Study On The Correlation Between Microstructure And Corrosion Resistance Of New Bearing Steel

In this thesis,microalloyed bearing steel GCr15Si1 Mo V is designed by adding alloying elements Si,Mo and microalloying element V,on the basis of traditional bearing steel GCr15.The designed new bearing steel was isothermal treated near Ms temperature,and the isothermal martensite and low-temperature bainite were obtained.The austenization temperature was increased to 1100℃ on the basis of the original heat treatment to analyze the influence of different heat treatment processes on the material properties.The microstructure of microalloyed bearing steel was characterized by SEM and XRD,and the relationship between microstructure and mechanical properties of microalloyed bearing steel was analyzed by combining a series of mechanical properties experiments such as hardness,impact and tensile.The corrosion resistance of the experimental steel was compared and analyzed in the simulated seawater solution(3.5%Na Cl),and the polarization curve,electrochemical impedance spectrum and Bode diagram of each experimental steel were obtained.The corrosion potential,corrosion current density and impedance were obtained by Tafel fitting and equivalent circuit fitting.The relationship between specific phase composition and corrosion resistance of bearing steel is analyzed based on the results of microstructure experiment,and the influence of austenitizing temperature on corrosion resistance of materials is further analyzed.The experimental results of microstructure and mechanical properties show that a s mall amount of bainite phase and some residual austenite exist in the microstructure of i sothermal martensitic bearing steel with a long time isothermal.The content of residual austenite in low temperature bainite bearing steel is significantly higher than that in isoth ermal martensite bearing steel.SEM images show that there are a small amount of undis solved spherical carbides on both surfaces,and the content of undissolved carbides decre ases.Compared with martensite bearing steel,low temperature bainite bearing steel has higher residual austenite content,and the bainite structure itself has good plasticity and t oughness,so the low temperature bainite bearing steel has both high toughness and high strength while maintaining high hardness.With the austenitizing temperature rising to 1100 ℃,the grains of the experimental steel are relatively coarse,and the carbon content in the matrix increases.Although the hardness of the experimental steel is improved,the plastic toughness is poor.The electrochemical experiment results show that the corrosion current density of low-temperature bainite bearing steel is lower than that of martensitic bearing steel,and its impedance value is much higher than that of martensitic bearing steel.This indicates that the bainite structure at low temperature has better corrosion resistance.With the increase of austenitizing temperature,the corrosion resistance of the experimental steel is not improved although the surface carbide is reduced.The main reason is that the increment of austenitizing temperature increase the secondary precipitates,leading to the enhancement of intergranular corrosion.