Laser Cutting 20 mm Steel: Speeds, Gas, Focus and Power
PCL Group application engineering, Jinan · Updated 20 October 2026
20 mm carbon steel is where fiber laser cutting stops being routine and starts needing the right set-up. A 6 kW source cuts it with oxygen at about 0.6 m/min; a 12 kW source cuts it at 1.5 m/min with oxygen or 1.8 m/min with compressed air. The figures below come from the cutting data of Maxphotonics, the laser source fitted as standard on PCL machines, and show what changes with power, gas, nozzle and focus.
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Cutting data for 20 mm carbon steel
Q235B carbon steel, Maxphotonics MFMC multimode sources with 100 µm fiber. All rows are marked by the laser maker as suitable for continuous production.
| Source | Gas | Speed (m/min) | Pressure (bar) | Focus (mm) | Nozzle |
|---|---|---|---|---|---|
| 6 kW | Oxygen | 0.6–0.65 | 0.6–0.8 | +2.5 to +3.5 | Double 4.0 |
| 8 kW | Oxygen | 0.8–1.1 | 0.6–0.8 | +11 to +12 | 1.4 |
| 10 kW | Oxygen | 1.3 | 0.6–0.8 | +11 to +12 | 1.4–1.6 |
| 10 kW | Air | 1.4 | 16–18 | −11 to −12 | Single 6.0 |
| 12 kW | Oxygen | 1.5 | 0.6–0.8 | +11 to +12 | 1.4–1.6 |
| 12 kW | Air | 1.8 | 16–18 | −11 to −12 | Single 5.0/6.0 |
| 15 kW | Oxygen | 1.5 | 0.6–0.8 | +11 to +12 | 1.4–1.6 |
| 15 kW | Air | 2.5 | 16–18 | −11 to −12 | Single 5.0/6.0 |
Pressure measured at the cutting head. Oxygen 99.99%, nitrogen 99.999%. The 6 kW oxygen row uses 2.2–2.6 kW of output with a large nozzle and 1.5 mm stand-off; the other oxygen rows run at full power with 0.8 mm stand-off, air rows with 0.5 mm. Results depend on the machine, cutting head, plate and gas supply, so use these as starting values, not guarantees. Other materials and thicknesses: fiber laser cutting speed chart.
Choosing power
- 6 to 8 kW: 20 mm is a production thickness, but only with oxygen and at 0.6 to 1.1 m/min. Fine if 20 mm is occasional work.
- 10 to 12 kW: oxygen speed roughly doubles, and air cutting becomes an option. The usual choice when 20 mm is regular work.
- 15 kW and above: oxygen speed hardly rises further (1.5 m/min at both 12 and 15 kW), but air cutting jumps to 2.5 m/min. Extra power pays off through air cutting, not oxygen.
On beams and channels, the PCL H-beam laser is offered at 12, 20 and 30 kW; the 20 kW version cuts flanges up to 40 mm.
Oxygen or air
Oxygen
- 0.6–0.8 bar; the oxygen burns the steel and adds energy
- Smooth, slightly oxidised edge
- Works from 6 kW
- Speed levels off above 12 kW
Compressed air
- 16–18 bar; the gas only blows the melt out
- Faster from 10 kW, much faster at 15 kW
- Low gas cost, but needs a large, dry, oil-free compressor
- Edge can carry a little dross
Nitrogen gives an oxide-free edge but at 20 mm it is slower than air and uses a lot of gas; at 8 kW the data show 0.8 m/min. For painted or galvanised parts the thin oxide film from oxygen cutting is normally removed by blasting anyway.
Nozzle and focus
- Oxygen: focus well above the plate surface (+11 to +12 mm at 8 kW and up), small nozzle (1.4 to 1.6 mm), low pressure. A wide kerf lets the oxygen flow and the slag clear.
- Air: focus deep into the plate (−11 to −12 mm), large single-layer nozzle (5.0 to 6.0 mm), high pressure and a close 0.5 mm stand-off.
- 6 kW oxygen works differently: reduced output, a 4.0 mm double nozzle and focus at +2.5 to +3.5 mm give a stable matt-finish cut.
- Keep the nozzle centred and the protective window clean. At 20 mm, an off-centre nozzle shows as one rough side and a dirty window as a cut that suddenly fails.
Piercing thick plate
Piercing 20 mm takes far longer than sheet and throws molten metal back at the nozzle. Staged or pulsed piercing with the head raised protects the nozzle. Put pierce points in scrap, use a short lead-in, and on parts with many holes check whether pierce time, not cutting time, sets the cycle.
Edge faults and fixes
| Fault | Likely cause | Fix |
|---|---|---|
| Heavy dross on the bottom edge | Speed off the chart; focus wrong; worn nozzle | Return to the reference speed and focus, replace the nozzle |
| Burnt or melted corners | Heat builds up where the head slows down | Corner power ramp or cooling; small loops on sharp corners |
| Rough lower half of the edge | Not enough gas reaching the bottom, focus too high for air cutting | Check pressure at the head and nozzle size, lower the focus |
| Cut does not go through | Power loss: dirty window or lens, wrong nozzle | Clean or replace the protective window, check beam centring |
| Gouges along the edge | Rust or scale on the plate, low oxygen purity | Clean plate surface, check oxygen purity |
Holes in 20 mm plate
As a working rule, holes close to or larger than the plate thickness cut round and square. Smaller holes need slower, pulsed settings and should be checked on both faces. Cut holes before the outer contour so the part cannot move. For structural work, see laser-cut bolt holes tested against drilling.
Video: one laser line cutting heavy plate and H-beams for a structural steel plant.
Machines for 20 mm and thicker
High-power fiber laser for thick plate, where air cutting pays off.
Beds up to 12 m, 1 to 30 kW, for full mill plates and long parts.
12, 20 or 30 kW; flanges up to 40 mm at 20 kW, with 5-axis bevels.
FAQ
What laser power is needed to cut 20 mm steel?
A 6 kW fiber laser cuts 20 mm carbon steel with oxygen at about 0.6 m/min. 12 kW raises that to 1.5 m/min with oxygen or 1.8 m/min with air, and 15 kW reaches 2.5 m/min with air (Maxphotonics reference data).
Is oxygen or air better for cutting 20 mm steel?
Oxygen gives a smooth edge and works from 6 kW. Air is faster from about 10 kW and much faster at 15 kW, with low gas cost, but can leave a little dross.
What focus and nozzle are used for 20 mm oxygen cutting?
At 8 kW and above, a 1.4 to 1.6 mm nozzle, 0.6 to 0.8 bar oxygen and focus about 11 to 12 mm above the surface. Air cutting uses a 5 to 6 mm nozzle, 16 to 18 bar and focus 11 to 12 mm into the plate.