Nitriding Basics: Gas and Plasma Nitriding Without a Quench

A different hardening principle

Unlike carburizing, nitriding does not rely on a phase transformation. Parts are heated to roughly 500–580 °C — below the steel's tempering and transformation temperatures — in a nitrogen-supplying environment (ammonia gas, or a nitrogen plasma). Nitrogen diffuses in and forms fine, hard nitrides with the alloying elements in the steel. Because nothing is quenched afterwards, distortion is minimal and finished parts can be treated.

The two zones

A nitrided surface has two distinct regions:

• Compound layer ("white layer") — a thin surface film of iron nitrides, typically a few micrometres to a few tens of micrometres thick. It is very hard, resists scuffing and corrosion, but can be brittle; on precision or fatigue-loaded parts it is often limited or removed by lapping.
• Diffusion zone — beneath it, nitrogen in solution and alloy-nitride precipitates raise hardness gradually back to core level. Total case depths are typically about 0.2–0.7 mm, much shallower than carburized cases, because the lower temperature slows diffusion and cycles already run tens of hours.

Surface hardness depends strongly on the steel: aluminium- and chromium-bearing grades can exceed roughly 1000 HV, while plain carbon steels gain little more than surface protection — they lack the nitride-forming elements.

Steels that nitride well

Nitriding steels are alloyed deliberately with aluminium, chromium, molybdenum and vanadium:

• 1.8550 (31CrMoV9) — the classic dedicated nitriding steel, supplied pre-hardened.
• 4140 / 42CrMo4 — widely nitrided for shafts and gears where moderate case hardness suffices.
• H13 (1.2344) — hot-work dies are routinely nitrided (or nitrocarburized) to resist soldering and heat checking in die casting service.
• Stainless and maraging grades have their own low-temperature variants to avoid harming corrosion resistance or the aged structure.

Process rules that matter

1. Pre-harden first. The core is quenched and tempered before nitriding, and the final tempering temperature must sit safely above the nitriding temperature — otherwise the cycle itself over-tempers the core.
2. Stress-relieve after rough machining, so the nitriding cycle is not asked to do two jobs.
3. Gas vs plasma. Gas nitriding is the economical batch standard. Plasma (ion) nitriding offers better control of the white layer, works on stainless grades more readily, and can mask areas mechanically, at higher equipment cost.
4. No quench means no rescue. Surface defects, decarburization or a soft core going in will still be there coming out — preparation quality decides the result.

Nitriding versus carburizing

The two case processes are complements, not competitors. Carburizing gives deeper cases (commonly 0.5–2 mm class), higher load capacity and lower unit cost on high-volume gears, but it needs a quench, with the distortion and finish-grinding that follow. Nitriding gives shallower cases and lower contact-load limits, but the part can be finish-machined before treatment and emerges dimensionally stable, with better corrosion resistance and scuffing behaviour. The deciding factors are usually case depth required, distortion tolerance and whether the steel in question carries the nitride-forming elements nitriding depends on.