Abstract: Composite clad layer of VC-Cr₇C₃ was in situ synthesized on the surface of Q235 steel using laser cladding technology, and the effect of laser scanning speed on microstructure and mechanical properties of the clad layer was studied. The microstructure and properties of the clad layer were analyzed by means of scanning electron microscope, X-ray energy dispersive spectrometer and X-ray diffractometer. The results show that laser cladding technology can cause in-situ reactions between mixed particles of V, Cr and C to form a composite clad layer of VC-Cr₇C₃. It is mainly composed of black gray VC phase, gray Cr₇C₃ phase and {FeM} bonding phase, in which Fe and Cr can form Cr₇C₃ phase (M₇C₃). The laser cladding solidification shape control factor K and the distribution of C element result in a large amount of carbide equiaxed crystals at the top of the clad layer, with a reduced amount of carbide equiaxed crystals in the middle and less carbide at the bottom due to the low C content. The carbide shape is influenced by the laser scanning speed, with carbide dendrites appearing at 1 mm/s and carbide equiaxed crystals at 1.5 mm/s. At the same time, the grain size of the clad layer at 1.5 mm/s is significantly smaller than 1 mm/s. The above changes in the structure and composition of the clad layer cause its hardness to decrease as the layer depth increases. As the scanning speed increases, the hardness of the clad layer gradually increases, with the hardness of the clad layer being more than three times higher than that of the Q235 steel. The friction coefficient of the clad layer is 0.4 at 1.5 mm/s, which is lower than the 0.6 of the Q235 steel substrate. The significant lower wear loss of the clad layer compared to that of the Q235 steel substrate indicates that the composite clad layer of VC-Cr₇C₃ can be used to modify the surface of the carbon steel for high hardness and wear resistance.

Key words: laser cladding, in situ synthesis, VC-Cr₇C₃ composite layers, microhardness, wear resistance