Stainless steel is where most plasma shops get into trouble. The same machine, the same thickness — carbon steel comes out clean, switch to 304 and the edge turns black, dross welds itself to the bottom of the cut, and the weld shop starts reporting porosity. In most cases the power source is not the problem. The gas is. This guide compares compressed air, oxygen, nitrogen and argon-hydrogen mixes for stainless, gives you a thickness-based gas selection matrix, a workable parameter window, and an honest hourly cost comparison that includes grinding labour.
1. Why gas selection matters far more on stainless than on carbon steel
When you cut carbon steel with air plasma, the iron oxide formed at the cut face melts below the base metal temperature, so the gas jet blows it straight out and the residue solidifies as brittle, knock-off slag. Stainless behaves the opposite way:
- Chromium reacts with oxygen and nitrogen between roughly 500 and 800 °C, forming dense chromium oxide and chromium nitride layers.
- Those compounds melt above the base metal, so the jet cannot clear them. They re-solidify on the lower edge as the tough, tenacious "sugar dross" that eats angle grinder discs.
- Worse, chromium locked up in those compounds leaves the cut edge chromium-depleted, which directly reduces corrosion resistance and shows up later as weld porosity and inclusions.
So the whole of stainless gas selection comes down to one principle: either exclude oxygen and nitrogen from the cut zone, or use an inert, high-enthalpy jet to physically blow the molten pool clean — and keep time-at-temperature as short as possible. That is precisely why high-precision plasma systems run on nitrogen or argon-hydrogen for stainless.
For normative references, consult the thermal cutting resources published by the American Welding Society at www.aws.org and the surface quality classification defined in ISO 9013.
2. How the four process gases actually behave
| Gas | Cutting mechanism | Edge colour | Dross | Weld prep | Relative gas price |
|---|---|---|---|---|---|
| Compressed air | Oxidising exotherm + mechanical blow-out | Grey-black / dark brown | Heavy, tenacious, hard to remove | Grind back to bright metal | 1.0 (cheapest) |
| Oxygen O₂ | Strong oxidising exotherm, fastest travel | Deep black | Plentiful but brittle | Grinding mandatory; severe Cr depletion | 1.5–2.0 |
| Nitrogen N₂ | Inert shield + high-enthalpy mechanical blow-out | Bright silver | Minimal, flakes off | Often weldable as-cut | 3.0–5.0 |
| Argon-hydrogen Ar/H₂ | Reducing atmosphere + highest arc energy density | Near-mirror silver | Effectively none | Weld directly | 8.0–15.0 |
The takeaway is blunt: if the part will be welded or must resist corrosion, do not cut stainless with air plasma. The hidden cost of the air route — grinding labour, weld rework, corrosion claims — dwarfs the money saved on gas.
3. Gas selection matrix by plate thickness
| Thickness | First choice | Fallback | Typical machine class |
|---|---|---|---|
| 0.5–3 mm sheet | Nitrogen N₂ (FineCut nozzle) | Ar/H₂ where appearance is critical | 45–105 A high-precision |
| 3–12 mm medium | Nitrogen N₂ | Air (blanking only, full post-machining) | 130–200 A high-precision |
| 12–25 mm heavy | Nitrogen N₂ | Oxygen O₂ (speed first, grinding accepted) | 200–300 A high-precision |
| 25–50 mm plate | Nitrogen N₂ or N₂/O₂ mix | Oxygen O₂ | 300–400 A heavy duty |
| Above 50 mm | Oxygen O₂, or switch process | — | 400 A and up |
For shops running mostly stainless, specify a multi-gas high-precision power source from the start — for example the Max-200HPR or Max-300HPR. These carry dedicated air / O₂ / N₂ gas circuits with matched process libraries, so one machine covers carbon steel, stainless and aluminium instead of forcing a second purchase just for stainless.
4. A workable process window for nitrogen on stainless
Correct gas is step one; the parameter window decides the finish. Baseline for 12 mm 304 stainless at 200 A on nitrogen:
| Parameter | Recommended | Symptom when wrong |
|---|---|---|
| Cutting current | 8–10 A per mm of thickness | Too high: over-melted face, top-edge roll-over. Too low: incomplete cut, dross |
| Gas pressure | 0.55–0.75 MPa measured dynamic at the torch | Too low: poor blow-out, bottom dross. Too high: disturbed swirl, double arcing |
| Travel speed | Back-calculate from a bevel angle of ≤ 3° | Too fast: heavy drag lines, high-speed dross. Too slow: top-edge melt, wide kerf |
| Torch-to-work distance | 4–8 mm with THC arc-voltage height control | Too low: spatter destroys nozzle. Too high: arc spreads, bevel grows |
| Pierce height | 1.5–2 × normal cutting height | Too low: nozzle and shield destroyed in one pierce |
| Nitrogen purity | ≥ 99.95 % (vaporised liquid N₂ or PSA) | Oxygen above 0.5 % and the edge goes black again |
Tune it with a single-variable method: hold current and pressure, change only speed. Step up in 100 mm/min increments and watch the drag lines. The speed at which the drag lines run closest to vertical is your optimum.
5. Running the real numbers on gas cost
Most shops judge nitrogen by its unit price and miss the system cost. Here is a like-for-like comparison: 200 A class machine, continuous cutting of 12 mm stainless.
| Cost item | Air plasma | Nitrogen plasma |
|---|---|---|
| Gas consumption | ≈ 2 400 L/min compressed air | ≈ 60–90 L/min nitrogen |
| Gas cost per hour | ≈ ¥6 / h (compressor electricity) | ≈ ¥45–70 / h |
| Grinding time per metre of cut | 0.8–1.5 min | 0–0.2 min |
| Grinding cost (labour at ¥60 / h) | ≈ ¥60 / h | ≈ ¥6 / h |
| Rework rate from poor weld prep | 3 %–8 % | Below 1 % |
| Total cost per hour | ≈ ¥66 / h plus rework | ≈ ¥55–76 / h |
Read that bottom row carefully: once grinding labour is included, nitrogen is usually at parity with air — or cheaper — while winning outright on edge quality, weld yield and lead time. At volume, switching from cylinders to a liquid nitrogen tank or an on-site PSA generator cuts the nitrogen unit price by another 30 %–50 %.
6. Getting the gas supply right
Source
Cylinder bundles suit shops using under 2 000 m³ per month. Above that, a liquid nitrogen tank with vaporiser gives the lowest unit cost and the steadiest pressure. Mid-volume shops can use a PSA nitrogen generator, but purity drifts — fit an online oxygen analyser and alarm.
Pipework and regulation
Run the nitrogen main at DN15 or larger in stainless or degreased copper. Never share or adapt fittings between nitrogen and oxygen lines. Fit two-stage regulation plus 0.01 μm coalescing filtration at the machine and a pressure/flow check immediately before the torch.
Cooling
High-current nitrogen cutting puts real heat into the torch. Pair it with the MSW-20L industrial water chiller to hold coolant at 20–25 °C; it measurably extends consumable life on the MS-4000 FineCut plasma torch.
7. Frequently asked questions
Can I cut stainless with air plasma?
Physically yes, and we do not recommend it. Air leaves an oxide-nitride layer and a chromium-depleted edge, which hurts corrosion resistance and causes weld porosity. It is defensible only when a downstream operation removes the entire heat-affected zone. Any welding or corrosion requirement means nitrogen.
Why does my nitrogen cut come out yellow or blue?
Usually purity or trapped air. Verify purity at ≥ 99.95 %, purge the line properly, and check every fitting for leaks. Low pressure is the other common cause — the molten pool cools too slowly. Try +0.05–0.1 MPa and re-test.
Is nitrogen worth it for small batches?
Include grinding and rework and the answer is usually yes. For low-spec blanking where nobody cares about the edge, stay on air. For pressure vessels, food machinery or chemical equipment, make nitrogen the standard.
Should I go to argon-hydrogen?
Ar/H₂ gives a near-mirror edge you can weld as-cut, but gas costs two to three times nitrogen and hydrogen demands dedicated safety management — explosion venting, flashback arrestors, segregated cylinder storage. It pays only on appearance parts, mirror-finish work or high-value heavy plate.
Can one machine handle both air and nitrogen?
Yes. Multi-gas high-precision sources such as the Max-200HPR and Max-300HPR carry three internal gas circuits with matched parameter libraries. You switch gas type on the panel and, with a FineCut plasma torch fitted, move between carbon steel, stainless and aluminium in minutes.
8. Where to start
Roughly 80 % of the quality gap on stainless plasma comes from gas and parameters, not from how expensive the power source is. Fix the gas first, then the nozzle and parameters, then audit total cost. That is the only reliable way to avoid owning a machine you cannot run profitably.
If black edges, stubborn dross or weld porosity are costing you money, talk to our process engineers. Give us your thickness mix, alloy grades and throughput target and we will return a complete gas and machine specification. You may also find our plasma consumable life guide and our comparison of the three thermal cutting processes useful for building the full selection framework.
