Experiments and calculations on refining mechanism of NbC on primary M7C3 carbide in hypereutectic Fe-Cr-C alloy

Two hypereutectic Fe-Cr-C hardfacing alloys were manufactured, whose Nb contents are 0 and 1.2 wt% respectively. Their phase precipitation curves were calculated by Thermal-calc software and their phase structures were detected by X-ray diffractometer, which indicate that NbC precipitates previously to the nucleation of primary M7C3 carbide. The alloys were observed by metallographic microscope, which finds that the primary M7C3 carbides in the alloy with 1.2 wt% Nb are obviously refined. The chemical compositions of carbides were further measured by energy diffraction spectrum, which finds that NbC particles are distributed in the primary M7C3 carbides. The melting process of M7C3 carbide was simulated by molecular dynamics method and the melting point of M7C3 carbide is identified to be 1625±5 K. The melting behaviors of M7C3 embryos with different radii were simulated, and the solid-liquid interfacial energy of M7C3 carbide was calculated to be 3.312 J/m2 based on the classical nucleation theory. Two kinds of M7C3(0001)/NbC(111) interface models with different atomic stacking modes were built. By means of first-principles calculations, M7C3(0001)/NbC(111) interface I is combined by polar covalent/ionic bonds and metallic bonds, while M7C3(0001)/NbC(111) interface II is combined by ionic bonds and metallic bonds. The works of adhesion are 1.23 J/m2 and 2.24 J/m2 respectively. There exists a region where the interfacial energy of interface II is lower than the solid-liquid interfacial energy of M7C3 carbide. The above experimental and calculation results demonstrate that NbC particle is the heterogeneous nucleus of primary M7C3 carbide and thereby refines it.