316L stainless steel has emerged as one of the most extensively utilized stainless steels in the fabrication of marine-engineering equipment components. Components that operate in harsh environments are susceptible to damage and failure. To minimize surface damage caused by wear and corrosion, high-performance coatings are typically necessitated on components that operate in the offshore-platform environments. Ni-based/WC composite coatings prepared via laser cladding exhibit remarkable wear and corrosion resistance. Employing these coatings can be instrumental in enhancing the surface characteristics of 316L stainless steel, thus ultimately extending its service life in the demanding marine environment. However, challenges arise due to significant differences in the thermal expansion coefficient and thermal conductivity between Ni-based alloy powder and WC powder. Combining this with the substantial temperature gradient resulting from rapid heating and cooling during laser cladding renders the coating susceptible to significant residual thermal stresses, thus causing cracks to emerge in the coating. To mitigate these challenges and further enhance the coating properties, Ni-based WC/CeO₂ composite coatings with auxiliary treatments are prepared on a 316L substrate via laser cladding.

In this study, substrate preheating and laser melting are performed to optimize a Ni-based WC/CeO₂ composite coating. Three types of powders are uniformly mixed and preplaced on a polished substrate using a planetary ball mill. The coatings are prepared using a laser-cladding device with a laser power of 1200 W, scanning speed of 800 mm/min, spot diameter of 3 mm, and track spacing of 1.2 mm. The substrates are preheated on a heating plate, and the coatings are fused at room temperature (25±2)℃, 200 ℃, and 350 ℃ , whereas another set of samples are fused under the same preheating conditions. The same laser processing parameters are used for remelting, and the samples are continuously heated to maintain a fixed temperature during laser processing. The microstructures and chemical compositions are characterized via scanning electron microscope (SEM) and energy dispersive spectroscope (EDS), and crystal structures of the coatings are analyzed using X-ray diffraction (XRD). The microhardness of the cladding coatings is tested using a digital microhardness tester. The electrochemical corrosion property of the samples is evaluated using a three-electrode measurement system installed on an electrochemical workstation. Experimental samples are soaked in a solution of NaCl with a mass faction of 0.035 at a temperature of (25±1)℃ for 30 d. The friction coefficient of the coatings and the surface contour after the abrasion are tested using a tribometer. Subsequently, the wear and corrosion morphologies of the coatings are characterized using SEM.

Based on the results of flaw-detection experiments (Fig. 3), the number of cracks in the Ni-Based WC/CeO₂ composite coating decreases as the preheating temperature increases. Laser remelting further disintegrates the WC in the coatings and smoothes out the coatings, thus resulting in a more uniformly dense tissue distribution (Fig. 4). Based on the XRD patterns (Fig. 7), the coating primarily comprises the γ-Ni solid solution phase and carbide hard phase, and the preheating and laser melting minimally affect the phase composition of the coatings. The microhardness of the coating is shown in Fig. 8. The average microscopic hardness of the coating without auxiliary treatment is 820.3 HV. Preheating reduces the cooling rate of the melting pool, increases the grain growth time, and reduces the average microhardness to 660.1 HV. Laser remelting facilitates the further decomposition of WC and the even distribution of carbonates, thus causing the microhardness to increase slightly. The electrochemical properties of the coating are shown in Table 4 and Fig. 10. The preheating of the substrate reduces the passivation current density from 19.29 mA?cm^-2 to 17.72 mA?cm^-2. By increasing the overall resistance of the coating via laser remelting, the corrosion current is decreased from 11.51 μA?cm^-2 to 4.848 μA?cm^-2. Combining the above with the corrosion morphologies (Fig. 11), large areas of corrosion occur primarily near the cracks, and reduced fissures effectively inhibits corruption. As shown in Table 5 and Fig. 13, the wear volume of the coating without auxiliary treatment is 6.8×10^-3 mm³. Preheating increases the wear volume to 11.4×10^-3 mm³, whereas laser remelting reduces the wear volume to 6.1×10^-3 mm³. The wear morphologies (Fig. 14) indicate that the strengthening phases in the coating subjected to preheating and laser remelting are not easily removed during wear.

In this study, preheating and laser remelting auxiliary treatments are applied to prepare a Ni-based WC/CeO₂ laser cladding coating on a 316L stainless steel surface. Preheating reduces the number of cracks in the coating and decreases the coating hardness. Laser remelting reduces the electrical corrosion of the coating in the corrosive solution and improves its corrosion resistance. The WC particles of the coating subjected to remelting are removed easily during wear, and their wear resistance depends primarily on the strength of the Ni base. The reduced hardness decreases the wear resistance. Laser remelting can refine carbide in the coatings. In particular, laser remelting at a temperature of 350 ℃ can refine the carbides in the coating while improving the bonding of carbides with the Ni substrate. In summary, the coating subjected to two auxiliary treatments can effectively reduce cracks while improving both wear and corrosion resistance.