The results show that the graphitization annealing of white cast iron takes a long time. When annealing at 800 ℃, it usually needs 12 ~ 16 hours to obtain fully ferritized malleable cast iron. Accelerating the decomposition rate of cementite and increasing the graphite nucleation of gray cast iron are effective measures to improve the graphitization efficiency. After laser melting and impact of gray cast iron, gray cast iron can obtain fully ferritic matrix structure only by heating to 800 ℃ and holding for 4 hours. Compared with the sample without laser shock, the greater the deformation of the melting layer of gray cast iron after laser shock, the easier it is to obtain the matrix structure of complete ferrite in the modified layer after graphite annealing. At the same annealing temperature, the cementite in the melting layer after laser shock decomposes faster, the number of graphite nucleation of gray cast iron is more, and the graphitization process of gray cast iron is more sufficient.
The shape and size of gray cast iron cementite have a great influence on the graphitization annealing process. The finer the microstructure of gray cast iron cementite, the faster the decomposition process, thus accelerating the graphitization process. After laser shock of gray cast iron, the long strip structure of cementite in the melting layer is refined into short strip structure, and even part of cementite structure decomposes directly after laser shock. Therefore, the laser shock of gray cast iron increases the instability of cementite and accelerates the graphite annealing process of gray cast iron. Moreover, the laser shock of gray cast iron increases the dislocation accumulation around cementite, increases the interface energy of cementite and promotes the decomposition of cementite.