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S-Phase Explained: Plasma Nitriding of Stainless Steels and the Functional Double Layer

  • Writer: redesiht
    redesiht
  • Nov 17
  • 2 min read

Updated: Dec 1

What is a burger doing in the blog of a plasma nitriding furnace manufacturer? More than it seems. The stainless stack is a visual metaphor to explain what happens inside stainless steels when they go through plasma nitriding: layers working together in balance to bring out the best of the material.


After being ‘slow-cooked’ in a plasma nitrider, stainless steel is never the same again. Like in a master grill session, the process delivers a perfect seal: the surface is hardened while the valuable properties inside are locked in.


The core of the steel remains ‘juicy’ and ductile. The expanded austenite or S-phase, formed by nitrogen diffusion, acts like the seared crust of the meat: it provides firmness and wear resistance without drying out or turning brittle. On top, the chromium oxide passive film (Cr₂O₃) regenerates, acting like the bread that wraps and protects against corrosion.


The result: plasma nitriding turns stainless steels into a functional sandwich — ‘juicy’ at the core, sealed at the surface, and shielded against aggressive environments.


Infographic of a hamburger used as a metaphor for plasma nitriding of stainless steels, showing the intact core as the meat, the hardened S-phase as the seared layer, and the regenerated chromium oxide passive film as the protective bread.
The “stainless stack” infographic: a visual guide to plasma nitriding of stainless steels, showing the tough core, the hardened S-phase, and the regenerated chromium oxide passive layer.


When to Apply This HOT Seal?

  1. The right mechanical need: parts requiring surface hardness, reduced friction coefficient, or extended service life without dimensional distortion.

  2. Applications that benefit most: valves, seals, medical instruments, turbines, injection molds, precision components, and parts subject to wear in corrosive environments.

  3. The right stainless steel grade:

    • Martensitic and PH: plasma nitriding adds surface hardness while keeping corrosion resistance intact.

    • Austenitic: the real stars, where plasma nitriding achieves hardness once thought impossible.

    • Ferritic and duplex: gain improved resistance to localized wear and pitting corrosion.



Plasma nitriding of stainless steels: key advantages

Compared to other methods, plasma nitriding of stainless steels is unmatched because it combines two unsurpassable virtues in one process:

  • Low-temperature operation (350–450 °C / 662–842 °F), preventing unwanted phase transformations and keeping the core intact.

  • Preservation of corrosion resistance, since no chromium nitrides precipitate and the passive film remains untouched.


That is why plasma nitriding deserves to be seen as a true grill sensei: a master that knows when to sear, when to let rest, and how to bring stainless steels to perfection without ruining their essence.



This article is just a starter. To move from apprentice at the grill to a true sensei of surface hardening, download our eBook ABC of Stainless Steels – And why plasma nitriding is the right way to treat them without damaging their anti-corrosion shield.


📲 And if you want to know how plasma nitriding could be implemented in your production line, our engineers can walk you through the process in detail and recommend the furnace that best fits your needs: Contact us.


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