Formation mechanism of ultrafine M7C3 carbide in a hypereutectic Fe-25Cr-4C-0.5Ti-0.5Nb-0.2N-2LaAlO3 hardfacing alloy layer

In this work, 0.2 wt% N and 2.0 wt% nano-LaAlO3 additives were added into a hypereutectic Fe-25Cr-4C-0.5Ti-0.5Nb hardfacing alloy layer. The microstructure and formation mechanism of ultrafine M7C3 carbide in the alloy layer was investigated. The results showed that the large block primary M7C3 carbide were largely refined and uniformly distributed in the hardfacing alloy layer. The macrohardness of the alloy layer was significantly improved from 59.5 hazard risk category (HRC) to 65.6 HRC. In addition, a more uniform hardness distribution was observed. The standard Gibbs free energy (ΔG) of the primary phase (Ti, Nb)(C, N) in the N-containing alloy layer was −79,453 J/mol, which was much lower than that of MC which is −21,851 J/mol in the N-free alloy layer. In addition, the two-dimensional lattice misfits between LaAlO3 with M7C3, TiC, TiN, and (Ti, Nb)(C, N) were 10.02%, 11.32%, 9.65%, and 11.90%, respectively, which indicate that nano-LaAlO3, as their heterogeneous nucleation core, effectively promoted the nucleation of M7C3, TiC, TiN, and (Ti, Nb)(C, N). The refined (Ti, Nb)(C, N) can serve as a heterogeneous nucleation core for M7C3 carbide to further refine the M7C3 carbide. Hypereutectic Fe-25Cr-4C-0.5Ti-0.5Nb hardfacing alloy layer with a uniform microstructure was obtained with a M7C3 carbide average size of less than 0.5 μm.