Gas nitriding — the workhorse and its limits

At nitriding temperature, NH₃ dissociates to N₂ and H₂. The fraction that has dissociated at the furnace outlet is the dissociation percentage. It is measured two ways in commercial practice. The dissociation pipette is a glass burette over water that absorbs un-dissociated NH₃; the operator reads the volume change. It is slow, manual, and accurate to about ±2-3%. A hydrogen sensor (thermal conductivity or mass spec) reads the H₂ fraction in real time and converts to dissociation continuously; this is what closed-loop shops run.

Dissociation percentage by itself does not specify the atmosphere. Flow rate matters, retort loading matters, the steel surface area exposed to the gas matters. What dissociation gives you, combined with flow rate, is a usable estimate of the nitriding potential:

Kn = p(NH₃) / p(H₂)^1.5

Kn is the modern control parameter, set by AMS 2759/10 for aerospace work. Kn in the 3-5 range with low dissociation drives ε-phase compound layer growth. Kn in the 0.5-1.5 range with higher dissociation drives γ' formation or, at the low end, no compound layer at all. The Lehrer diagram maps Kn vs temperature to predict which phase forms. A shop running closed-loop Kn control sets a target, the hydrogen sensor reads actual, and a controller meters NH₃ flow to hold the setpoint. A shop running open-loop bleeds NH₃ at a fixed flow and lets the retort dissociate however it dissociates that day, which depends on furnace cleanliness, retort age, and load size.