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Lesson 05·Process variants

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

Why a 420 stainless mold cavity that gas nitride ruins comes out of a plasma cycle intact, what the four plasma levers actually do, and which parts the process is the correct call for.

10 min readLesson 5 of 12

Tying it together

What this means on the shop floor

Nitralloy 135M precision gears. Plasma at 480-500°C, low N₂ ratio, 18-25 hours, compound layer ≤3 µm or none. Distortion budget is the deciding factor.

420 stainless plastic mold cavities. Plasma at 480°C, sputter-clean cycle first, target 0.10-0.20 mm case, compound layer ≤2 µm to preserve polish. Gas and salt-bath cannot break the Cr₂O₃ passivation reliably and should not be quoted on this substrate.

Thin-wall plastic mold cores (P20, H13). Plasma at 500-510°C, hydrogen-rich, 12-18 hours, compound layer minimized. The thin section will not survive a long high-temperature gas cycle without dimensional drift.

H13 forge dies, tight distortion spec. Plasma at 510-520°C, 20-25 hours, controlled compound layer 4-8 µm γ' dominant. A gas Floe cycle works on bulky dies; on long thin tooling, plasma holds dimension.

Pushback questions for a plasma vendor

  1. Is the bias DC or pulsed, and what duty cycle range can you run for parts with deep blind features or sharp internal corners?

  2. What is the N₂/H₂ ratio in your standard recipe for my substrate, and how do you adjust it if I want the compound layer suppressed?

  3. What pre-cycle sputter-clean step do you run for stainless grades, and what is the chamber pressure and bias during the sputter step versus the nitriding step?

  4. Can you provide a witness coupon from the same load showing compound layer thickness, phase, and the full diffusion-zone hardness traverse?

Common confusions

Plasma nitriding is not ion implantation. Ion implantation is a line-of-sight, high-vacuum, MeV-range process used in semiconductor doping and specialty surface modification, with case depths measured in nanometers. Plasma nitriding is a low-vacuum diffusion process with case depths measured in tenths of a millimeter. They share a name root but are not the same process.

Pulsed DC is not always required for simple geometries. Plain bushings and flat blocks can run on older steady-DC plasma without arcing. Pulsing matters when the geometry has deep blind holes, sharp internal corners, or thin web sections that concentrate the discharge. Match the equipment generation to the part geometry rather than to a default vendor preference.

Compound-layer-free requires the right recipe and the verification. A vendor who claims compound-layer-free but does not specify gas ratio, temperature, and the post-cycle witness-coupon cross-section is documenting a marketing line. The recipe is a high-H₂, low-temperature cycle, and the only way to confirm zero compound layer is a sectioned and etched coupon under the microscope.

Up next: FNC vs nitriding.

Sources