Study On Microstructure And Properties Of Gray Cast Iron After Laser Surface Melting-laser Shock Processing-graphitization Annealing

Gray cast iron is widely used in the modern mechanical industry because of its good casting property,machinability,damping property and wear resistance,such as machine bed,engine block and so on.However,the flake graphite structure in the gray cast iron matrix seriously affects the tensile and corrosion properties of the gray cast iron,thus limiting the further application of the gray cast iron.Therefore,in this paper,aiming at solving the problem that the flake graphite in the gray cast iron matrix causes the poor mechanical properties of the gray cast iron,the surface of the HT200 gray cast iron was modified by the composite strengthening technology of laser surface melting,laser shock processing and graphitization annealing(LSM-LSP-GA).The microstructure,tensile and corrosion properties of the modified layer under different parameters were studied and analyzed.The phase transition mechanism,grain refinement mechanism,graphite morphology evolution mechanism and property strengthening mechanism of the modified layer were clarified.The following main conclusions were obtained:(1)XRD,SEM and TEM were used to observe the microstructure of the LSM-LSP-GA modified layer of gray cast iron.The results showed that the flake graphite in the original specimen gradually changed into flocculent graphite,and the pearlite in the matrix changed into ferrite after LSM-LSP-GA composite experiment.At the same time,there were a lot of dislocation structures in the modified layer of the specimens.The formation and aggregation of such dislocation structures made a large number of subgrain boundaries appear in the modified layer,and the original coarse grains were divided into new fine grains.In the GA process,carbon precipitated from cementite and diffused.Nucleation occurred at grain boundaries and dislocation defects,and graphite nucleus turned into flocculent graphite.(2)The tensile tests at room temperature were carried out for the specimens at each stage of LSM-LSP-GA composite experiment.The results showed that the tensile strength of gray cast iron increased from 177.32 MPa to 219.29 MPa and the elongation increased from 0.51% to 1.38% after the composite experiment,whichindicated that the tensile property was improved.The tensile fractures of the specimens were all quasi-cleavage fracture,which were mainly characterized by tearing ridges,river patterns and dimples.The tensile fracture mode was brittle fracture which was characterized by quasi cleavage fracture.Because the flake graphite in the modified layer changed into flocculent graphite,the stress concentration phenomenon disappeared.Therefore,the generation of crack sources reduced during the stretching process.The amount of subgrain boundaries in the modified layer of the specimen increased and the microstructure was refined.The above phenomena effectively hindered the growth and expansion of cracks,and then the tensile property of the specimen was strengthened.(3)The electrochemical corrosion experiments of the specimens at each stage of the LSM-LSP-GA composite experiment were carried out.The results showed that the corrosion property of gray cast iron was significantly improved after the composite modification experiment,which was mainly manifested on the positive shift of the potentiodynamic polarization curve and the expansion of the impedance spectrum circuit.The electrochemical corrosion surface morphology of the specimen was great and the size of corrosion pits decreased.After the composite modification experiment,the gray cast iron surface formed a dense grain refinement layer,and there was flocculent graphite with strong corrosion performance in the layer.The corrosion property of the specimen was strengthened.