It is found that the distribution of covalent electron pair number and covalent bond energy was nonuniform in carbides Fe 3C, (Fe,Cr) 3C and (Fe,Mn) 3C, in which the energy difference between the strongest and the weakest bonds is very great. A criterion of covalent bond breaking during the heat treatment of M 3C pattern carbide is presented as follows. If the heat energy %E-%w from heat treatment is higher than the diffusion activation energy %Q% of carbon atom in γ_Fe, the less the covalent bond energy %E% α is, the earlier the covalent bond α will be broken. According to the criterion, why the breaking of (Fe,Mn) 3C network was easier than that of Fe 3C network and why the breaking of (Fe, Cr) 3C network was more difficult than that of Fe 3C network are explained. It is indicated that Mn promoted the carbide to be spheroidized and Cr hindered the carbide from spheroidization during the heat treatment of carbides spheroidization. It is found that the distribution of covalent electron pair number and covalent bond energy was nonuniform in carbides Fe 3C, (Fe, Cr) 3C and (Fe, Mn) 3C, in which the energy difference between the strongest and the weakest bonds is very great. A criterion of covalent bond breaking during the heat treatment of M 3C pattern carbide is presented as follows. If the heat energy % E - % w from heat treatment is higher than the diffusion activation energy % Q % of carbon atom in γ_Fe, the less the covalent bond energy % E % α is, the earlier the covalent bond α will be broken. According to the criterion, why the breaking of (Fe, Mn) 3C network was easier than that of Fe 3C network and why the breaking of (Fe, Cr) 3C network was more difficult than that of Fe 3C network are explained. It is indicated that Mn promoted the carbide to be spheroidized and Cr hindered the carbide from spheroidization during the heat treatment of carbides spheroidization.