Modification And Refinement Of Bearing Steel With Graphene Coated Multiphase Nano Ceramic Particles

As the most typical bearing steel,GCr15 high carbon chromium bearing steel has excellent mechanical properties,such as high fatigue strength,high hardness and wear resistance,good impact toughness and dimensional stability,etc.GCr15 bearing steel is mainly used for the preparation of rolling bearings and rolling rings.Bearing is known as"the joint of equipment",widely used in road vehicles,precision equipment,high-speed train,aerospace and large energy machinery and other important industries and fields.The stability of structure and performance of bearing steel directly determines the safety and efficiency of equipment.Improving the performance of GCr15 will promote the development of high-end bearing industry,thus greatly promoting the development of modern industry.In this paper,the microstructure of GCr15 will be optimized by inoculation to improve the comprehensive mechanical properties of GCr15.Based on the principle of material thermodynamics,Nb C-Ti C@Graphene/Fe and VC-Ti C@Graphene/Fe inoculants were prepared by in situ reaction and high energy ball milling.The average grain size of the inoculants was below 30 nm,and the ceramic nanoparticles were surrounded by graphene and amorphous components.The ceramic nanoparticles in the inoculant have high surface energy and strong adsorption capacity.The adsorption of inclusions by ceramic nanoparticles reduces the content of inclusions in the grain boundary.The in situ ceramic nanoparticles are based on iron and have good wettability with GCr15,which makes them nucleate uniformly in molten steel.In this thesis,the optimization mechanism of inoculation treatment on the as-cast and heat treated structure of bearing steel was studied.The nucleation rate of as-cast structure was increased by inoculation treatment,and the pearlite layer spacing and grain size were significantly reduced.After forging and annealing,the best heat treatment process was obtained by quenching and tempering at various temperatures,and finally the fine aphanitic martensite structure was obtained.After inoculation,the grains were refined,the phase composition was changed,and the Cr₇C₃ with better properties was produced.In the inoculant,graphene reacts with Cr in situ to form Cr₇C₃,and a small amount of austenite exists at the edge of the particles.The austenite in GCr15 after inoculation is distributed around the martensite sheet,which greatly improves the impact toughness.It is found that Fe Cr phase,martensite phase and Cr₇C₃phase are coplanar.Fe Cr nucleates on Cr₇C₃,and martensite can nucleate on Cr₇C₃ and Fe Cr.In this paper,the relationship between the macro-performance and microstructure of GCr15 bearing steel after inoculation and heat treatment was studied.After the modification and refinement of two inoculants,the structure and properties of GCr15were optimized and improved,while the structure and properties of bearing steel with Nb C-Ti C@Graphene/Fe inoculant were better.After inoculation and heat treatment,the hardness of GCr15 increased from 59.9 HRC to 61.2 HRC.Under the same experimental conditions,GCr15 contains more ceramic particles,resulting in higher hardness and easier resistance to friction and wear.The test results showed that the wear rate decreased by 0.2155%.The tensile strength of GCr15 increased by 259.18 MPa and reached 2247 MPa after inoculation.After inoculation,the impact toughness of bearing steel increased by 150.91%to 125.33 J/cm².