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The NEXT-Phase™ process, step by step
Low-temperature nitro-carburizing works because of one narrow thermodynamic window — and one hard problem at the surface of every stainless part. Here is how we handle both.
The window
Cold enough that chromium cannot react. Hot enough that carbon and nitrogen still move.
Chromium is what makes stainless steel stainless, and it is also what makes stainless steel hard to harden. Above roughly 500 °C, chromium becomes mobile enough to react with incoming nitrogen or carbon and precipitate as CrN or Cr23C6. Those precipitates are hard, but they consume the chromium that was maintaining the passive film. The part gets harder and starts to rust.
Below that threshold, chromium is effectively frozen in place while interstitial nitrogen and carbon still diffuse at useful rates. We run inside it, and we do not leave it. The cycle itself is low and slow — low temperature and long diffusion time are the whole point — but your parts are still back in 5–7 days.
Why nitro-carburizing rather than one or the other. Nitrogen gives the highest surface hardness — measurements up to 1450 HV are reported for nitrided S-phase — but it diffuses more slowly and is less thermally stable. Carbon builds a deeper, more forgiving gradient, typically to around 1000 HV. Co-diffusing both lets us put a very hard nitrogen-rich zone at the surface and carry it into the bulk on a carbon-rich gradient, which is why case depths beyond 30 µm are achievable without a hardness cliff.
Temperature comparison
Where our process sits against conventional case hardening.
The bars show the process temperature range for each treatment. The shaded region is where chromium carbide and nitride precipitation becomes a practical risk in austenitic stainless steel — the reason conventional case hardening cannot be used on corrosion-critical parts.
Process temperature by treatment
Ranges as reported in the peer-reviewed literature. Conventional treatments are shown for comparison only.
Process sequence
Six controlled stages. The second one is the whole trick.
Incoming inspection and cleaning
Parts are checked against the drawing, identified and batch-tagged, then cleaned to remove cutting fluid, drawing compound, handling oils and machining residue. Diffusion is a surface reaction: anything left on the part shows up as a soft spot or an uneven case.
Depassivation and activation
Every stainless part arrives wearing a chromium oxide film only a few nanometres thick, and that film is an almost perfect barrier to nitrogen and carbon. It has to be chemically stripped and the bare metal held reactive until diffusion begins. This is the step that separates a uniform, repeatable case from a patchy one, and it is where the process engineering lives.
Nitro-carburizing
Carbon and nitrogen are co-diffused from a controlled gas atmosphere at temperature. Time, temperature and atmosphere composition set the case depth, the hardness profile and the nitrogen-to-carbon ratio through the case.
Controlled cool
Cooling is controlled to hold the supersaturated solid solution intact. There is no quench, no phase transformation and no distortion mechanism, which is why finished parts can be treated without a corrective machining operation afterwards.
Repassivation
The passive chromium oxide film is re-established over the treated surface. Because chromium was never removed from solid solution, the film that reforms is the same film the alloy had originally.
Verification and release
Process coupons run with every batch. We measure micro-hardness at the surface, take a hardness traverse to establish case depth, and section coupons for metallography to confirm a continuous case with no precipitation at grain boundaries. Records are retained against the batch.
Honest comparison
Against the alternatives you are probably weighing.
| NEXT-Phase™ nitro-carburizing | Conventional nitriding | Tungsten carbide thermal spray | PVD / DLC coating | Hard chrome plating | |
|---|---|---|---|---|---|
| Nature of layer | Diffusion zone in the base metal | Diffusion zone plus compound layer | Sprayed coating, ground to size | Deposited coating | Deposited coating |
| Process temperature | ≤ 450 °C | 500–580 °C | Spray jet above 2000 °C; substrate kept cool | 200–450 °C | ~ 60 °C |
| Surface hardness | 1000–1500 HV | 1000–1200 HV | 1050–1350 HV | 1500–3000 HV | 800–1000 HV |
| Layer thickness | 10–50 µm | 50–300 µm | 75–380 µm | 1–5 µm | 10–100 µm |
| Corrosion resistance | Retained | Severely degraded | Depends on porosity, binder and sealing; can corrode at the interface | Depends on pinholes and pinhole corrosion | Degraded at cracks; hexavalent Cr concerns |
| Delamination risk | None — no interface | Compound layer can spall | Bond-limited; can chip under impact | Adhesion-limited | Can crack and lift |
| Internal bores, threads, blind holes | Treated uniformly | Generally yes | Line of sight only | Line of sight only | Poor throwing power |
| Dimensional change | Negligible | Growth plus distortion risk | Adds coating; grinding required | Adds coating thickness | Adds plating thickness |
| Effect on fatigue life | Improved by compressive stress | Variable | Can reduce it | Can reduce it | Reduces it |
Full comparison with tungsten carbide thermal spray
Values for competing processes are typical published industry figures given for orientation, not vendor-specific claims.
Technical FAQ
The questions engineers ask us first.
If your question is not here, ask it directly — we would rather have the technical conversation than send a brochure.
Ask a process engineerWill this really not affect corrosion resistance?
That is the entire design constraint of the process. Corrosion resistance in stainless steel depends on chromium being dissolved in the matrix and available to form a passive film. Because we stay below the temperature at which chromium can diffuse and react, no chromium compounds precipitate, no chromium-depleted zones form, and no sensitisation occurs. The literature reports cases where pitting resistance actually improves, attributed to dissolved nitrogen at the surface. We confirm it on your alloy with corrosion testing when the application warrants it.
Is it a coating? Can it flake off?
No. Nothing is added to the surface. The hardened zone is the original alloy with carbon and nitrogen dissolved into it, so there is no interface between coating and substrate and therefore no adhesion failure mode. It cannot chip, peel or delaminate. Hardness falls off gradually into the bulk, so the case is supported rather than sitting on soft material.
Will my finished, tolerance-critical parts still be in spec?
In almost all cases, yes. Case growth is at the micron scale and the process temperature is far below any distortion or tempering threshold, so parts are normally treated fully finished with no post-treatment machining. For very tight tolerances or thin sections we measure before-and-after on trial parts and give you the actual numbers rather than a general assurance.
Does it change the surface finish or the colour?
No meaningful change to either. Roughness is essentially preserved — a mirror-polished part comes back mirror-polished — and there is no colour shift, unlike black oxide or many coatings. This matters for sanitary, optical and cosmetic surfaces.
How deep can the case be?
Typically 10–50 µm depending on alloy and treatment time. Depth grows roughly with the square root of time, so doubling depth costs roughly four times the cycle. In practice we select the shallowest case that solves the wear problem, because deeper is not automatically better.
What is the temperature limit in service?
Expanded austenite is a metastable phase, so it will eventually decompose into chromium nitrides or carbides if held hot enough for long enough. Reported incubation times for nitrogen-rich S-phase run from minutes at 500 °C to thousands of hours at 300 °C, and carbon-rich S-phase is more stable still. For continuous service we would want to see your operating temperature and duty cycle before committing.
Can it be applied selectively, to only part of a surface?
No. Masking is not possible with this process, so the entire part is treated. In practice there is little downside: the case adds no measurable dimensional change, leaves finish and colour essentially unchanged, and keeps the corrosion resistance of the alloy, so surfaces that did not need hardening are simply harder too. If a region must be machined or welded after treatment, tell us at quotation so we can advise on sequence.
Can NEXT-Phase™ replace tungsten carbide thermal spray?
On stainless and other corrosion-resistant alloys, often yes. HVOF tungsten carbide is typically 1050–1350 HV; NEXT-Phase™ reaches 1000–1500 HV0.05 in the alloy itself. You lose the spray, the grinding and the bond line, keep the corrosion resistance, and harden bores and threads a spray gun cannot reach. Thermal spray remains the better choice where you need to rebuild worn dimensions or carry very heavy abrasive loads through a thick layer. Send us the part and the failure mode and we will tell you which side of that line it falls on.
What is the lead time?
Typically 5–7 days from receipt of parts. The treatment itself is low and slow by design, but because it is a batch process with no plating, spraying or grinding afterwards, the total turnaround is short. Tell us if you have a hard date and we will confirm at quotation.
Can treated parts be welded or machined afterwards?
Welding after treatment is not recommended: the heat-affected zone will exceed the stability limit of the phase and locally destroy both the case and the corrosion benefit. Welding should be completed before treatment. Machining after treatment removes the case in the machined area, so any finish machining should also come first.