Microstructure distribution and bending fracture mechanism of 65Mn steel in the laser surface treatment

Abstract

Laser surface treatment of 65Mn steel was conducted using a high-power diode laser. Based on the numerical simulation of the temperature field, the distribution of the microstructure and the peak temperature at various depths were analyzed. An empirical model was proposed to predict the hardened depth. The bending property and fracture mechanism were studied. The results indicated that the martensitic microstructure was coarser near the outer surface of the hardened layer owing to the higher peak temperature. The peak temperature decreased exponentially with increasing depth and increased linearly with laser power and the reciprocal of the square root of the scanning rate. The maximum hardness of the laser-hardened layer was approximately 950 HV, and the self-tempering effect decreased the maximum hardness. Because of the higher brittleness of the hardened layer and the concentration of deformation near the hardened layer, the bending properties deteriorated after laser surface treatment. However, the hardened layer increased the resistance to early bending deformation. Tempering at 150 °C slightly increased the bending strength. The bending fracture consists of three stages: first, tensile load induces the initiation of microcrack, and the crack propagates in an intergranular manner near the crack source; second, the crack propagates in a transgranular manner, and the fracture morphology exhibits quasi-cleavage; and finally, the fracture morphology changes to a ductile fracture in the substrate region.