Nitriding

The basics

Nitriding diffuses nitrogen into the surface of steel at relatively low temperatures (350-590°C / 660-1100°F). No quench afterward — the hardness comes from the nitrogen, not from a quench. Per ScienceInsights and PatSnap's process surveys, the result is two distinct zones in the treated surface:

• Compound layer (white layer): Outermost, typically 4-8 µm thick. Composed of iron nitrides (ε-Fe₂₋₃N and γ'-Fe₄N). Looks white under a microscope, hence the name. Very hard, very corrosion-resistant, but it can be brittle.
• Diffusion zone: Sits underneath the compound layer. Typically 50-200 µm deep. Nitrogen content drops from ~20% at the surface to ~4-7% deeper in. This is where most of the load-carrying happens.

Combined surface hardness after nitriding is typically 60-70 HRC. The substrate underneath stays close to its pre-nitride hardness — nitriding doesn't soften the core because the process runs below tool steel tempering temperatures.

Two main flavors: gas vs. plasma

• Gas nitriding uses decomposing ammonia (NH₃) in a sealed furnace. Runs at 450-590°C with cycles of 40-60 hours. Equipment is relatively simple. Compound layer tends to be thicker and less controlled — you get what you get.
• Plasma nitriding (also called ion nitriding) uses a low-pressure glow-discharge plasma to deliver nitrogen at 350-560°C. Cycles are shorter than gas. The big advantage: dramatically more control over the compound layer. You can grow a thinner one, or run a recipe that produces essentially no compound layer at all — just the diffusion zone. Lower temperatures also mean less distortion of the part.

If you don't care about precision and you just want hard surfaces, gas is fine. If you care about compound-layer control, distortion, or running on grades that can't tolerate higher temperatures, plasma earns its higher cost.

What this means on the shop floor

• Nitriding doesn't require post-process heat treatment. You don't have to re-harden anything afterward. That's a big practical advantage for complex tools where you can't afford the distortion of a hardening quench.
• The compound layer is both the strength and the weakness. Hard and corrosion-resistant, but brittle under impact. If your application has shock loading (like cold heading), a thick compound layer cracks and spalls. For impact-prone work, specify plasma nitriding with a compound-layer-free or compound-layer-thin recipe.
• Don't try to PVD-coat a nitrided part with the compound layer intact. PVD adhesion on a compound layer is unreliable. If you want both treatments, sequence matters: nitride first, polish off the compound layer, then coat. The diffusion zone underneath is excellent substrate for coatings because it's already hardened.
• Lead times are long. Gas nitride cycles are 1.5-2.5 days plus furnace queue. Plasma is faster but still measured in days, not hours.

The next lesson covers carburizing, which is often confused with nitriding because both add elements to the surface. They're different processes for different jobs.