| In this paper,the surface deep gradient strengthening study is carried out for the typical heavy-duty gear Fe-Cr2Ni4 alloy material.By precisely controlling the heat treatment process,the quenching-carbon partition-precipitation strengthening method is used to increase the strength and toughness with optimizing the toughness matching at the same time.Thus,the gradient-strength anti-wear layer with excellent property is obtained.The microstructure,lattice structure,dislocation configuration and the phase boundary evolution of gradient strengthening layer are analyzed to explore the strengthening mechanism the anti-wear layer of the Fe-Cr2Ni4 alloy by means of field emission scanning electron microscopy(SEM),electron backscatter diffraction(EBSD),field emission transmission electron microscopy(TEM)and the X-ray diffraction(XRD).In order to further elucidate the influence mechanism of the microstructure of the gradient strengthening layer against contact fatigue wear on the macroscopic performance,this paper uses the computer aided software such as Hypermesh/Abaqus to simulate and analyze the fatigue stress field based on the real organization of the high-strength and tough gradient material.The stress field distribution of the strengthening layer is obtained,and then the failure mechanism of the fatigue wear of the gradient strengthening layer is explored.At the same time,in order to systematically study the microscopic mechanism of deep gradient strengthening layer obtained by high carbon potential atom diffusion,this paper further combines the first principle calculation and molecular dynamics simulation to study the solid solution and the migration of single interstitial carbon atoms in face-centered cubic lattice.Then,the spatial distribution of multi-gap carbon atom system and its influence on carbon atom migration and diffusion is carried out.The non-uniform stability of multi-gap atomic system is also demonstrated from the perspective of molecular dynamics.The main conclusions are as follows:The high carbon potential carburizing-quenching-tempering heat treatment on the surface of Fe-Cr2Ni4 alloy was carried out,and a gradient strengthening layer with excellent toughening performance was obtained.For the high carbon Fe-Cr2Ni4 alloy strengthening layer with different tempering time,it shows different strengthening mechanisms,and the competition between martensitic reverse transformation and carbon partitioning strengthening,martensite intrinsic strengthening and second phase strengthening is dominant.The fatigue wear of the relatively low carbon content strengthening layer is mainly local small stripping,the surface of the stripped layer is gradually enlarged,and then the unpeeled area is swallowed.The martensite of the high carbon partitioning layer is better in resisting deformation.The fatigue crack source between the subsurface martensite sheets is also not easily generated.However,the martensite strips of the high carbon precipitation strengthening layer are not deformed substantially,and the fatigue crack source is not easily generated between the lath bundles inside the martensite,but the brittle fatigue crack of the surface layer still exists.Under the contact cyclic load,the fatigue dislocation slowly moves step by step,without directly bypassing the precipitation phase,which leads to a dislocation plug near the precipitation phase with hindering fatigue dislocation slipping.Under the cyclic contact stress field the strengthening effect of the precipitation phase is significantly larger than the traditional condition with static stretching.Due to the intrinsic strengthening of the martensite lath bundle,the fatigue stress between the martensite regions with the original austenite grains as the unit increases.A large fatigue tensile stress in the boundary will be produced and then it will be developed to the fatigue crack along the crystal.By controlling the finer precipitated phase size,strengthen the martensite lath bundle can be obtained with greater plastic toughness which can also avoid grain boundary to show a greater anti-fatigue performance.By the distribution of migration energy of carbon atoms in the interstitial micro-region,the nonlinear migration diffusion path and preferred orientation of carbon atoms between the octahedral gaps in the face-centered cubic iron lattice are obtained.This preference in multi-gap atomic systems still exists.The interstitial atoms arranged in the preferred orientation are in a low energy state,and during atomic migration,the repulsive force between the interstitial atoms will cause them to more easily migrate in the above-described nonlinear manner to diffuse,thereby increasing the diffusion rate.In the Fe-C lattice,during the process of obtaining a deep strengthening layer by diffusion with a high carbon potential,there is still a spatially non-uniform stable state of the interstitial carbon atoms,which further promotes the rapid diffusion of the interstitial carbon atoms in the austenite face-centered cubic lattice.In this paper,from the multi-scales of atomic transition-lattice evolution-tissue transition-stress distribution-macroscopic performance,the relationship among the strength and toughness matching of the strengthening layer,the contact fatigue performance and the microstructure of the high-strength and tough Fe-Cr2Ni4 alloy is discussed.The related results will provides new ideas and theoretical references for the strength and toughness of the gear material,which is of great significance for improving high load capacity of the hard tooth surface of the heavy duty gear under extreme working conditions and prolonging the service life. |
PDF Full Text Request