Thermal reconstruction and properties of surface microstructure of gray cast iron

The combination of laser melting, laser shock and graphitization annealing process of gray cast iron was applied to the study of surface modification of gray cast iron. Through the combination of simulation and experiment, the processing process and process parameters were optimized, the microstructure and graphite morphology of gray cast iron surface after different modification processes were studied, and the properties were tested. The main conclusions are as follows:

(1) ABAQUS simulation software is used to analyze the laser melting process of gray cast iron. The size of molten pool and the temperature distribution of molten layer under different laser power and laser scanning speed are compared and analyzed. According to the simulation results, the laser power has a greater impact on the temperature and size of molten layer, and the scanning speed has a smaller impact on the temperature and size of molten layer, When the laser power is 1500 W and the laser scanning speed is 5 mm / s, the fused layer with better temperature and better size can be obtained. In the process of laser melting of gray cast iron, the surface of gray cast iron can rise rapidly to 2364 ℃ within 0.3 s, which is far higher than the melting point of gray cast iron (1270 ℃). When the laser spot leaves, the temperature of molten pool decreases rapidly within 1 s, and a large temperature gradient is generated around the molten pool, which is the main reason for the residual tensile stress of molten layer. When the laser power is 1500 W, the maximum longitudinal residual tensile stress of gray cast iron melting layer is 240 MPa and the maximum transverse tensile stress is 173 MPa. Increasing the laser melting power, the tensile stress in gray cast iron melting layer increases significantly. By simulating the laser melting process, the laser melting parameters were optimized.

(2) The change of residual compressive stress in the melting layer during laser shock is simulated and analyzed. After laser shock, the residual tensile stress in the melting layer of gray cast iron is completely transformed into compressive stress. With the increase of laser shock energy, the value of residual compressive stress in the melting layer of gray cast iron increases significantly. Under 5 J laser shock energy, the maximum compressive stress in the melting layer of gray cast iron is – 220 MPa, With the increase of laser shock energy, the increase of fusion solidification lamination stress of gray cast iron slows down. When the laser impact energy is 3 J, the residual compressive stress distribution depth is 0.3 mm. With the increase of laser impact energy, the deeper the induced compressive stress distribution depth is. When the laser impact energy is greater than 5 J, the residual tensile stress in the heat affected zone in the depth direction is gradually transformed into compressive stress. When the laser impact lap rate is lower than 30%, the value of longitudinal residual compressive stress on the surface of the fused layer fluctuates greatly. In order to reduce the fluctuation of residual compressive stress, a larger laser impact lap rate should be selected as far as possible in the experiment.

(3) The laser melting experiment of HT200 gray cast iron was carried out. The changes of macro morphology and molten pool size of gray cast iron melting layer surface under different laser power and laser scanning speed were compared, and better laser melting process parameters were obtained. When the laser power is 1500 W and the laser scanning speed is 5 mm / s, the fused surface quality is the best, and the width and depth of the molten pool are uniform. The melting zone of gray cast iron is mainly composed of coarse cementite and network eutectic ledeburite. The long strip cementite is connected and distributed continuously, and the flake graphite is completely decomposed in the melting zone of gray cast iron. Heat affected mainly consists of cellular ledeburite, lamellar martensite and retained austenite. The graphite in the heat affected zone is partially refined and decomposed, and there is secondary graphite precipitation between the graphite. The maximum surface hardness of gray cast iron after laser melting is 700 ~ 730 hv0 2, the friction coefficient is about 0.15. Compared with the original sample, the hardness of the melting layer of gray cast iron is significantly improved and has better wear resistance.

(4) The causes of surface cracks in the melting layer are analyzed. There is a large residual tensile stress in the melting layer of gray cast iron after laser melting. The maximum residual tensile stress on the surface of the melting layer is measured to be 275 MPa through experiments, and because there is hard brittle cementite with poor toughness in the melting layer of gray cast iron. The experimental study of laser shock on the surface of laser melting layer shows that after laser shock, the residual tensile stress in the melting layer of gray cast iron is completely transformed into residual compressive stress, which effectively hinders the initiation and propagation of cracks in the melting layer. After laser shock, the size of cracks in the melting layer of gray cast iron is reduced. After 3 J laser energy impact, the surface of gray cast iron melting layer is 215 downward μ Large plastic deformation occurs in the depth of M, the cementite structure in the deformation layer is seriously deformed, and the distance between cementite dendrites is shortened. With the increase of laser impact energy, the deformation depth of gray cast iron melting layer increases, the cementite structure breaks and decomposes into short cementite structure. The refinement of cementite structure increases the strength of the fused surface layer, reduces the generation of surface cracks, and provides conditions for shortening the graphitization annealing time. After laser shock, the hardness of laser melting impact deformation layer of gray cast iron further increases, and the average hardness value is 1000 ~ 1200 hv0 At the same time, it has good wear resistance. The friction coefficient of the surface of gray cast iron after laser melting and impact is 0.11. Under high friction load, the wear mark width of the surface of gray cast iron after laser melting and impact is reduced to 15 μ m. Moreover, the wear marks are shallow, and the cracks and spalling on the worn surface are greatly reduced compared with the samples after laser melting.

(5) After laser melting and impact treatment, the surface structure of gray cast iron is cementite matrix. Due to the high hardness and brittleness of cementite, the performance of modified layer of gray cast iron is limited. Through graphitization annealing, the coarse cementite structure in the melting layer is transformed into fine flocculent graphite, so as to improve the surface properties of gray cast iron. By comparing different annealing temperature experiments, after heating to 800 ℃ and holding for 3 hours, fine ferrite matrix and uniformly distributed fine flocculent graphite can be obtained in the modified layer. With the increase of annealing temperature, the ferrite grain size, graphite size and quantity of the modified layer after annealing increase, and there is a trend of growth and connection. Graphitization annealing process improves the plasticity of laser melting impact layer of gray cast iron, reduces the initiation of surface crack of modified layer in the process of wear, and makes the modified layer have better wear resistance. Laser shock before graphitization annealing can effectively refine the cementite structure in the laser melting impact layer of gray cast iron, accelerate the decomposition of cementite in the graphitization annealing process, effectively accelerate the graphitization process and shorten the graphitization annealing time. At the same time, laser shock increases the nucleation position of graphite in the process of graphitization annealing, so that the number of precipitated graphite increases, the morphology is refined and the distribution is more uniform.

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