Thermal Cutting, Gouging, and Related Processes
Thermal Cutting, Gouging, and Related Processes
This domain tests oxy-fuel cutting, plasma cutting, air carbon arc gouging, and related methods. You need to select the process for the metal, set the equipment, and recognize cut quality and safety issues.
Core rules
- Oxy-fuel cutting works on steel because iron oxidizes exothermically. The preheat flame brings the steel to ignition temperature; the oxygen jet burns the iron and blows slag out. It is not suitable for aluminum or stainless because refractory oxides prevent a clean cut.
- Set a neutral flame for cutting, use the correct tip size for thickness, and maintain the right standoff. Kerf is the width of the cut; drag is the lag of the exit point behind the entry point. A slight drag is normal; excessive drag means speed is too high or the tip is wrong.
- Plasma cutting uses a constricted arc and high-velocity gas. It cuts stainless, aluminum, and carbon steel. Use the correct consumables, current, gas, and cut speed; piercing thick plate requires a pierce height and delay.
- Air carbon arc gouging uses a carbon electrode and compressed air to remove metal. It is typically DCEN, with the carbon electrode negative, to keep the electrode cooler and put heat into the work. Set current and air pressure for the electrode size, and maintain the correct angle.
- Safety: protect against sparks, slag, and radiation; keep combustible materials away; plasma and gouging produce noise, fume, and UV.
Worked example
A 6 mm stainless steel plate must be cut. Oxy-fuel cutting is a poor choice: stainless forms chromium oxide, which has a melting point above the base metal, so the oxygen jet cannot cut cleanly. Plasma cutting is the practical choice. Select the correct nozzle and current for 6 mm, set the cut height, and use the recommended speed. The reasoning is material chemistry, not just thickness.
Traps
- Trying to oxy-fuel cut aluminum or stainless.
- Using an oxidizing flame for cutting.
- Setting plasma current too high and producing a ragged kerf.
- Gouging with the wrong polarity or too much current.
- Leaving slag on a cut edge before welding.
- Piercing thick plate at the normal cut height.