Research On The Vacuum Catalysis And Performance Improvement Mechanism Of Composite Rare Earth Pretreatment Of 20Cr2Ni4A Steel By Supersonic Fine Particles Bombarding

20Cr2Ni4A steel is a high-quality low-carbon alloy steel that is widely used as a key transmission component in mechanical equipment under harsh working conditions such as heavy load and high-speed after carburizing treatment.However,the comprehensive performance,production efficiency,energy conservation and consumption reduction of the existing carburization treatment process for 20Cr2Ni4A steel are no longer able to meet the increasingly stringent service performance requirements and sustainable development concept of transmission parts with the rapid development of the mechanical manufacturing industry and the upgrading of mechanical equipment.Therefore,it is urgent to develop new carburizing technologies and processes that improve the service performance of transmission parts while further improving carburizing efficiency and reducing production energy consumption.This article starts with the pretreatment technology of supersonic fine particles bombarding（SFPB）nano composite rare earth and vacuum carburization technology.Taking advantage of surface nanomaterialization and rare earth catalysis,various new pre-treatment processes of supersonic fine particles bombarding（SFPB）nanoization-rare earth composite have been designed to establish an optimized combination of rare earth and surface mechanical nanoization catalysis mechanism.The mechanism of the promotion of carburization and performance improvement under 900°C×240 min vacuum carburization conditions by high strain rate nanomaterialization and different introduction pathways and survival states of rare earth elements has been studied.The influence of SFPB pretreatment process characteristics on the formation mechanism of gradient nanocrystalline layers has been studied.A three-layer gradient nanocrystalline layer composed of a severely plastic deformation layer（MPD）,a slightly plastic deformation layer（SPD）and a matrix layer（Matrix）obtained through pretreatment.The average grain diameter at the top of the severe plastic deformation layer is 17 nm.The expansion of plastic deformation towards deep layers has a step evolution.Dislocations propagate towards deeper layers along sliding steps after the accumulation of grain dislocations leads to a decrease in orientation factor.Based on Gaussian fitting,it is confirmed that with the development of plastic deformation,the deformed grains form a preferred orientation with low index crystal planes（001）perpendicular to the surface distribution.The formation mechanism of nanocrystals under SFPB conditions is as follows:dislocation proliferation and entanglement form dislocation walls and cellular dislocations;The movement of dislocations causes the formation of layered grains parallel to the surface in the cellular structure;The formation of dislocation walls,adiabatic shear bands and twinning divides the layered grains into subgrains;The increase in orientation differences between subgrains leads to the formation of two-dimensional nanocrystals;The stacking of two-dimensional nanocrystals forms nanoscale layered grains parallel to the surface,which are further divided by subgrain boundaries such as adiabatic shear bands and twin boundaries until smaller nanocrystals are formed;The dynamic recrystallization of some deformed grains with high energy storage leads to further reduction of nanocrystalline size.The influence of gradient nanocrystalline layer structure and its evolution mechanism during austenitization on carbon atom diffusion was studied.Statistical methods were used to analyze the distribution of carbides in SFPB pre-treatment samples.It was found that after SFPB pretreatment,the carburized layer thickness（effective hardening layer depth）and the number of carbides in the vacuum carburized 20Cr2Ni4A steel at 900°C significantly increased,and the size of carbides significantly decreased.The permeation promoting mechanism of SFPB pretreatment is as follows:Firstly,the adsorption of activated carbon atoms on the surface is enhanced by relying on rough surfaces,sliding steps,and Fe₃O₄ nanoparticles;The second is that the defects such as grain boundaries and dislocations generated during the nanoscale process increase the channels for rapid diffusion of carbon atoms;The third is the preferentially oriented ferrite grains formed during the surface nanocrystallization process,which are transformed into austenite with the densest arrangement plane perpendicular to the surface during the austenitization process.The spacing between the densest arrangement planes is the largest,which is conducive to the diffusion of carbon atoms.The formation of lanthanum-bearing nanocrystalline layers and their effects on carburizing behavior and performance improvement were studied.After lanthanum-bearing SFPB pre-treatment,a lanthanum-bearing nano alloy layer with gradient structure was obtained on the surface of 20Cr2Ni4A steel.Based on the distribution of La element,it has been confirmed that the diffusion of La element is consistent with the process of nanomaterialization.The rate of La element diffusion towards deeper layers gradually increases with the formation of surface nanocrystalline layers.La₂O₃ first undergoes fragmentation and cold welding on the surface of the sample,forming submicron particles.At the same time,the sliding steps,grain boundaries,and dislocation walls formed during the nanoscale process promote the diffusion of La elements into deeper layers.After pre-treatment with lanthanum-bearing SFPB,the thickness of the carburized layer increased by 18.4%.This is mainly due to the improved surface adsorption of lanthanum-bearing SFPB and the provision of fast diffusion channels,thereby promoting the deep diffusion of carbon atoms.The vacuum infiltration effect and wear behavior of LaFeO₃ perovskite/nanocrystalline composite pre-treatment layer were experimentally studied.After composite pre-treatment,a composite layer with deep submicron particles LaFeO₃ and nanostructures was formed on the surface of 20Cr2Ni4A steel,which increased the thickness of the vacuum carburized layer by23.4%at 900°C.Based on the surface valence state of the carburized composite layer,the accelerating mechanism of the composite layer was analyzed.The oxygen vacancies provided by LaFeO₃ increase the surface activity of the sample,as well as the increase in Fe³⁺caused by composite pre-treatment,which is the main reason for the increase in surface adsorption and the thickness of the carburized layer.The wear mechanism of the presence of composite layers on different layer depths was studied using wear test results of eutectoid layer surfaces at different depths.The wear mechanism from surface to interior changes from abrasive,delamination,and adhesive wear to oxidation and adhesive wear.The presence of rare earths promotes the formation of surface oxides,and the oxygen vacancies provided by LaFeO₃perovskite improve the anti stripping and crack resistance of the worn surface oxide film,thereby reducing the risk of delamination and improving surface wear resistance.