Microstructure and interfacial stress of M2 coating prepared by UHSLC

The M2 HSS coating was prepared on the surface of 42CrMo steel substrate using ultra-high speed laser cladding (UHSLC). The microstructure and characteristics of the M2 coating were investigated by scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), energy dispersive spectrometer (EDS), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The projected contact area of Bohrer-nanoindentation was measured and corrected to calculate in combination with atomic force microscopy (AFM), and the residual stress distribution of the M2 coating in the direction of thickness was obtained. The results showed that the M2 coating prepared by UHSLC had a low dilution rate and formed a cladding interface with a width of about 1 ~ 2 μm, and the interface metallurgy was well bonded. The M2 microstructure near the interface was predominantly equiaxial, with a large amount of lamellar martensite in the intermediate zone, and the subsurface zone consisted of intracrystalline nanoscale acicular martensite + grain boundary "basket net"-like carbides. The "basket net"-like carbides were composed of unstable V₄C₃ and unstable Cr₃(W₁₀C₃)₂, in accordance with parallel orientation relationships of \overline5 \;\overline15\; 3_\mathrmV_4 \mathrmC_3 \|110_\mathrmCr_3\left(\mathrmW_10 \mathrmC_3\right)_2and (015)_\mathrmV_4 \mathrmC_3 \|(\overline1 \; 15)_\mathrmCr_3\left(\mathrmW_10 \mathrmC_3\right)_2 . The projected contact area of Bohrer-nanoindentation introduced by pile-up was corrected and the obtained information on residual stresses at the M2 cladding interface was in good agreement with the G&S (Giannakopoulos & Suresh) energy method. After the residual tensile stress near the M2 cladding interface reached a peak value of about 300 MPa, it showed a steep decrease in stress within the heat affected zone (HAZ) of the 42CrMo steel and subsequently entered the compressive stress state.