Research On Heat Treatment Technology,Microstructure And Mechanical Properties Of Carburizing Bearing Steel For High-speed Railway

The high-speed railway has been developing rapidly in our country,but the high-speed railway bearing with the speed of over 200 km/h is entirely dependent on imports.It is of great significance to independently research and develop high-speed railway bearing steel technology and realize localization of key bearing materials for high-speed railway trains with speed of 200?250 km/h.Based on the National High Technology Research and Development Program(863 program):key technology development of bearing steel for major equipment,the heat treatment process of 20CrNi2MoV carburized steel for high-speed railway bearing was studied.Through the reasonable matching of microstructure and mechanical properties for carburizing layer and core,the heat treatment after carburizing process was obtained.The heat treatment technology of secondary quenching+cryogenic treatment+low temperature tempering was put forward,which further improved the wear resistance and contact fatigue performance of the experimental steel,and the heat resistance and strengthened durability of the test bearing passed the bench test.The main works are as follow:(1)On the basis of G20CrNi2MoA steel used for common railway bearing,the chemical composition of 20CrNi2MoV steel used for high-speed railway bearing was optimized according to the general idea of decreasing Cr and increasing Ni,adding microalloying elements Nb and V.The chemical composition(mass fraction)was as follows:C:0.18%?0.24%,Si?0.2%,Mn?0.2%,P?0.01%,S?0.005%,Cr:0.4%?0.8%,Ni:1.5%?2.5%,Mo:0.1%?0.3%,Nb:0.02%?0.05%,V:0.1%?0.3%,the rest of iron.(2)An improved heat treatment before carburizing process was proposed.The billet after forging was forced air cooled(cooling rate was more than 10 ?/s)to 650?700 0C,then cooled to 450?500? in furnace,finally cooled to room temperature in air.After carburizing pretreatment process,the banded structure grade of billet was less than 1.5,and the microhardness was 237?247 HV.The improved heat treatment before carburizing process improved the microstructure uniformity and cutting performance.(3)Different carbon contents in the carburizing layer and core of the experimental steel lead to different phase behaviors.Therefore,the phase transformation behavior of carburized layer and core was studied respectively.The Ac1 and Ac3 temperatures of carburizing layer were 762? and 812? respectively,the Ac1 and Ac3 temperatures of core were 768? and 838? respectively.On the condition of oil cooling(60?/s),with increasing quenching temperature from 820? to 900?,the Ms of carburized layer was to lower from 395? to 364?;the Ms of core was down from 676? to 554?.(4)The microstructure and mechanical properties of the heat treatment after carburizing of the experimental steel were studied.Through the reasonable matching the microstructure and mechanical properties of the carburizing layer and core,the heat treatment after carburizing process(secondary quenching+low temperature tempering,Q+T for short)was obtained as follows:the secondary hardening temperature was 900? and holding time was 45 min,the tempering temperature was 180? and tempering time was not less than 2 h.The mechanical properties were:microhardness of carburized layer was 825 HV;core yield strength was 1089 MPa,tensile strength was 1305 MPa and impact energy was 92 J.(5)The process of adding cryogenic treatment between secondary quenching and low temperature tempering was proposed.The effect of the secondary quenching +-80 ?cryogenic treatment+low temperature tempering(Q+C80+ T)and the second quenching+-196? cryogenic treatment+low temperature tempering(Q+C196+ T)processes on microstructures and mechanical properties of the carburized layer and core has been investigated.Compared with Q+T process,the retained austenite content in the carburized layer was decreased by 13.3%and the carbides content was increased by 3.8%under Q+C80+T process.The ratio of high angle grain boundary in core was increased by 9.61%and the retained austenite in core was decreased by 4.8%.The mechanical properties of experimental steel under Q+C80+T process are as follows:hardness of carburized layer was 848 HV;The yield strength and tensile strength of the core were 1180 MPa and 1530 MPa,the hardness of the core was 407 HV,and the impact toughness and fracture toughness of the core were 86.4 J and 121.8 MPa·m1/2,respectively.Compared with Q+T process,the hardness of carburized layer was increased by 3.7%,the hardness of core was increased by 10.9%,yield strength and tensile strength of the core were increased by 8.4%and 16.9%respectively,but the impact toughness and fracture toughness of the core were decreased by6.6%and 4.3%respectively.Compared with Q+T process,the retained austenite content in the carburized layer was decreased by 14.8%and the carbides content was increased by 8.9%under Q+C196+T process.The ratio of high angle grain boundary in core was increased by 12.32%and the retained austenite in core was decreased by 5.6%.The mechanical properties of experimental steel under Q+C80+T process are as follows:hardness of carburized layer was 886 HV;The yield strength and tensile strength of the core were 1250 MPa and 1550 MPa,the hardness of the core was 410 HV,and the impact toughness and fracture toughness of the core were 84.2 J and 122.1 MPa·m1/2,respectively.Compared with Q+T process,the hardness of carburized layer was increased by 8.3%,the hardness of core was increased by 11.7%,yield strength and tensile strength of the core were increased by 14.8%and 18.5%respectively,but the impact toughness and fracture toughness of the core were decreased by 8.9%and 4.1%respectively.Q+C80+T and Q+C196 +T processes promoted microstructure refinement and carbides precipitation,reduced the retained austenite content,thus significantly improved the microstructure.The mechanical properties of the experimental steel met the requirements of high-speed railway bearing steel.(6)The wear behavior of the experimental steel under different heat treatment after carburizing was studied.Compared with Q+T,the wear rates of Q+C80+T and Q+C196+T samples were reduced by 17%and 25.5%respectively.Under the process of Q+C196+T,the precipitation of carbides in the carburized layer reduced the content of carbon and alloying elements in the matrix.In addition to improving the hardness of the carburized layer,the toughness of the martensite matrix was also improved,thus greatly improved the wear resistance.The wear rate is reduced to 1.17 × 10-5 mm3/m for Q+C196+T samples.(7)The contact fatigue properties of the experimental steel under different heat treatment after carburizing were studied.Compared with the Q+T process,the rated life L10 and median life L50 of the experimental steel under Q+C196+T processing were both improved,which were 0.198949 × 108 times and 1.095821 × 108 times,respectively.The slope parameter b of weibull distribution decreased from 1.2658 to 1.1041,and the stability of fatigue life was enhanced.(8)The bench test of high-speed rail bearing samples under Q+C196+T process was carried out.The duration of thermal properties experiment was 240 h and the highest temperature of bearing was below 90?.The duration of durability strengthening test was 384h,and the bearing ran smoothly.The bearing has passed the thermal performance and durability strengthening performance assessment,and has met the requirements of high-speed rail bearing.