Study On Strengthening And Toughening Mechanism And Property Optimization Of 18Ni (350) Ultra-High Strength Maraging Steel

18Ni series maraging steels are widely used in many fields.Among them,18Ni(350)ultra-high strength maraging steel plays an important role in various fields by virtue of its ultra-high strength of 2000 MPa.With the continuous development of high-tech industry in China,higher requirements have been put forward for the matching of strength and toughness of materials.Effective methods to improve the toughness of materials while ensuring high strength has brought into focus of current development.Therefore,the strength and toughness matching of 18Ni(350)ultra-high strength maraging steel were improved by optimizing conventional heat treatment process,refining grain by cyclic phase transformation and adding appropriate amount of rare earth La.Firstly,the properties and structures of the solution and aging process were explored,and the changes of the structure and properties of the materials under different solution treatment temperatures,different aging temperatures and different aging time were explored.When the solution treatment temperature increases gradually,martensite matrix relaxes gradually,the size of the martensite laths become coarser,the boundaries of the martensite laths become blurred and the toughness of the materials decreases gradually.As the aging temperature increases,the peak aging time arrives in advance,the precipitated strengthening phase was coarsened and the strength and toughness grade decrease.When the aging treatment time is gradually longer,the aging phase of the material changes constantly,and the precipitated phase disperses and was not coarsened at peak aging,and the strength and toughness match is well.After orthogonal experiment and comparison of different variables,the optimal conventional heat treatment process was determined as follows: 800℃×1h+480℃×16h.Secondly,a cyclic phase change process was introduced on the basis of the conventional heat treatment process.The fine grain strengthening mechanism was used to further improve the strength and toughness matching of the material by adjusting the grain size.The effects of different cyclic phase change times and different cyclic cooling methods on the microstructure and properties of the material were studied.When the number of cyclic phase change time increases gradually,the strength and toughness grade of the material increases gradually,and the grain size decreases gradually and becomes more uniform.When the number of cycles exceeds 4,the refining effect is no longer obvious.As the cooling rate of cyclic phase change gradually increases,the grain size of the material becomes higher and higher,and the strength and toughness of the material also gradually increase.The optimal cyclic cooling method is water cooling with the fastest cooling rate.In conclusion,the optimal cyclic phase transition process is water cooling after 4 cycles of phase transition.Finally,through adding appropriate amount of rare earth La to improve the type and size of material inclusions,the effects of different contents of rare earth La on the structure and properties of materials,as well as on the size,morphology and types of material inclusions were studied.The addition of appropriate amount of rare earth makes the distribution of inclusions more uniform,and the species and morphology of inclusions are also changed.With the decrease of the number of large inclusions,the inclusions are changed from irregular angular blocks to spheres.Subsequently,the stress concentration is reduced,and the matrix lath is refined,which guarantees the high strength and toughness level of the material.