How to achieve a bright, polished edge when laser cutting steel plates?

To achieve an oxide-free, highly reflective cut surface (known as “bright-face cutting”) when laser cutting steel plates (typically carbon steel), the key is to completely prevent oxidation during the cutting process and to use finely tuned cutting parameters that thoroughly blow away the molten material. Ordinary air cutting will form a rough, gray surface with an oxide layer. To achieve a bright surface cut, follow these key steps and principles:

Core Principle

The essence of bright-surface cutting is “melting cutting” rather than “oxidation cutting.”

Oxidation cutting (ordinary air): the use of oxygen as an auxiliary gas, iron and oxygen intense exothermic reaction (combustion), produce a large amount of iron oxide slag, cutting surface rough, gray black.

Melt-cutting (mirror-finish cutting): Uses high-purity nitrogen (N2) or argon (Ar) as the assist gas. The inert gas is isolated from oxygen, the laser only melts the metal, and the high-pressure gas blows away the pure metal solution to obtain a silver bright surface without oxidation.

Key elements and steps to achieve bright surface cutting

1. Auxiliary gas: high purity nitrogen must be used

• Purity requirements: ≥ 99.99%(usually 4 nines or higher). Insufficient purity can cause trace oxidation, affecting the surface finish.
• Pressure requirements: very high. Depending on the thickness of the plate, a stable pressure of 15-20bar (about 1.5-2.0MPa) or even higher is usually required. Need to use a large flow of liquid nitrogen tank or nitrogen generator, ordinary air compressor can not meet.
• Function: blow away molten metal, cool cutting seam, isolate oxygen.

2. Laser power and mode: need high power and high quality beam

• Power requirements: Achieving bright surface cutting usually requires higher laser power. For example, cutting 6mm carbon steel bright surface with a 4000W laser will be easier and more effective than cutting ordinary plates with the same thickness. Insufficient power will cause the cutting speed to be too slow and heat accumulation, which will affect the quality.
• Beam quality: single-mode laser or quasi-single-mode laser because of its more concentrated energy density, smaller spot, in the bright surface cutting than multi-mode laser has inherent advantages, can obtain a finer, brighter section.

3. Cutting parameters: fine adjustment

The following parameters need to be finely optimized according to the equipment and gas conditions:

• Cutting speed: Compared with oxygen cutting, the speed will be significantly reduced. If the speed is too fast, the melt will not be blown clean, and the back will hang slag; if the speed is too slow, the heat input is too much, and the plate may overheat and change color.

Nozzle selection and height:

• Nozzle diameter: usually choose a larger nozzle diameter (such as φ3mm or φ4mm) to adapt to the high pressure and large flow of nitrogen to ensure that the air flow is stable and covers the entire slit.
• Nozzle height: It needs to be precisely controlled, generally slightly higher than when cutting with oxygen to form a good air curtain protection.
• Focus position: The focus is usually set on the surface of the plate or slightly below, and the best position needs to be determined experimentally to obtain the narrowest and brightest slit.
• Peak power/frequency modulation: For some thicker plates, the use of appropriate pulse cutting mode helps to control the heat input to obtain a better bright surface effect.

4. Board requirements

• Surface cleaning: oil, rust and coating on the surface of the plate will affect the cutting effect and should be cleaned in advance.
• Uniform material: Uneven material results in inconsistent glitter effects.

Operation process summary

1. Preparation: Make sure to use high-purity nitrogen and high-pressure gas supply system. Clean the sheet surface.

2. Select nozzle: install a large diameter nozzle suitable for nitrogen high pressure cutting (such as φ3mm).

3. Setting parameters: Select “Nitrogen cutting” or “Bright surface cutting” mode in the cutting control software. Enter the sheet material (carbon steel) and thickness.

4. Key points of reference adjustment:

• The assist gas was set to N2.
• The gas pressure is adjusted to the high pressure range (for example, for 6mm carbon steel, try from 18bar first).
• Significantly reduce the cutting speed (may be oxygen cutting with the thickness of the speed of 1/3 to 1/2).
• Adjust the focus and nozzle height appropriately.

5. Trial cutting and optimization: first use small graphics or scrap edges for trial cutting. Observation section:

• If the upper part is bright, the lower part is distributed black or hanging slag: it may be too fast or insufficient air pressure.
• If the whole section is yellow or oxidized: the purity of nitrogen is not enough or the pressure is too low, and oxygen is mixed in.
• If the cutting is not transparent or the slag is severely adhered: insufficient power, too fast or the focus position is wrong.

6. Formal cutting: after the completion of parameter optimization, the formal cutting.

Considerations and Limitations

• Higher cost: a large amount of high-purity nitrogen is consumed, and the cutting speed is slow, and the unit cost is much higher than that of oxygen cutting.
• Thickness limit: The bright surface effect becomes worse as the thickness increases. It usually works best on carbon steel medium and thin plates (1-12mm). When the board thickness exceeds the equipment capacity, it is difficult to obtain a perfect bright surface.
• Equipment requirements: the laser power, beam quality, machine stability, gas supply system have higher requirements.
• Main application: used for the appearance, welding, spraying quality requirements of the workpiece, such as elevator panels, precision mechanical parts, appearance parts, etc., to avoid the follow-up grinding process.

Simply put, to cut out the bright surface, remember the three elements: high pressure and high purity nitrogen, high power and good beam, low speed fine tuning parameters. It is recommended that you obtain a basic parameter table for a specific model and thickness from the equipment manufacturer and use this as a basis for fine-tuning on site.