As the substrate for nucleation of primary austenite in hardfacing metals, the effectiveness of RE inclusions and the most common inclusions such as Al2O3, SiO2 and MnO in hardfacing metals of medium-high carbon steelswas analyzed and calculated in detail. The calculation based on the theory of planar lattice misfit shows that Ce2O3,La2O3 and Ce2O2S, instead of SiO2, Al2O3, MnO and CeS, are effective as the heterogeneous nuclei of primary austenitein medium-high carbon steels. As the substrate for nucleation of primary austenite in hardfacing metals, the effectiveness of RE inclusions and the most common inclusions such as Al2O3, SiO2 and MnO in hardfacing metals of medium-high carbon steels were analyzed and calculated in detail. The calculation based on the theory of planar lattice misfit shows that Ce2O3, La2O3 and Ce2O2S, instead of SiO2, Al2O3, MnO and CeS, are effective as the heterogeneous nuclei of primary austenite medium-high carbon steels.