The metallurgy you actually need — Fe-N phase diagram in plain English

Kn is the modern control parameter for gas nitriding:

Kn = p(NH₃) / p(H₂)^(3/2)

Units are atm^(-1/2). It is the thermodynamic driving force for nitrogen transfer from the gas atmosphere into the steel surface, derived from NH₃ ⇌ ½N₂ + 3/2 H₂. A vendor running closed-loop Kn control measures hydrogen partial pressure in the retort with an H₂ sensor, calculates Kn in real time, and adjusts ammonia flow to hold a setpoint.

The Lehrer diagram plots Kn versus temperature with three regions: α (diffusion zone only), γ' (γ'-only compound layer), and ε (ε-dominant compound layer). For pure iron at 520°C, the α/γ' boundary sits near Kn ≈ 0.3 and the γ'/ε boundary near Kn ≈ 5. Alloying shifts the boundaries, but the structure of the diagram holds.

In practice: Kn of 0.2 grows essentially no compound layer, sometimes specified for impact tooling where the brittle layer is a liability. Kn of 1-3 grows γ'. Kn above 5 grows ε. AMS 2759/10 codifies Kn ranges by class. A vendor who says "Kn-controlled" but cannot tell you their setpoint and their H₂ sensor calibration is running open-loop nitriding under a Kn-controlled label.