Bainite Structure Control And Performance Optimization Of 42CrMo Steel

In this paper,the chemical composition,heat treatment process,microstructure characterization and performance testing of the experimental steel 42 CrMo are studied.Jmat-pro software is used to examine the 42 CrMo steel cooling transformation process.Based on this,the continuous and isothermal cooling transformation curves of the experimental steel is measured.The microstructures and characteristics under different cooling rates and isothermal temperatures are observed and analyzed by using OM,SEM,hardness tester and other equipments.In particular,the relationship of bainite transition interval,types,characteristics and content of bainite with the hardness is obtained.Metallographic structures are controlled through the design of heat treatment process,the relationships between the microstructure and hardness,toughness,wear resistance are established.The results of the continuous cooling transition curve of the experimental steel 42 CrMo show that Ac1 is 743°C and Ac3 is 792°C.In the range of experimental cooling rate,there are four transition zones of ferrite,pearlite,bainite and martensite.When the cooling rate is greater than 3°C/s,heterogeneous microstructure which characters of sheaves of upper bainite and plates of lower bainite are obtained.With the increasing of cooling rate,the volume fraction of martensites increased,and the content of bainite decreased in the mixed structures of bainite and martensite,the hardness is also on the rise.When the cooling rate is 20°C/s,dual phase structures of a martensitic matrix and(3%~5%)bainite are obtained.The graph of isothermal cooling transformation show that the isothermal temperature range is from 410℃ to 500°C for upper bainite transformation,and the temperature of lower bainite transformation is between 310°C and 410°C,which microstructure illustrated the composite of spiculate bainite(and granular bainite)and lath martensite.With the decrease of the isothermal temperature,the content of martensite increased.The large number of needle-like weissner structures between 560°C and 590°C are related to the uneven microstructure and coarse grains of the experimental materials.According to the experimental results of quench-tempering heat treatment process,the quench heating temperature is 820°C and 18% water-based quench medium is adopted for cooling.The microstructure is mainly composed of martensites matrix with the volume fraction of 20.3.% lower bainite.After tempering at 560°C,its comprehensive mechanical properties are well matched.The results of isothermal heat treatment show that the complex structure of lower bainite and martensite could be obtained during keeping at the temperature between 320°C and 380°C.A large number of acicular bainite are obtained during isothermal heat treatment at 320°C which the time is more than 3 min,and the hardness of bainite/martensite mixed structure is greater than 45 HRC which possessing better wear resistance.The microstructure is characterized by acicular bainite at 350°C isothermal temperature for 1 min,with a hardness of 38 HRC and expressing excellent toughness.With the increase of isothermal temperature,the shape of bainite changed from tiny plates to bundles,and the bundle size increased with the extension of isothermal time led to the decrease of hardness and toughness.