Research On The Morphology Reconstruction Under Thermal And Force Effect And Properties On Gray Cast Iron Surface

Gray cast iron was widely used in mechanical parts because of its good mechanical properties.However,the carbon content was very high in the gray cast iron,and most of the carbons were distributed in the matrix in the form of flaky graphite.The continuity of iron matrix was severely cut off by flaky graphite,which had damage to the strength and toughness of the iron.Therefore,in order to improve the properties of gray cast iron,the morphology of the graphite needs to be changed or refined.In this paper,the effects of laser remelting process,laser shock process and graphitization annealing process parameters on microstructure and mechanical properties were discussed through simulation and experiments.The changes of residual stress and flake graphite transformation to cementites after laser remelting and laser shock process were analyzed.The transformation of cementites to flocculated graphites during graphitization annealing process has been studied.And the properties of the modified layer were tested.The purpose to change the graphite morphology and matrix structure of gray cast iron has been realized,and the the properties of gray cast iron has been effectively improved.The finite element software ABAQUS was used to simulate the temperature and stress fields of gray cast iron during laser melting process.The results showed that,the temperature of laser molten pool increased rapidly to 2380°C,in a short period of time,which was far beyond the remelting point of the gray cast iron.As the laser power increased,the molten pool temperature also increased.The molten pool cooled rapidly after the laser spot left,which produced residual tensile stress.The larger the laser power was,the more obvious the residual stress changed.In order to reduce the influence of residual tensile stress on the property of gray cast iron during laser remelting process,the laser shock processing was adopted.The simulated results showed that laser shock processing can effectively change the state of residual tensile stress after laser remelting,which induced the transformation of residual tensile stress to residual compressive stress.With the increase of laser shock energy,the compressive stress gradually increased.The experimental results showed that,the surface of the remelted layer was relatively smooth and the molten pool was better under the laser power of 1500 W and scanning speed of 5 mm/s.It can be known from the SEM results that the flake graphites were decomposed after laser remelting process.The microstructure of the remelted layer were composed of cementites and ledeburites and the heat affected zone was mainly composed of lamellar martensite,retained austenite,and undecomposed flake graphite.The microhardness of the remelted layer surface were greatly improved because of cementites,which was up to 750 HV0.2.Subsequently,laser shock was applied to induce plastic deformation in the melted surface.The larger laser shock power was,the deeper the deformation depth was.The microstructure of cementites in the deformation layer was refined and decomposed.The residual tensile stresses in the remelted layer were changed after laser shock process,which can effectively reduced the generation and expansion of the cracks in the surface layer.The microhardness of remelted layer were further increased after laser shock process.The maximum microhardness was increased to 1100 HV0.2,and the wear resistance was obviously improved.The application of modified layer were limited after laser remelting and laser shock process because of the hard and brittle microstructure of cementites,which can be reduced by graphitization annealing treatment.It can transform the cementites into fine flocculated graphites to achieve the purpose of property improvement.The effects of graphitization annealing temperature on the microstructure transformation of the remelted layer matrix was analysed,and the graphite morphology was studied by experiments.The laser shock process accelerated the decomposition of cementites and promoted the graphite nucleation.The cementites were completely decomposed,and the precipitated graphite converted into small globular after heat treatment at 800 °C for 3 hours.The microhardness of the modified layer was 425 HV0.2 after graphitization annealing treatment,and wear resistance of the modified layer was improved.