Study On The Influence Of Pearlite Phase Transition Process On Its Microstructure And Mechanical Properties

Effects of the deep undercooling(DUIT)on pearlite microstructure and properties,and pearlite microstructure on crack formation and expansion were studied in this paper by rapid phase change apparatus DIL805 high purity helium.The pearlite microstructure and properties were explored by Leica DMI5000 M intelligent scanning microscope,scanning electron microscopy(SEM),field emission transmission electron microscopy(TEM)and INSTRON8501 electro-hydraulic servo universal testing machine.The effects of DUIT(deep undercooling temperature and deep undercooling time)on the microstructure and mechanical properties of pearlite were investigated.The experimental results show that:(1)After the undercooling and then isothermal,the number of large-angle grain boundaries of pearlite microstructure was significantly higher than that of direct isothermal samples,and the size of pearlite colony was refined,moreover,with the decreasing of undercooling tempreture and the extending of isothermal time the more obvious,pearlite lamellar spacing has a tendency to refine.(2)The colony size and grain size of the pearlite are the important structural parameters that affect the plasticity of the pearlite material.The plasticity of the pearlite material depends mainly on the grain size and the colony size.When the grain size is kept constant,the elongation increases with the decrease of the size of the pearlite.The plasticity index of the undercooled sample is better than that of the direct isothermal sample,and the strength is mainly affected by the lamellae spacing.When the lamellae spacing has little change,tensile strength has no significant changes.(3)The failure mechanism of the tensile specimen is the dimple fracture,fiber area range of the undercooled sample is wider than the direct isothermal sample,while the radiation area is smaller than the latter,and the dimple size is more than the direct isothermal sample Large and deep.What’s more,tear area is smaller than the direct isothermal sample,the number of secondary cracks reduced.(4)There are four kinds of crack expansion mechanism in the pearlite: 1)expansion of perpendicular to the cementite;2)shear fracture with the cementite into a 45 °;3)tearing up along the ferrite and cementite Phase interface;4)cracking along the boundary of pearlite colony.When the stress intensity level is low,the crack initiation and expansion resistance of the fine grain are lower than that of the coarse grain.With the increase of the stress intensity level,the fine grain is superior to the coarse grain,and the number of initiation cracks is more than that of large grain samples,while the cracks in the coarse grain samples are wider.In the coarse pearlite colony,when the crack spread to the boundary of pearlite colony,it will be crossed directly and then continues to expend into the next one,crack propagation path is more straight.On the contrary,the crack spreads along the boundary of pearlite colony in the smaller size of pearlite colony and the path of the expansion has many bends.Under the condition of coarse lamellae spacing,the crack is torn along the interface between the ferrite and the cementite.While lamellar spacing is small,crack expansion perpendicular to the ferrite and cementite.(5)Increasing the pressure of compressed air,that is,to improve the cooling rate,the yield strength and tensile strength of the wire are increased,while the plastic index is not changed;the mechanical properties of air-cooled sorbite steel wire is close to that of lead bath.