Microstructure And Property Optimization Of 35CrMo Steel Based On Ultra-Fast Cooling

At present,the hardness of 35CrMo alloy steel bar exceeds the standard,which needs to be solved urgently.This paper is based on the study of gear steel 35CrMo.The hot deformation behavior,the continuous cooling behavior of supercooled austenite,and the influence of rolling process parameters on the microstructure evolution and hardness are studied and analyzed in order to optimize the rolling processes.And then optimize the micro structure and property of 35CrMo gear steel.The main work of the thesis is as follows:(1)The hot deformation behavior of high temperature austenite is studied through single-pass compression experiments,in order to analyze the influence of deformation temperature and strain rate on the flow stress curve.The recrystallization process of austenite of 35CrMo steel is also studied by experiments.(2)The continuous cooling transformation curves of supercooled austenite of 35CrMo steel under static and dynamic conditions are drawn by using the phase transformation point measurement method,which combines thermal expansion method,microstructure observation and microhardness analysis.Obtain the effects of deformation and cooling rate on the transformation of microstructure by data comparison and analysis.The results show that to some extent the deformation can enlarge the transformation range of ferrite and pearlite.However,due to the addition of alloying elements such as Cr and Mo,the critical cooling rates of bainite of 35CrMo steel under dynamic and static conditions are both less than 0.3?/s.(3)According to the GB requests,the heat treatment processes of raw material as rolled bars are carried out by tube furnace.The microstructure evolution and hardness distribution of the 35CrMo steel under different heat treatment processes are compared and analyzed in order to obtain the reason of hardness exceeding standard of alloy steel bar after heat treatment.The addition of Mo element will enhance the stability of the quenched structure so that and the structure becomes tempered martensite after tempering under high temperature,which causes the hardness of experimental steel exceeds the standard after heat treatment.(4)Thermal simulator is used to simulate different rolling and cooling process schemes,in order to study the influence of various process parameters on microstructure evolution and hardness distribution.It has been proved that the ultra-fast cooling process can refine grains obviously,obtain the ferrite structure with higher volume fraction,and reduce the hardness of the experimental steel significantly.(5)Two-stage rolling deformation can achieve the purpose of promoting austenite recrystallization and grain refinement.Through the process scheme of deformation-ultrafast cooling-deformation,The hardness of 35CrMo gear steel is successfully reaches the requirements for alloy structural steels of GB/T 3077-2015.