The principle of laser heating surface quenching is the same as that of conventional heat treatment, but its heating time is very short, the area is small, and the cooling time is extremely fast. That is, using laser as a heat source, a small area of the metal surface is rapidly heated to austenitize it, and then rapidly cooled for quenching and strengthening. Both theory and practice have confirmed that the surface temperature and thermal penetration depth are proportional to the square root of the laser irradiation duration. Therefore, by appropriately adjusting the spot size, scanning speed, and laser power, the surface temperature and thermal penetration depth can be controlled. When the laser beam leaves the heated surface, the heat there is quickly conducted to the remaining cold parts of the surface, equivalent to self-quenching, without the need for other rapid cooling measures. During laser beam scanning, the power density can also be adjusted by changing the amplitude and frequency of the beam oscillation, thereby controlling the hardening layer depth and coverage area.

Since the laser spot or the amplitude of beam oscillation is very small, heating can only be achieved by scanning the beam one by one on the surface of the part. To prevent the hardened part of the previous strip from being tempered and softened by the heat at the edge of the subsequent scanning strip, efforts should be made to make the energy distribution at the beam edge or oscillation surface as steep as possible. This can be achieved using a grating.

Similar to induction heating surface quenching, the microstructure of general steel after laser surface quenching is also divided into the surface fully quenched zone, the layered incompletely quenched zone, and the unquenched core zone.

Compared with conventional heat treatment, laser heating surface quenching has the following characteristics.

1. Extremely fast heating speed, with minimal thermal deformation of the workpiece. Due to the high power density of the laser, the heating speed can reach up to 10^10 °C/s, the heat-affected zone is small, and the thermal deformation of the workpiece is minimal.

2. Very fast cooling speed. If the workpiece mass is sufficient, the cooling speed can reach 10^23 °C/s; self-quenching does not require a cooling medium, relying on heat conduction from the surface to the interior.

3. After laser quenching, the workpiece surface obtains a fine martensitic structure, with high surface hardness and wear resistance.

4. Since the laser beam scanning (heating) area is very small, it can process workpieces with complex shapes (such as small grooves, blind holes, small holes, thin-walled parts, etc.) or partial processing very precisely. It can also perform different treatments on different parts of the same area as needed.

5. No heating medium is required, and no gas is emitted to pollute the environment, which is beneficial for environmental protection.

6. Energy saving, with a clean workpiece surface. No grinding is required after treatment, and it can be used as the final finishing process for the workpiece.

7. Laser heating surface quenching can achieve personalized customization and flexible processing. For metal parts of different materials, shapes, and sizes, personalized customization and flexible processing can be achieved through laser heating surface quenching technology. At the same time, this technology can also achieve automated production, improving production efficiency and reducing costs.

In summary, laser heating surface quenching technology is a metal surface treatment technology with high hardness, high precision, high efficiency, and environmental protection characteristics. It is increasingly widely used in the field of metal manufacturing and processing and has become one of the important development directions of current metal heat treatment technology. With the continuous advancement of science and technology and the continuous expansion of application fields, laser heating surface quenching technology will continue to improve and develop, bringing more innovation and transformation to the metal manufacturing and processing industry.