Study on graphitization annealing of laser modified surface of gray cast iron

Although the surface hardness of gray cast iron after laser melting and impact is greatly improved, the microstructure toughness of cementite in the laser melting and impact layer of gray cast iron is poor, which limits the improvement of comprehensive properties of gray cast iron surface. Therefore, reducing the hard brittle cementite phase in the melting impact layer of gray cast iron is of great significance to improve the properties of gray cast iron. Through graphitization annealing process, the microstructure of cementite in the modified layer can be decomposed, the cementite can be transformed into flocculent graphite, and the matrix of the modified layer can be transformed into ferrite to improve the toughness of the modified layer. After graphitization annealing, the size, morphology and quantity of graphite obtained in the modified layer and the change of matrix structure are the main factors affecting the properties of the modified layer.

(1) After solid-state graphitization annealing, the hard brittle cementite in the original structure decomposes, graphite gradually precipitates, and the cementite matrix gradually changes into ferrite. Graphitization annealing temperature has a great influence on the matrix structure and the size, morphology and quantity of graphite after annealing. If the annealing temperature is low, the cementite in the laser melting impact layer of gray cast iron is not completely decomposed, and there is a large number of secondary cementite distribution on the matrix. The amount of precipitated graphite is small, which is distributed between cementite dendrites in point shape. With the increase of annealing temperature, cementite decomposes completely and gradually changes into ferrite grains, the amount of precipitated graphite also increases gradually, the size of graphite increases and tends to be spherical. When the temperature exceeds 800 ℃, the growth of precipitated ferrite grains and graphite phase becomes larger, and the shape of graphite tends to deteriorate. The modified layer with fine ferrite grains, good graphite shape and uniform distribution can be obtained after annealing at 800 ℃ and holding for 3 hours. After annealing at this temperature, the hardness of the modified layer is 422hv0 Compared with the modified layer of gray cast iron after laser melting and impact, the friction coefficient is 0.34, and the wear resistance is worse than that of gray cast iron.

(2) Laser shock can effectively refine the cementite structure in the laser melting impact layer of gray cast iron, accelerate the decomposition of cementite during graphitization annealing, effectively shorten the time required for graphitization annealing and accelerate the graphitization process. After laser shock, the graphite nucleation position of the modified layer increases, the amount of precipitated graphite increases and the graphite morphology is refined. The precipitated graphite is mostly distributed in ferrite grains, which reduces the harm to ferrite grain boundaries.

In the process of graphite annealing of gray cast iron, the control of annealing temperature and time is the key. Controlling reasonable annealing temperature and holding time will have an important impact on the microstructure produced after annealing, the degree of homogenization of microstructure, the morphology of graphite, the quantity and distribution of graphite, and then affect the performance of modified layer of gray cast iron. The main factors determining the graphitization annealing speed are the number of precipitated graphite cores and the diffusion ability of carbon atoms in austenite. Laser shock can promote the decomposition of cementite and accelerate the diffusion of carbon atoms. This chapter mainly studies the changes of the structure of the modified layer under different annealing temperatures and the accelerating effect of laser shock on graphitization annealing, and tests the properties of the samples after graphitization annealing.