Research On Flakes' Generation Mechanism By Macro-meso Cross Scale Methods In Heavy Forgings

The research on the flakes of large forgings has been for many years at home and abroad.All free forgings that need more than 1 000 tons of forged hydraulic press can be called large forgings.Large forging is the core components of large complete sets of equipment,the succeed product rate of heavy forgings is lower,which belonging to the State Science and Technology Development Plan proposed urgent need to address cutting-edge technology.When heavy forgings in high-temperature forming and cooling process,the hydrogen will be separated out and aggregated in forgings internal micro defects and lead to cracks initiation because of high hydrogen pressure,which called hydrogen embrittlement(also called flakes).Flakes is an internal crack in steel.During the process of high temperature forming and its cooling,the hydrogen evolution of the steel causes the molecular hydrogen to precipitate and segregate into the microdefects and microcracks inside the forging.Large forgings are often used in critical locations of large equipment,with high loads and large work intensity.So,flaks produced by the high pressure and micro-cracks can easily make the whole forging fracture.Therefore,the white spots are regarded as "cancer" for large forgings.The initiation of flakes in large forgings involves a series of complex problems such as the concentration of stress in the mesophase grains and the cohesion of hydrogen atoms between the grains.Macro finite element is difficult to solve these problems.So,the establishment of mesoscopic model is very necessary.Large forgings are large in size,and the mesoscopic model is computationally complex and the calculation of the large area is relatively unstable.Therefore,by means of representative volume element(RVE),we connect the macro scale and mesoscopic scale.The simulation results of the dangerous area in the macroscopic model are put into the mesoscopic model as the boundary condition.In this way we have deeper simulation study,which is called as macro-meso cross scale research.In the establishment of mesoscopic simulation model,Voronoi microstructure model and crystal plastic finite element method are used.In the finite element software ABAQUS,the crystal plastic finite element method is combined with the finite element simulation by means of the preparation of the material subroutine UMAT.The VASP is used to calculate the microscopic parameters required for the calculation of the plastic finite element of the finite element method based on the density functional theory and the method of plane plane wave.In the aspect of microstructure modeling,the microstructural uniformity control and grid parameterization are realized,and the computational complexity and instability of the microstructure model can be reduced effectively.Two-dimensional and three-dimensional Voronoi microstructure simulation can be realized.According to the hydrogen pressure theory,hydrogen and internal stress are the main factors of white point.In this paper,Cr5 steel,a typical steel forging type,is selected as the research object.The thermo-physical parameters of Cr5 material are measured by advanced modern materials such as Gleeble 3500 and TC-7000.And the simulation process of forging heat treatment of large forgings was carried out.The microstructures and residual stresses of the forging heat treatment were obtained.The stress sensitive area under the WHF method for forging was analyzed by using the macro-meso-scale cross-scale simulation method.The risk region of the shear slip between the individual crystal grains calculated by the mesoscopic model can be echoed with the macroscopic model.Under the problem that the hydrogen content in large forgings is difficult to be accurately measured,the hydrogen diffusion coefficient of Cr5 steel is studied by electrochemical hydrogenation.The hydrogen bubbling and fracture morphology of hydrogen after hydrogen filling are observed by scanning electron microscopy.The mechanical properties and meso-morphology of the large forgings material under hydrogen conditions were analyzed.And a hydrogen-diffusion model with macro-scale and a hydrogen trap model under mesoscopic horizon are established.The study on the expansion and expansion of hydrogen in metals is studied.After the combination of microstructure observation of Cr5 specimen after electrochemical hydrogen charging and the progress of research at home and abroad,it is put forward that the hydrogen bubbling of Cr5 steel is closely related to the surface oxide layer.The depth of the hydrogen bubbling is the thickness of the oxide layer,which is generated between the oxide layer and the Cr5 specimen.Based on the size observed by mesoscopic scanning electron microscopy,a mesoscopic model of hydrogen bubbling initiation and expansion was established.The initiation and expansion of hydrogen bubbling has been studied.Hydrogen bubbling produced by electrochemical hydrogenation is an extreme manifestation of hydrogen embrittlement.To study the initiation of hydrogen bubbling,but also to guide the initiation of white spots.The extended finite element method was used to simulate the initiation of white spots in the large forgings.The crack propagation of different orientations was studied.The finite element method was used to study the microstructures of the microvoids in the Voronoi microstructure model.Hydrogen internal pressure state simulation simulation.The shear strain and slip shear stress of 12 slip systems in Cr5 steel are obtained.The prediction of the crack propagation direction of circular holes was realized by using the plastic plastic finite element method.