Development of a continuous laser hardening method without tempering zone

Laser hardening is one of the most promising method to improve wear resistance and fatigue strength of steel parts, due to its high speed, noncontactive processing and controllability of heat input. Because the laser power available is limited, the region of the irradiated area at one time is not large enough. Therefore it is very difficult to harden a larger area. In order to obtain a uniformly hardened area larger than beam spot size, conventional repeated irradiation methods have frequently been attempted. In conventional manner, a laser beam is irradiated on the region adjacent to the preprocessed area, which causes a tempering zone on its edge, and that breaks the uniformity in hardened area. To solve this problem and to have an uniformly hardened large zone, a new method has been developed. The main feature of our method is to control the heat input on the surface of a work piece and to form a suitable heat pattern, using divided laser beams obtained by a newly developed optical system, that is, concave beam-splitting mirror and truncated ptramid mirrors. A laser beam is split and scanned over the surface of the work piece one after another to keep the temperature of the processed area over the A₁ transfomation point, and to maintain the cooling rate necessary for hardening work piece. A 15kW CO₂ laser and six truncated pyramid mirrors are used for experiments to evaluate the new method. A large surface hardened without tempering on a carbon steel plate was achieved. This newly developed method would be very effective for continuous hardening of a very large steel plate without tempering zone.