Research On Crushing Behavior Of Alumina Inclusion In Steel During Forging

The existence of non-metallic inclusions in metal parts severely weakens the mechanical properties of the matrix.Therefore,large forgings for high-end equipment have strict limits on the size and distribution of the internal non-metallic inclusions.The current smelting technology cannot completely remove the non-metallic inclusions in the steel,and forging deformation is the main way to break and disperse the large-sized hard inclusions that are more harmful in the steel ingot.Therefore,research on the crushing and dispersion behavior of hard inclusions in forgings during forging can provide corresponding theoretical basis and technical support for solving the problem of forging scrap caused by hard inclusions in production.In this paper,the typical hard Al₂O₃ inclusions in steel are taken as the research object and the sintered Al₂O₃ inclusion samples with different porosity were prepared by hot pressed sintering method.Series of thermal compression experiments under deformation conditions were carried out on sintered Al₂O₃ inclusion samples through Gleeble-3800 thermal/mechanical simulation tester with a special heating and compression device.The relevant thermophysical properties of Al₂O₃ inclusions with different porosity were obtained,and the prediction model of high temperature crushing strength of Al₂O₃ inclusions related to porosity was established.The results show that the porosity of sintered Al₂O₃ inclusions is negatively correlated with sintering temperature.The high temperature crushing strength of Al₂O₃ inclusions is positively correlated with strain rate and negatively correlated with porosity and deformation temperature,and is more sensitive to deformation temperature.Higher Z value or lower porosity will lead to the transformation of fracture mode from intergranular fracture to transgranular fracture.The high temperature crushing strength prediction model of Al₂O₃ inclusion was embedded into the finite element simulation software,and the representative volume model of Al₂O₃ inclusions with intergranular Cohesive elements embedded was established basing on Voronoi structure.Taking the high temperature crushing strength of Al₂O₃ inclusion as the critical criterion of its crushing behavior,the failure parameters of the Cohesive unit were calibrated.The cross-scale sub-model method was used to analyze the influence of various factors on the fracture deformation of Al₂O₃ inclusions in the forgings.The results show that the simulated true stress-strain curves of Al₂O₃ inclusion has the highest coincidence with the experimental true stress-strain curves when the k=2.The increase of deformation temperature or the decrease of deformation speed will promote the crushing of near dense Al₂O₃ inclusions.With the transition from small deformation zone to large deformation zone,the difficulty of inclusion crushing decreases gradually.The more regular the shape of the inclusion and the lower the porosity,the more difficult for it to break in the forging process.Metal samples with Al₂O₃ inclusions were prepared by the combination of powder metallurgy technology and 3D printing cladding technology.A series of forging experiments were carried out on the above samples and metal samples with primary Al₂O₃inclusions under deformation conditions,the effects of various factors on the crushing and dispersion behavior of Al₂O₃ inclusions were supplemented,and the above finite element simulation method was verified.The results show that the higher the porosity of Al₂O₃inclusion is,the more complex the deformation path is,and the more prone it is to crushing and dispersion deformation in the forging process.The process scheme of reducing deformation temperature or increasing deformation speed can significantly improve the crushing and dispersion degree of Al₂O₃ inclusions with higher porosity,on the contrary,it is more conducive to the crushing of Al₂O₃ inclusions with lower porosity.The forging deformation experiments of metal samples containing primary Al₂O₃inclusions further verify the accuracy of the above finite element simulation method and experimental research.Based on the research foundation of Al₂O₃ inclusion crushing and dispersion process,in order to further evaluate the relevant properties of forgings containing inclusions,304L metal samples dispersed with Al₂O₃ inclusions were prepared by sintering.The effects of the content and size of dispersed Al₂O₃ inclusions on the mechanical properties of metal matrix were studied through relevant mechanical property tests.The results show that the tensile damage of specimens with dispersed Al₂O₃ inclusions is positively correlated with the content of inclusions,and decreases first and then increases with the increase of the particle size of inclusions.The increase of inclusion content and particle size will promote the transformation of fracture mechanism from ductile fracture to brittle fracture.In the high temperature compression test,the stress softening index of the samples with dispersed Al₂O₃ inclusions increases with the increase of inclusion content and particle size.With the increase of inclusion particle size,the softening mechanism of the sample changes from the mixed softening mechanism of dynamic recrystallization softening combined with interface crack softening to the interface crack softening mechanism.