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

Salt-bath nitriding is a molten cyanate-carbonate salt bath at 550-590°C with the parts immersed in it. The active species is cyanate (CNO⁻), which decomposes at the steel surface to deliver both nitrogen and carbon to the ferrite. Carbon enters the lattice alongside nitrogen, which makes essentially every modern salt-bath process a ferritic nitrocarburizing (FNC) process rather than pure nitriding. Straight nitriding in a salt bath is rare in commercial practice. Lesson 6 covers FNC chemistry in depth; for this lesson the operating fact is that the salt bath delivers nitrogen and carbon together in roughly 1-3 hours of immersion, against 30-60 hours for a gas retort to grow a comparable compound layer.

Three commercial process names dominate. Tufftride is the original Durferrit process, now run under HEF (Houghton/Quaker chemistry). Melonite is the Aalberts Surface Technologies / Advanced Heat Treat designation for the same family with proprietary salt chemistry. Nu-Tride / Meli-1 is the Kolene line. The trade names refer to specific salt formulations and operating windows, not to fundamentally different processes. All run molten cyanate around 570°C and produce an ε-dominant compound layer over a shallow diffusion zone.