| Carburizing technology was one of the most widely used surface strengthening technologies to improve the surface hardness,wear resistance and fatigue performance of bearing and gear parts.The carburizing process was formed by the diffusion and transformation of carbon elements under high temperature conditions.However,its diffusing rate was slow,the structure was relatively coarse and the comprehensive performance and service life were needed to be further improved.Therefore,it is urgent to effectively regulate the forming process,structure and performance of carburizing layer according to the characteristics and problems of carburizing technology.At present,carburizing technology and compound strengthening technology are effective methods to make up for the lack of single carburizing strengthening and improve the comprehensive performance of carburizing layer.In this paper,the vacuum carburizing was the main study subjects.First of all,the two methods of rare earth ion implantation and surface nanocrystallization(supersonic particle bombardment,SFPB)were used used as catalytic treatment to improve the speed,structure and properties of carburizing layer.The effects of SFPB and rare earth implantation on the valence state of elements and structure of vacuum carburizing were investigated,and the mechanism of promoting and strengthening vacuum carburizing was revealed.Then,on basis of the concept of surface composite strengthening,ion implantation technology was used to prepare a composite strengthening layer with gradient structure and properties along the cross-section so as to enhance the wear resistance and fatigue resistance of carburizing layer.Meanwhile,the structure and mechanism of compound strengthening layer were studied systematically.A mixture structure of cryptocrystalline martensite,residual austenite and carbide was formed on the surface of 9310 gear steel at vacuum carburizing temperatures of 925℃ and 905℃.However,when the carburizing temperature was 925℃,the carbon concentration,carbide size and residual austenite content of the carburized layer were higher,and the distribution of the forming phase was more uniform.Thus,the higher surface hardness,and hardness gradient along the section of carburized layer become more slowly.The SFPB method was used to vacuum carburizing for catalytic treatment to accelerate the rate and refine the structure of vacuum carburizing layer.The morphology of martensite was obviously changed,which was a mixed structure of the lath martensite as the main component and fine twin martensite as the auxiliary.The texturization phenomenon of the carburized layer structure was obviously weakened and had better isotropy with the impact of SFPB.At the same time,the hardness and residual compressive stress of vacuum carburized layer was effectively improved.After the SFPB,nanometer crystal with different orientations and a large number of dislocations were formed on the surface of the gear steel,which promoted the short circuit diffusion of carbon elements and formed a large carbonization potential gradient,thus playing a role in promoting the infiltration.The vacuum carburizing process is controlled by implantation of rare earth.Rare earth implantation could accelerate free-carbon diffusion,increase the carburizing speed,obviously refine the structure and make the distribution and orientation of the carburized layer more uniform.The catalytic effect of rare earth implantation mainly was four aspects:Firstly,the La ion by implantation existed in the form of simple substances and sosoloid,which purified the surface of the specimen to accelerate the cracking of C2H2.Secondly,a small amount of rare La2O3 were existed on the the surface of carburized layer,which could occur transformation process of La2O3→LaFeO3 to promote the release of carbon and its diffusion inward.Thirdly,radiation damage and lattice distortion were introduced by the rare earth implantation to provide a short-circuit channel for carbon diffusion during carburizing.Fourthly,the trend of Fe-C bond formation in the austenite was weaken,thus making carbon atoms more inclined to form Cottrell atmospheres and increasing carbon concentration gradient in the carburized layer to promote the diffusion of carbon elements and form LaC2.After rare earth implantation,the surface/section hardness,residual compressive stress and tribological properties of the vacuum carburized layer were significantly improved due to the fine grain enhancement of martensite phase and the second enhancement of carbides.Ti/Zr-N ion implantation technique was used to prepare gradient hardening layer with compact structure and good mechanical properties on the surface of carburized layer.The surface of the composite reinforcement layer was a thin layer rich in hard nano-ceramics phase of TiC,TiN and ZrN,ZrC,respectively.Among them,a large number of ZrO2-C amorphous structure could be formed in Zr-N implantation carburized layer.For Zr-N implantation carburized layer,the radiation damage and lattice distortion of near-surface induced by implantation increased the hardness and residual compressive stress,and the better structure with strong and toughness enhanced the improvement of mechanical properties.The results of contact fatigue test showed that the fatigue life of Zr-N ion implanted composite strengthening layer was 10.2%higher than that of carburized strengthening layer under the condition of 0.95 confidence and 0.5 reliability. |
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