Plasma / ion nitriding — control, distortion, and the parts that need it

There are three classes of parts where plasma is the correct technical answer, not just an alternative. Each maps to a substrate or geometry the operator can identify on the print.

Compound-layer-free is actually achievable. A high-H₂ plasma recipe (typically 80% H₂ or higher) at 480-510°C grows nitrogen diffusion zone with little or no detectable compound layer. The Aalberts and Nitrex process literature both describe this as a routine recipe class. Gas nitriding can approach it at very low Kn, but holding open-loop Kn below 0.2 across a 40-hour cycle is hard, and salt-bath cannot do it at all. Compound-layer-free is the right answer for parts that get a follow-on PVD coat or a finish polish where any compound layer would be removed and wasted anyway.

Distortion budget is lower. Plasma at 480-510°C runs 50-80°C below a typical gas cycle. Heat-up and cool-down are slower because the vessel is at vacuum and heat transfer is radiation-dominated. There is no quench. For a long thin punch or a precision gear, the dimensional swing is small enough that finish-grind stock can shrink from 75 µm per side to 25 µm per side. On parts where finish-grind through the case is a real risk, that margin is the difference between a usable part and scrap.

Stainless works. Austenitic and martensitic stainless steels (304, 316, 420, 440C) carry a passivating Cr₂O₃ film. Gas and salt-bath atmospheres cannot reliably penetrate it. Plasma sputtering physically removes the oxide during the cycle and keeps it removed, exposing fresh chromium-bearing metal to the active nitrogen. Below ~450°C on austenitic grades, plasma produces the expanded austenite (S-phase) layer at HV 1000-1400 with corrosion resistance intact. On 420 martensitic stainless mold cavities, the same sputter-clean is what makes nitriding work at all.