Study On Thermal Deformation Behavior And Strengthening And Toughening Mechanism Of High Strength And High Permeability Carbon Steel C61

This work mainly focued on the hot deformation behavior and strengthening and toughnening mechanism of a new high strength, toughness carburizing gear steel-C61. The high temperature thermoplastic features were investgated using Gleeble-3800 thermal simulation testing machine. The mechanical properties, evolution of microstructure and mechanism of strengthening and toughening were explored by means of mechanical performance testing, optical microscope (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and physical and chemical phase, which lays a theoretical and pratical foundament for the engineering application of caburizing gear steel-C61. The main results obtained are as follows:The activation energy Q of hot deformation of the testing steel was 414.543kJ/mol. Based on the high temperature flow stress curve, the hot deformation equationwas established, the relationship between dynamic recrystallization critical strain and deformation temperature and strain rate was constructed, and Relationship between mean size of dynamic recrystallization grain and Z parameters was established at a temperature range of 850～1150℃ and strain rate range of 0.01～10s-1. The hot deformation map was established, which was based on the dynamic material model.The existence of undissolved M6C and M23C6 carbides in matrix after quenching at austenizing temperature lower than 950℃ caused the reduction of strength and toughness of tested steel. On the other hand, the carbides particles would dissolve ataustenizing temperature higher than 950℃ but the prior austenite boundarycoarsened. When quenched at temperature higher than 1000℃, the prior austenite boundarycoarsened severely leading to the rapid reduction of toughness. Therefore, the appropriate austenizing temperatures ranged from 950～1000℃.The optimum heat treatment system of the testing steel was oil colding after heat preservating at 950℃ for 1 hour, air colding after cooling treatment at-73℃ for 1 hour and air colding after tempering at 482℃ for 16 hours. The excellent combination of strength and toughness of tested steel was achieved under these conditions. The ultimate tensile strength of tested steel treated at this process was 1625MPa with the yield strength reaching at 1556MPa and the Sharpy V impact energy was 82J. The fine prior austenite boundary and dispersive precipitation of numerous M2C carbides in lath martensite with high dislocation density primarily contributed to the excellent combination of strength and toughness. Meanwhile, the precipitation of thin film of reverted austenite in martensite matrix played an important role to promote the toughness of tested steel.Carburizing and austeniting process of C61 steel can be carried out simultaneously. The surface hardness of C61 steel was 750HV after carburizing at 1000℃ for 4 hours, direct quenching, cooling 1 hour in liquid nitrogen, and tempering at 482℃ for 16 hours. The matrix structure of surface was lamellar martensite; the matrix structure of transition layer was mixed microstructure of lath martensite and lamellar martensite; the matrix structure of the center was lath martensite and hardness was 485HV. The thickness of the carburzing layer is up to 1.6mm.