Molecular Dynamics Simulation On Surface Nanocrystallization And Carburizing Process Of Low Carbon Steel

The use of surface nanocrystallization technology to prepare nanometer layers on the surface of metal materials can effectively improve the surface properties of metal materials and extend their service life.Nanocrystallization pretreatment before carburizing experiments can reduce the carburizing temperature,increase the carburizing rate,prolong the service life of equipment and workpiece fixtures,increase the hardness of parts and play a role in composite strengthening.However,the mechanism of the microscopic catalytic effect of nanocrystallization pretreatment on carburizing is not yet clear and it requires further research.In this paper,surface nanocrystallization of low carbon steel 20Cr2Ni4 WA was pretreated by supersonic fine particle bombarding(SFPB)and carburization experiments were carried out at different temperatures.Optical microscopy and transmission electron microscopy were used to observe the microstructure of the surface and cross-section of the material.The phase structure of the nanolayer was analyzed using X-ray diffractometry.And the surface hardness+ and cross-sectional hardness of the material were measured with a micro-hardness tester.Then the effect of nanocrystallization on the microstructure,properties,and carburization process of the specimens was analyzed.Nanocrystallization processes was simulated by using molecular dynamics simulation software Lammps to analyze the formation mechanism of microscopic defects during nanocrystallization.The diffusion process of carbon atoms in defects was simulated by using Materials Studio to analyze the effect of defect structure on carburizing process.The results show that the supersonic particle bombardment treatment can form a nanometer layer with a grain size below 30 nm on the surface of the low carbon steel20Cr2Ni4 WA,refine the microstructure and significantly increase the surface hardness of the material.In the nanometer simulation process,the plastic deformation occurs on the surface of the material under the external force and the crystal defects formed internally.Vacancies,a/2<111>-type,2/a<110>-type and a<100>-type dislocations are successively generated.The number of defects gradually increases with the increase of bombarding time and bombarding speed.Compared with ordinary carburizing,nanocrystallization pretreatment helps increasethe thickness and carburizing rate of the carburized layer,which refines the carburized layer structure,increases the hardness of the carburized layer and achieve the same carburized layer structure at a lower carburizing temperature.In the process of simulating carburizing,the diffusion coefficient of carbon atoms increases significantly due to the presence of defects.And the promotion effect increases with the increase of defects.The promotion effect of the defect types on carbon diffusion from the largest to the smallest is grain boundaries,dislocations and vacancies.In defect structure,the diffusion capacity of carbon atoms increases with increasing temperature and decreases with increasing pressure.