band at the surface. The phases are not always distinguishable optically, but the structure is.
An ε-only compound layer with a clearly porous outer zone (visible as a darker, channeled band in the outer 30-50% of the white layer) is the FNC signature. A vendor running QPQ will show the same structure with a thin dark oxide skin on top.
A compound layer with two distinct sublayers, an outer ε on an inner γ' separated by a visible boundary, is straight gas nitriding at moderate Kn. The boundary is the spalling plane discussed in Lesson 2.
A thin, uniform, dense compound layer with no obvious porous zone and no internal boundary is either a controlled γ'-only straight nitride at low Kn or a plasma nitride with low compound-layer growth.
When the optical reading is ambiguous, EDX on the compound layer settles it. Carbon above ~0.5 wt% in the compound layer means FNC. Carbon at the matrix background level means straight nitriding. XRD of the compound layer gives the phase directly, but EDX is the faster confirmation in a job-shop lab.
Quick check
A microsection shows a uniform 14 µm white band with no internal boundary and a visible channeled darker zone in the outer 5 µm. EDX on the compound layer reads 0.8 wt% carbon. Which process produced it?