Study On Microstructure Evolution And Tempering Process Of 18Cr2Ni4WA Steel By Vacuum And Low-pressure Carburizing

18Cr2Ni4WA steel is a low-carbon alloy steel with excellent toughness and excellent combination.After carburizing,the steel has the characteristics of external strength and internal toughness.It is widely used to manufacture engines and other large machinery under high-speed and heavy-load conditions.In this paper,a series of studies are conducted on the evolution of the microstructure of the carburized layer during vacuum low-pressure carburizing and postcarburizing heat treatment of 18Cr2Ni4WA steel.Experiments are performed on the carburizing process at high and low carbon concentrations and the corresponding post-carburizing heat treatment process.The evolution of the layer structure and hardness during heat treatment,and the thermodynamic evolution of the layer structure during high temperature tempering was calculated.The main research results are as follows:(1)Study the grain growth behavior of 18Cr2Ni4WA steel under different heating conditions.The results show that the grain size grows from 7.8 ?m to 10.0 ?m at a temperature of 850?,and the grain grows slowly;at a temperature of 1050?,the size of the grains grows from 23.1 ?m to 35.3 ?m.and the grain grows faster.Therefore,the optimum vacuum lowpressure carburizing temperature of 18Cr2Ni4WA steel was determined to be 950?.(2)The relationship between the microstructure evolution and hardness changes during the carburizing and quenching-low temperature tempering of the low-carburized layer was studied.The results show that the microstructure after carburizing and quenching is similar to the microstructure after carburizing and quenching+low-temperature tempering;the maximum reduction of the surface hardness of the carburized layer after low-temperature tempering is significantly reduced from HV691 to HV621.During the low temperature tempering process,the decrease of martensite lattice distortion contributes to the hardness of the carburized layer,so that the hardness of the carburized layer is significantly reduced.(3)The evolution of the microstructure and properties of the carburizing and quenchinghigh temperature tempering-second quenching-cold treatment-low temperature tempering process of the high-carburized layer was studied.The results show that the structure is coarse and contains a lot of retained austenite after carburizing and quenching,and a large amount of cementite precipitates after tempering at high temperature.Partial cementite is dissolved after second quenching,and the retained austenite is less than that after second quenching.The maximum hardness value of the surface of the carburized layer changed from HV678 to HV353 to HV713 and then to HV745.The final maximum hardness of the carburized layer was HV717 and the effective hardened layer depth was 1.8 mm.(4)The driving force of the carburized layer structure during the high temperature tempering process was calculated.The calculation results show that the driving force for martensite decomposition to ferrite and cementite is-1.268 kJ/mol,the driving force for residual austenite decomposition ferrite and cementite is-0.499 kJ/mol;The driving force for the transformation core of cementite is-8.235 kJ/mol,the driving force for the transformation core of residual austenite to cementite is-1.835 kJ/mol;the driving force for transformation of residual austenite to martensite is 0.794 kJ/mol.It is thus obtained that during the hightemperature tempering process,only the decomposition reaction of the parasite occurs,and no martensite transformation occurs;during the high-temperature tempering and cooling process,the residual austenite transforms to martensite.