Laser Cutting Gas: Nitrogen, Oxygen or Air

Choosing the wrong assist gas changes cutting speed by up to ten times on the same machine.

Carbon steel: thin and thick behave in opposite ways

From the cutting speed tables (3000 W machine):

Carbon steel, nitrogen / air
 1 mm 28 – 35 m/min
 2 mm 16 – 20 m/min
 3 mm and above: no longer suitable

Carbon steel, oxygen
 2 mm 3.8 – 4.2 m/min
 6 mm 2.2 – 2.5 m/min
 10 mm 1.0 – 1.3 m/min
 20 mm 0.4 – 0.55 m/min

The numbers make oxygen look slow and therefore worse. The reverse is true: on thick plate, oxygen is the only gas that cuts at all. The oxidation reaction feeds extra heat into the kerf. Nitrogen has no such reaction.

On thin sheet, nitrogen or air is nearly ten times faster and leaves a clean edge that needs no oxide removal before welding or painting.

Stainless and aluminium

Stainless and aluminium are cut with nitrogen almost without exception. Oxygen oxidises and discolours the edge and degrades the weld that follows.

Stainless, nitrogen, 3000 W machine
 1 mm 28 – 35 m/min
 3 mm 7 – 10 m/min
 6 mm 2 – 2.7 m/min

On the shop floor

• Mostly thin carbon steel under 3 mm: consider a compressed-air system. Running cost is far below bottled nitrogen.
• Plate above 6 mm in the mix: an oxygen line is mandatory.
• Stainless in the mix: nitrogen is required and cannot be substituted.