Gases for Plasma Arc Cutting

Virtually all plasma cutting of mild steel is done with one of three gas types:

• Nitrogen with carbon dioxide shielding or water injection (mechanized)
• Nitrogen-oxygen or air
• Argon-hydrogen and nitrogen-hydrogen mixtures

The first two have become standard for high-speed mechanized applications. Argon-hydrogen and nitrogen-hydrogen (20% to 35% hydrogen) are occasionally used for manual cutting, but dross formation may be a problem with the argon blend. Dross is a tenacious deposit of re-solidified metal attached at the bottom of the cut. A possible explanation for the heavier, more tenacious dross formed in argon is the greater surface tension of the molten metal. The surface tension of liquid steel is 30 percent higher in an argon atmosphere than in nitrogen. Air cutting gives a dross similar to that formed in a nitrogen atmosphere.

Pure nitrogen is sometimes used to increase torch consumable life (up to 50% greater when compared to air) but it will reduce the cutting capacity of the plasma by approximately 25%. In other words, a plasma rated at cutting one inch with air can cut 3/4" with nitrogen. Surface oxidation with N₂ is substantially reduced when compared to air. This is useful when cutting stainless and nickel alloys.

Oxygen is being more widely applied as a plasma gas with mechanized cutting. Rare earth consumables have been developed which help to maintain acceptable consumable life while enhancing cutting speed on mild steel. Note: Nitrogen may be required to initiate the arc.

The plasma jet tends to remove more metal from the upper part of the work piece than from the lower part. This results in cuts with non-parallel cut surfaces which are generally wider at the top than at the bottom. The use of argon-hydrogen, because of its uniform heat pattern or the injection of water into the torch nozzle (mechanized only), can produce cuts that are square on one side and beveled on the other side. For base metal over three inches thick, argon-hydrogen is frequently used without water injection.