| Six kinds of Fe-based alloy coatings with different content of Cr and Ni were produced onto a medium carbon steel by laser cladding to improve its wear and corrosion resistance. The morphology, microstructure, phase constitution, hardness, wear and corrosion resistance were studied using optical microscopy, X-ray diffraction(XRD), microhardness test, wear test and electrochemical workstation.The results show that the laser-clad coatings are free of cracks and pores and well-bonded with the substrate. The microstructure of the coatings is mainly composed of fine dendrites. The Fe-15Cr-0.8Ni and Fe-15.6Cr-2.54 Ni coatings consist of martensite. A small amount of retained austenite is found in the Fe-16.2Cr-4.28 Ni coating. The Fe-16.8Cr-6.02 Ni and Fe-17.4Cr-7.76 Ni coatings are composed of a mixture of austenite and martensite. The Fe-18Cr-9.5Ni coating is single austenite phase.Coatings with higher content of austenite have lower hardness and the Fe-15.6Cr-2.54 Ni coating has the highest hardness because of solid solution strengthening. Wear test show that the hardness of the coatings has the most important effect on the wear resistance, as a result, coatings with higher hardness has better wear resistance. On the other hand, the wear resistance of the laser-clad coatings is also affected by the work-hardening of austenite and its stress-induced transformation into martensite. This is the reason why the wear weight loss of the Fe-16.8Cr-6.02 Ni coating doesn't increase sharply although its hardness is much lower than that of the Fe-16.2Cr-4.28 Ni.Polarization behavior in 3.5wt.% Na Cl solution shows that the corrosion resistance of the laser-clad coatings is much better than that of the substrate with corrosion potential 0.25 V higher and corrosion current 19 times lower. The coatings has similar corrosion with 304 stainless steel. What's more, all the coatings shows passivation during polarization test while the substrate doesn't. Coatings with higher content of Cr and Ni have higher pitting potential. Fe-17.4Cr-7.76 Ni coating and 304 stainless steel show similar pitting potential, and the pitting potential of Fe-18.Cr-9.5Ni coating is about 0.138 V higher than that of the 304 stainless steel although these two alloys have similar elemental composition.Polarization behavior in 40 g l-1 Na OH + 8 g l-1 Na2 S solution show that the current peak near the corrosion potential is mainly due to the dissolution of Fe and is prevented by the passivation of Ni, forming Ni S2. As the anodic potential increases, Ni S2 begins is oxidized, corresponding to the current peak in the secondary active–passive transition. The secondary current peak is prevented by the passivation of Cr. As a result, Fe-15Cr-0.8Ni and Fe-15.6Cr-2.54 Ni coatings which has low Ni content show only one primary passivation, while other coatings show primary and secondary active-passive transition zones. And higher content of Ni, lower critical current densities and higher secondary current peaks. Fe-17.4Cr-7.76 Ni coating has similar corrosion resistance with 304 stainless steel and Fe-18Cr-9.5Ni coating has best corrosion resistance.In engineering application, it is important to select coatings with proper hardness, wear resistance and corrosion resistance according to the service condition. Till now, the Fe-15Cr-0.8Ni?Fe-15.6Cr-2.54Ni?Fe-16.2Cr-4.28 Ni and Fe-18Cr-9.5Ni coatings have been successfully applied in repairing and strengthening valves, and gear shafts. |
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