Studying On Strengthening And Tougheninng Mechanism Of Carburing,Boronizing And C-B Compound Infiltration Of 20CrMnTi Steel With EET Theory

As kind of low alloy steel,20CrMnTi steel is widely used in the fields such as high-speed and medium load gear,axis and piston etc.,with impact and friction at the same time.Surface treatment of carburizing and boronizing must be done for meeting the properties demand of hard outside and toughness inside.Solid powder carburizing and boronizing,boron carbide surface modified treatment were proceeded,the strengthening and toughening mechanism of surface treatment of 20CrMnTi steel was studied by experiment and EET calculation in this paper.Experimental research show that the infiltrated layer microstructure is needle temper martensite after carburizing at the temperature of 900⁹60℃.When carburizing temperature is relatively low,no cemetite exists in the infiltrated layer.Granular cemetite is found in the infiltrated layer with 930℃ carburizing for 8 hours while cemetite appears interrupted net after 960℃ carburizing for 8 hours.When carburizing time is relatively short,no cemetite exists in the infiltrated layer.Granular cemetite is found with 930℃ carburizing for 6 hours while cemetite appears interrupted net after 930℃ carburizing for 6 hours.Boride layer can be formed in the surface of 20CrMnTi steel with boronizing for 6 hours at the temperature of870⁹60℃.FeB phase exists outside of boride layer after boronizing for 6 hours at 930℃,and boride layer becomes thick gradually with boronizing temperature increasing and boronizing time prolonging.Infiltrated layer hardness gradually reduces from surface to inside with carburizing for 4¹0 hours at the temperature of 900⁹60℃,while the hardness gradient of Infiltrated layer reduces gradually with carburizing temperature increasing and carburizing time prolonging.Infiltrated layer hardness increases first and then reduces from surface to inside with boronizing for 4¹0 hours at the temperature of 870⁹60℃,that means Infiltrated layer hardness appears the peak value.The higher boronizing temperature is,the longer boronizing time is,the bigger the boronizing layer hardness is.EET calculation and analysis show that after carburizing of 20CrMnTi steel,the reason of the increase of strength and hardness in the surface lies in the quantity of crystal cells likeα-Fe-C-Cr with bigger binding force increases but the quantity of crystal cells likeα-Fe-Cr lessens in the infiltrated layer,so the quantity of stronger bonds increases.The reason that abrasive resistance increases lies in the precipitation of the phases with bigger binding force like ε-Fe₃C and alloy ε-（Fe₂Cr）₃C in the infiltrated layer.In the mean time,the reason of toughness reducing should be attributed to the phase interfaces with bigger interface electron density difference like α-Fe/α-Fe-C-Cr increasing,the phase interfaces with smaller interface electron density difference like α-Fe/α-Fe-Cr lessening while the interface binding force is small and α-Fe/α-Fe-C-Cr phase interface with bigger interface electron density difference increases.EET calculation and analysis show that the reason that boronizing can improve the hardness and abrasive resistance of surface layer in 20CrMnTi steel lies in the precipitation of phases like FeB and Fe₂B whose binding forces is far bigger than that of the matrix.The reason that the hardness of FeB is bigger than that of Fe₂B lies in its parameters bigger than that of Fe₂B respectively such as the binding force of the strongest bond,the binding forces of the main bonds structure connection and covalence electron density.The reason that the brittleness of FeB is bigger than that of Fe₂B should be attributed to these factors such as the covalence bond space distribution of FeB more nonuniform,having stronger covalency of main bonds structure in FeB while the main bonds of Fe₂B have stronger metallicity.The forming-bond ability of FeB is close to that of Fe₂B,so Fe₂B prior formed easily changes to FeB that makes the brittleness of the boronizing layer in 20CrMnTi steel increases.