Wrap-up
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Gas nitriding — the workhorse and its limits
How dissociating ammonia builds a case in a sealed retort, what the Floe 2-stage cycle actually does, and the difference between a controlled and uncontrolled gas nitride process.
Tying it together
What this means on the shop floor
Forge die (H13, Nitralloy 135M): deep case wanted, 0.4-0.8 mm, with a compound layer thin enough to survive impact. 2-stage Floe cycle, target compound layer 6-10 µm γ' dominant, 60-80 hour total.
Stamping die in tool steel (D2, A2): case depth target 0.20-0.40 mm, compound layer can be thicker and ε-rich for adhesive wear resistance against galvanized stock. Single-stage cycle at moderate Kn, 40-50 hours.
Plastic mold (P20, 420 stainless): shallow case, 0.15-0.30 mm. Thin compound layer (3-6 µm) or compound-layer-free if the mold will be polished or PVD-coated downstream. Single-stage cycle, 30-40 hours, low Kn.
Gear (Nitralloy N, 31CrMoV9): 0.30-0.60 mm case, controlled compound layer or none, distortion budget tight. Floe cycle if depth is the priority. Plasma is often a better fit for precision gears, which is Lesson 5.
Aluminum extrusion die (H13): 0.20-0.50 mm case, compound layer kept thin or removed in finish to avoid spalling against the flowing billet. Floe cycle, then a light finish polish over the bearing surface.
Pushback questions for a gas nitride vendor
Single-stage or 2-stage cycle, and what is the temperature and time of each stage?
What Kn are you targeting in each stage, and how is the atmosphere controlled: open-loop flow, dissociation pipette readings, or a hydrogen sensor on a closed loop?
What compound layer thickness and phase (ε, γ', or mixed) do you expect to deliver on my substrate, and can you provide a witness coupon cross-section from the same load?
If the answers are "we run our standard cycle," "we set the flow and check it occasionally," and "the compound layer is whatever it is," the certificate will document furnace time, not a controlled case.
Common confusions
Gas nitriding versus gas nitrocarburizing (FNC). Both run in a sealed retort with NH₃ atmosphere. FNC adds a carbon source (typically CO₂, propane, or endogas) so the compound layer takes up carbon as well as nitrogen, shifting toward ε-Fe₂₋₃(N,C). Vendors sometimes quote "nitriding" and deliver FNC, or vice versa. Lesson 6 covers the distinction in depth. For this lesson: if the atmosphere has a carbon source, it is not pure nitriding.
Controlled versus uncontrolled gas nitriding. Both run at the same temperatures with the same NH₃ feedstock. The difference is whether Kn is measured and held to a setpoint. A vendor's website that mentions "Nitreg," "Nitralloy potential control," "ZeroFlow," or AMS 2759/10 compliance is signaling Kn control. A vendor's website that says "gas nitriding services" with no atmosphere terminology is signaling something else.
Up next: salt-bath nitriding.