The influence of carbides on the microstructure, grain growth, and oxidation resistance of nanostructured carbides-strengthened cobalt-based multi-track laser-cladding layers

Abstract

Cladding layers were prepared on carbon steel using nanostructured carbide-strengthened cobalt (Co)-based alloy (using nanostructured tungsten carbide as hard phases) as a deposited material by multi-track laser cladding. In addition to Co-based alloy, the deposited materials contained nanostructured chromium carbide (Cr3C2), vanadium carbide (VC), or Cr3C2 and VC. Compared with the substrate, all cladding layers had an improved oxidation resistance with an increased time (0–50 h) at 800 °C. Cr2C3 had a higher oxidation-resistance performance than VC and the mutual reaction of Cr2C3 and VC. The constitution and phases of the cladding layers indicated hexagonal-close-packed-α and body-centered-cubic-β cobalt and hard carbides, such as CoCx, V8C7, and Cr3C2, and some decarburization as Co3W3C and WC1-x. Elemental inter-diffusion occurred between the different phases as indicated by transmission electron microscopy and electron backscattered diffraction. Mixed carbide has a larger misorientation than the cobalt matrix, and electron backscattered diffraction has considerable potential to evaluate plastic deformation. The average grain size of the hard carbides was much larger than that of the original deposited material. The carbides grew rapidly through interfacial diffusion, boundary migration, and coalescence, which followed the Kirkendall effect and Ostwald-coarsening mechanism. This research helps to understanding the microstructure, oxidation resistance, grain growth, and interfacial behavior in multi-track laser-cladding layers.