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Lesson 04·Process variants

Salt-bath nitriding (Tufftride / Melonite / QPQ) — fast, dirty, effective

What a 90-minute cyanate-bath FNC cycle delivers compared to a 60-hour gas retort, what QPQ's oxide step actually adds, and which dies belong in a salt pot and which do not.

9 min readLesson 4 of 12

Tying it together

What this means on the shop floor

High-volume FNC on carbon and low-alloy stamping dies is the textbook fit. Cycle times in hours, not days, and an ε-rich compound layer that survives galling against galvanized and coated stock. This is where Tufftride and Melonite earn their volume.

Forge dies are not a salt-bath job. Deep case requirements and thin γ'-dominant compound layer targets put forge service outside the salt-bath envelope. Lesson 3's Floe gas cycle or Lesson 5's plasma recipe is the right call.

Precision plastic mold cavities are not a salt-bath job. Thermal shock and dimensional growth move the cavity outside tolerance, and there is no finish-grind option without removing the working layer.

Aluminum extrusion dies are sometimes a salt-bath QPQ job and sometimes a gas Floe job. Volume and geometry decide. Sectional dies with simple bearing surfaces work in the bath; complex hollow profiles with blind features go to gas.

Pushback questions for a salt-bath vendor

Which salt-bath process are you running by trade name (Tufftride, Melonite, Nu-Tride, or in-house), what is the operating temperature, and what is the cycle time for my part?

Is this a straight salt-bath FNC or QPQ, and if QPQ, what is the polish step and the oxidizing-bath time and temperature?

What compound layer thickness, phase, and porous-zone fraction do you target on my substrate, and what diffusion-zone depth do you expect?

How do you handle parts with blind holes or recessed features for rinse-out, and what is your cyanate disposal stream?

Common confusions

Salt-bath nitriding is not just a faster gas nitride. The chemistry is different (cyanate delivering nitrogen and carbon), the resulting compound layer is ε-rich with porous outer zone, and the diffusion zone is shallow. A salt cycle and a gas cycle producing "the same case depth" rarely produce the same microstructure or service life.

QPQ is not a corrosion coating sitting on top of an unrelated nitride. The polish and oxidation steps are integral to the corrosion claim. Strip the oxide or grind through the polished compound layer and the salt-spray numbers do not apply.

Salt bath is regulated, not banned. It is fully legal under permitted operations and Kolene, HEF, and Aalberts run audited cyanate waste streams. Buyers who reject salt-bath on environmental grounds without checking the actual vendor's permits are sometimes rejecting their best metallurgical option on a misread of the chemistry.

Up next: plasma and ion nitriding.

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