O₂ vs N₂: what assist gas does to your edge — and your invoice

A fiber laser melts a narrow line of metal; a jet of gas blasts the melt out of the kerf. That gas isn’t just a cleaner-upper — the one we choose changes how fast we can cut, what the edge looks like, whether you can paint it straight away, and part of the running cost. There are two we use, and they pull in opposite directions.

Oxygen doesn’t just clear the cut — it burns with the steel, and that reaction adds heat of its own. That extra energy lets us cut thick mild steel faster and thicker than nitrogen can. The catch: the reaction leaves a thin oxide skin on the edge — the tell-tale dark, slightly rough face — which has to be cleaned off before painting or welding, or the finish won’t key and the weld can go porous.

Nitrogen is inert — it blows the melt out without reacting, so the edge comes off bright, clean and oxide-free, ready to paint or weld with no extra prep. It’s the only right answer for stainless and aluminium, which would discolour or oxidise under oxygen. The trade-off is money: nitrogen cuts more slowly and uses a lot of gas at high pressure, so a nitrogen cut costs more per metre than an oxygen one.

The deciding question is almost always what happens to the edge next — so tell us if it’s getting painted, welded or left on show.