Step 2 of 4
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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.
Step 2 of 4What you get out of the pot
A typical salt-bath FNC cycle on a low-alloy stamping die runs 90 minutes at 570°C and produces a 10-25 µm compound layer, ε-rich (Fe₂₋₃(N,C)), with a porous outer zone, over a 0.1-0.4 mm diffusion zone. Surface microhardness on a low-alloy substrate runs HV 0.05 around 600-800 in the compound layer. On nitriding-grade alloy steel (H13, Nitralloy) the diffusion-zone microhardness climbs to HV 0.5 around 900-1100 from the alloy-nitride precipitates discussed in Lesson 3, but the diffusion-zone depth stays shallow compared to a long gas cycle. Salt-bath is built for compound-layer-dominant performance on shorter case depths, not for deep cases on heavy hot-work tooling.
Quick check
A part comes back from salt-bath FNC with a 20 µm ε compound layer and a 0.15 mm diffusion zone. The application is a forge die intended to run at 1100°C billet contact with deep mechanical loading. Will this case carry the service?