Oxygen or nitrogen — which assist gas should I use?
Updated:
Short answer
Oxygen burns as it cuts, so it is faster and cheaper on mild steel and reaches greater thickness, but leaves an oxidised edge. Nitrogen only blows the melt away, giving a clean, weldable, corrosion-resistant edge on stainless and aluminium at higher pressure, lower speed and much higher gas cost.
How to decide in practice
| Symptom | Likely cause | First action |
|---|---|---|
| Edge finish matters less than cost. | Part will be painted or galvanised | Oxygen on mild steel: fastest and cheapest route. |
| Oxide layer would need grinding. | Part will be welded or stays visible | Nitrogen: pay the gas, save the rework. |
| Corrosion resistance and appearance are part of the spec. | Stainless or aluminium of any thickness | Nitrogen, high pressure, focus below the surface. |
| Nitrogen becomes impractically slow and expensive. | Plate above 20 mm | Oxygen with low pressure and ramped piercing. |
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Assist gas comparison
| Oxygen | Nitrogen | Compressed air | |
|---|---|---|---|
| Mechanism | Exothermic reaction adds energy | Inert, blows melt out | Mostly nitrogen, some oxidation |
| Edge | Oxidised, dark | Clean, weldable | Lightly oxidised |
| Typical use | Mild steel, thick plate | Stainless, aluminium, visible parts | Thin non-critical parts |
| Pressure | Low: 0.5–5 bar | High: 10–22 bar | Medium: 8–15 bar |
| Gas cost | Low | High | Lowest |
| Speed on 6 mm mild steel | 3–4 m/min | Slower, rarely used | 2–3 m/min |
Related questions
- Can I mix gases — pierce with oxygen and cut with nitrogen?
- Yes, and on thick mild steel with a clean-edge requirement it is common. The machine switches gas after the pierce; it costs a little cycle time and saves failed pierces.
- Why is my oxygen-cut edge sometimes dark and sometimes blue?
- Colour reflects the oxide thickness, which follows heat input. A blue edge usually means more heat: slower speed, higher power or accumulated heat in a dense nest.