The Behavior Of Low Temperature Bainite Transformation Of High Carbon Steel And The Effect Of Tempering On Microstructure And Properties

The purpose of this dissertation is to investigate isothermal transformation kinetics,microstructure characteristic and mechanical properties of medium carbon steel processed by austempering at different temperatures.Different types of bainite microstructures were obtained by different austempering.The microstructures were studied,and the qualitative relationship between microstructures and mechanical properties of various bainite microstructures were studied,and the qualitative relationship between microstructure and properties was established.The isothermal transition curve and the continuous cooling transition curve of the experimental steel were simulated by using J Mat Pro simulation software.Ac1,Ac3 and Ms were measured by DIL402 C thermal expansion apparatus and Gleeble 3800 thermal simulation tester.The phase transformation points Ac1,Ac3 and Ms of the experimental steels were 780 °C,820 °C and 220 °C.The bainite transformation interval is 220~380 °C.When the isothermal quenching at 240~320 °C,quenching at 340~380 °C,bainite structure coarsening and branching.With the increase of isothermal quenching temperature,the hardness and tensile strength decrease.And the yield strength decrease at first.The yield strength first decreases and then increases.After 240 °C isothermal quenching,the hardness and strength were 608 HV and 2137 MPa.With the increase of isothermal temperature,the volume fraction of retained austenite gradually increased.The volume fraction increased from 22.68% to 40.96%.The carbon content of the retained austenite is first increased and then decreased,and the carbon content of the baintic ferrite is gradually decreased.With the increase of the isothermal quenching decreased.With the increase of isothermal temperature,the elongation of the experimental steel increased first,reached the extreme value at 340 °C,and then decreased.With the increase of isothermal temperature,the impact energy of the experimental steel increased first and then decreased,reaching a maximum of 61 J at 300 °C.A series of tempering treatments were carried out on 300 ? and 340 ? samples.After tempering treatment,the strength and hardness of test steels changed little.The hardness of test steels after high temperature tempering decreased.The impact toughness of test steels at high temperature Temper brittleness occurred in the tempering stage.