How Thick Can You Cut with Plasma Underwater?

Plasma Underwater Cutting Thickness

Plasma Arc Cutting Underwater

Underwater plasma cutting is nothing new – it was first done in the mid-1970s. But it still has many advantages that lead many fabricators, steel service centres and manufacturers to prefer it over dry cutting. An "Air Curtain" or "Bubble Muffler" is used to create a pocket of air around the front end of the plasma torch, shielding the arc from the water. But not every thickness of every material can be cut underwater. So, what are the limits, and what happens if you push them?

Carbon Steel

Mild steel can be cut underwater up to 2 inches thick. This is achieved at 450 Amps with Oxygen plasma, and the plate is submerged below 2–3 inches of water. Less than 2 inches will not reduce the noise and arc glare below acceptable limits.

Fortunately, 2 inches is the piercing limit for plasma cutting on mild steel, so there is really no need to push the limit. If you did try to go thicker and edge start, say all the way up to 3 inches thick, you will see a significant reduction in cut speed just to make the cut, as well as a lot more dross on the bottom of the cut.

Stainless Steel and Aluminium

Stainless Steel and aluminium up to 2 inches thick can be cut underwater. This is achieved at 600 Amps with Nitrogen plasma or up to 720 Amps with water-injection plasma. As with mild steel, the plate is submerged below 2–3 inches of water level. Less than 2 inches will not reduce the noise and arc glare below acceptable limits.

Underwater cutting should not be done with flammable gases, such as Argon-Hydrogen (H35) or Methane (CH4), because excess gas can become trapped under the plate, causing an explosion hazard.

Considerations When Plasma Cutting Underwater

There are a few things to be aware of when cutting underwater:

• When cutting underwater, you may need to reduce the cut speed on some materials. However, on most materials 1 inch thick and under, the same speeds are used when cutting both above water and underwater.
• The air curtain must be installed correctly, and pressure set appropriately, or else the cut edge quality may be adversely affected.
• Due to the possibility of plate movement, plate thicknesses under 1/4 inch are not recommended for underwater cutting.