Experimental Investigation and Performance Characterization of Laser Quenching on 40Cr Steel by Disc Laser with Various Overlap Ratios

Conducting multiple overlapping laser quenching experiments and material microstructure characterization on 40Cr gear steel using a disc laser holds significant importance in terms of adjusting laser quenching parameters and achieving high-quality quenched layers. In this article, we have conducted a comparative analysis of the macro morphology, microstructure, and phase composition of the phase transition layer of 40Cr steel using Axio Vert A1 Zeiss microscope, Thermo ScientificTM Apreo scanning electron microscope, and X-ray diffractometer. By utilizing the QNESS-Q10M microhardness tester and friction-wear testing machine, we have conducted a performance analysis of the quenched layer, revealing the distribution of hardness and friction-wear properties of the quenched layer. The results indicate that the cross-section of the phase transition layer is crescent-shaped, with the area of the laser-quenched surface in the latter stage being greater than that in the former stage. Among them, the fully transformed region is composed of strip martensite, acicular martensite, and a small amount of residual austenite. The heat-affected zone is composed of a mixture of bainite, ferrite, and pearlite. The matrix is composed of ferrite and pearlite, while the tempered zone is composed of tempered martensite. After laser quenching, the surface hardness of 40Cr gear steel exceeds 800 HV, but in the overlapping area, it decreases stepwise to approximately 424 HV in the tempered zone. Higher overlay rates result in a more uniform surface hardness of the phase transformation hardening layer, but also increase the size of the tempering softening zone, leading to defects on the gear surface. Conversely, lower overlay rates decrease the size of the tempering softening zone, but also affect the uniformity of the surface hardness. Simultaneously, the wear resistance of the workpiece is enhanced after laser quenching by the disc laser.