Abstract: The quenching treatment was researched on the tooth surface of the 40Cr steel rack by using LDF 4000-100 semiconductor lasers. The results show that under the rectangular facular sized 17 mm×5 mm, the distance of 375 mm from the laser quenching head to the center of the workpiece surface, 75° of laser beams incident tooth surface,the uniform depth of the hardened layer on the tooth surface can reach 1.2 mm, the microstructure of hardened layer is the fine and uniform distributed needle-like martensite and its hardness can reach up to 750 HV0.2 under the selected optimal parameter with laser heating temperature of 1400 ℃ and scanning speed of 10 mm/s. Finally, the excellent application effect has been acquired in industrial pilot production.

Key words: rack, microhardness, hardened layer, microstructure