Investigation On The Microstructure And Property Of An Aero Bearing Steel During Vacuum Low Pressure Carburizing And Subsequent Heat Treatment

Aero bearing martensite stainless steel must be carburized to improve surface hardness and wear resistance.Most of the existing studies have focused on the low temperature(<550℃)carburizing process,that is,the martensite or austenite matrix with over-saturated carbon can be obtained by inhibiting the precipitation of carbide.However,its disadvantages are that the depth of the carburized layer is shallow and the hardness gradient is large,which cannot well meet the needs of aero bearing steel.In recent years,vacuum low pressure carburizing process has gradually replaced conventional gas carburizing process because of its advantages of high carburizing temperature,high efficiency and avoiding internal oxidation and surface carbon black.For a new generation of aero bearing with 15Cr14Co12Mo5Ni martensitic stainless steel,this paper adopts the method of vacuum low pressure carburizing process was carried out based on the thermodynamic calculation Thermo-Calc carburizing process design,carburizing heat treatment,the microstructure evolution process,the heat treatment process(secondary quenching,the spheroidizing annealing)on microstructure and performance of carburizing and other research,aimed at the aero bearing steel vacuum low pressure carburizing principle and technical guidance,provide the basis for the industrial production of the main research content is as follows:(1)Based on the service performance requirements of 15Cr14Co12Mo5Ni steel,Thermo-Calc software was used to carry out the cementation process design.Through thermodynamic calculation,the variation rules of key parameters including phase composition(type and fraction),chromium content in austenite,chromium content in each phase with carbon content were obtained,so as to determine the optimal surface carbon content range at different carburizing temperatures to ensure the corrosion resistance of carburizing surface.In addition,in order to obtain sufficient stability of matrix structure and significant secondary hardening effect,post-seepage heat treatment adopts the process route of "secondary quenching+cryogenic cooling+high-temperature tempering".(2)The microstructure evolution of the whole process of carburizing heat treatment"vacuum low pressure carburizing+secondary quenching+cryogenic cooling+tempering+cryogenic cooling+ tempering" was studied.By means of stripping and carbon analysis,the trend of carbon content decrease with the depth of the carburizing layer was determined.By means of SEM,EBSD and XRD,the change rule of microstructure(carbide,phase composition and fraction,etc.)with the depth of permeation layer and heat treatment process was analyzed.The results show that,compared with the carburizing process,the secondary quenching can increase carbon content in austenite and the residual austenite fraction after quenching.By optimizing the surface carbon concentration,the surface hardness of the permeable layer can be fully improved while avoiding a lot of residual austenite.(3)The influence of key heat treatment processes including secondary quenching and spheroidization annealing on the microstructure(carbide,phase composition and fraction,etc.)and properties of the permeable layer was studied.The results show that carbon content and residual austenite in the hardened austenite can be increased by increasing the secondary quenching temperature,which can effectively eliminate the network carbide formed in the carburizing process and increase the depth of the effective hardening layer.The spherification process transforms the reticular carbide formed in the carburizing process into coarse carbide,which increases the effective depth of the carburizing layer and improves the surface hardness of the carburizing layer.