Effect of Nb2O5 on the cracking susceptibility of laser-clad Fe-based coating

The experiments of laser-clad Fe-based alloy powder doped with and without Nb2O5 on medium carbon steel 45 by powder feeding are carried out.A crack-free,fine and homogeneous Fe-based coating is obtained at a proper amount of Nb2O5-doped under suitable optimizing processing parameters.The effect on microstructure of different amounts of Nb2O5 to Fe-based coating is studied with scanning electron microscopy,optical microscopy and Xray diffraction.The cracking on the surface of the coatings is observed and the hardness in the cross-section of the clad layers is measured.It is reveled that lots and lots of fine dendritic crystal grown along different directions in the cladding layers of G312+0.6% Nb2O5(mass fraction) are uniformly dispersed in the matrix,which is a mixture containing γ(Fe,Ni) solid-solution and chromium carbide or boride.It is considered that the improvement of properties of the coating is due to the following two reasons.On the one hand,Nb atoms can enhance intercrystalline cohesion when they are located in the crystal boundary.On the other hand,particles of NbC,reduced from Nb2O5 by C at high temperature and deposited primarily from the alloy solution,become new crystalline nuclei and increase the nuclear rate in solidification,which leads to the dendritic turn to fine.The cracking susceptibility of the Fe-based coating doped a proper content of Nb2O5 can be reduced because of the decrease in amount of crisp parts in the layers and increase in tenacity of the coatings.