The Experimental Study On Transformation Kinetics And Transformation Plasticity Of 3Cr2Mo Steel

Steel 3Cr2Mo has a medium hardness, excellent machinability and mirror-polishing property, making it widely used in plastic dies or moulds of large-size and high dimension accuracy and complicacy. In this present study, the CCT/TTT diagrams, the mechanical properties of different microstructure, the transformation plasticity and the effects of stress on transformation kinetics for different types of transformation in this steel have been experimentally investigated, providing accurate and necessary input data for further computer simulation of quenching process.The transformation kinetics of steels is not only the main basis for developing heat treatment process, but also necessary input data for computer simulation of heat treatment process. The continuous cooling transformation (CCT) diagram and the isothermal transformation (TTT) diagram of 3Cr2Mo steel have been obtained and analyzed based on the dilatometric experiments with Gleeble-3500 thermal mechanical simulator. The CCT/TTT diagrams show that the critical cooling rate to get martensite is about 5℃/s, and it is inclined to get bainite, however, always with its incompleteness for this steel.The mechanical properties of steels are necessary input data to realize accurate computer simulation of quenching process. The mechanical properties of different kinds of microstructure (martensite, pearlite, bainite and austenite) in 3Cr2Mo steel, including thermal expansion coefficient, elastic modulus, plastic modulus and yield strength, have been determined by dilatometric experiments and high temperature tensile test. The results show that the mechanical properties differ greatly for different kinds of microstructures, and they are the functions of temperature.The transformation plasticity of martensitic transformation, bainitic transformation and pearlitic transformation have been investigated by dilatometric experiments. The corresponding parameter K in Greenwood- Johnson model are the functions of applied stress, and can be formulated as, K = 7.30843 E ? 5 + 6.18494 E ? 7σ, K = 7.56269 E ? 5 + 7.46058 E ? 7σ, and K = 1.32036 E ? 4 + 2.45691E ? 6σ, respectively. The effects of applied stress on transformation kinetics have simultaneously been studied. For martensitic transformation, the applied stress has no obvious effect on coefficientαwhich always has the value about 0.02355 while the M s increases with the increasing of applied stress. For bainitic transformation, parameter n in Avrami equation changes very little with applied stress and isothermal temperature, while parameter b increases with the increasing of applied stress. For the pearlitic transformation, parameter n decreases obviously with the increasing of applied stress, but parameter b increases with the increasing of applied stress in whole although it has relatively high data divergence. The incubation period for bainitic and pearlitic transformation shortens with the increasing of applied stress, indicating that the applied stress can shorten the incubation period of diffusion phase transformation.