Study On The Cementite Deformation Behavior And Its Microstructure Evolution Of Cold Drawn Pearlitic Steels

Cold-drawn,hypereutectoid pearlitic steel wires show maximal tensile strength above 6 GPa,thus making them the strongest bulk nanostructured materials.Although pearlite has a large potential in engineering applications,the microstructural origin of its extreme strength is not well understood.Hence in this work,SWRS82B cold drawn pearlitic steels with diverse stains were tested.The structures were analyzed and quantified via some dominant test methods including in situ scanning electron microscopy?SEM?,electron backscatter diffraction?EBSD?,high-resolution electron microscopy?HRTEM?,X-ray diffraction?XRD?,etc.Study the behavior of cementite deformation and its fracture behavior in undeformed pearlite,microstructure transformation behavior of small-strain cold drawn pearlitic steels and the relationship between microstructure parameters and anisotropic fracture mechanism.A new heat treatment process for obtaining high-strength multiphase steel was explored based on the original studies,and the microstructure transformation behavior during heat treatment was studied.The conclusions as follows:1)The results by in situ SEM tests of pearlite with?=0 showed that the deformed structure had three different types of shear bands:shear deformation in the pearlite colonies,at the interfaces of the pearlite colonies and at the phase boundary between ferrite and cementite.The orientations of the shear bands were nearly anisotropic with respect to the tensile axis.The path of shear deformation developed at the dislocation walls and was propagated by the offset of the cementite along the dislocation walls.As the dislocations cut perpendicularly through the cementite layers,the single-crystalline cementite platelets were divided into many nanometer-sized subgrains by the effects of the shearing stress.This process hindered dislocation movement and led to the accumulation of complex dislocations at the adjacent ferrite lamellae.Thus,the plastic deformation of the cementite layers was characterized by bulging until they fractured.Hence,three different shear models characterized the crack propagation process,which occurred in a linear fashion and could be considered the pearlite colonies as a displacement unit for forward expansion.2)A quantitative statistical analysis of the measured parameters of interlamellar spacing?ILS?and the thickness of cementite lamellae indicated a slight decrease during the testing and a relatively high deviation from the calculated values at the conditions of small-strain pearlitic steels with?<0.8.Calculations on the dislocation density increased from approximately 4.25×10¹⁴8)^-2 at?=0 to approximately4.33×10¹⁵8)^-2 and then to 5.81×10¹⁵8)^-2 with increasing cold drawing strain.As calculated using the misorientation angle across the dislocation boundaries and the dislocation boundary area per unit volume,the dislocation density generally increased consistently and agreed with the increasing tendency of TEM morphology calculated values.3)The microstructure parameters were measured of the cold drawn pearlitic steels up to a true strain of 1.6.The aspect ratio?was defined to describe the fracture mechanism based on statistics pertaining to major axis and minor axis of pearlitic colonies,and the fitting formula was established as follows:=0.40)?2?+0.65.At??2.75,the fracture mechanism shifts from cleavage to quasi-cleavage fracture due to dislocation generation,as the measured parameters of interlamellar spacing?ILS?showed no obvious decrease.At values of??2.75,ductile fracture mechanism becomes dominant due to a drastic decrease in ILS.The<110>ferrite fiber microtexture formed during the cold drawing process and the component exhibiting a gradient distribution from the surface to the central regions increased gradually with drawing.The crack propagation path was then deflected and formed a?V‘shape at??1.5.However,as carbon migrated from cementite to the ferrite near the interphase,the fracture path deflected again.Furthermore,two reasonable models were formulated to explain fracture crack forming and anisotropic fracture behavior.In addition,this study illustrated that the relative crystallographic orientation of the ferrite and cementite components followed the Bagarytski relationship.With increasing strain,the cementite layers transformed from single crystals into nanostructured polycrystals and even evolved an amorphous structure at the interface at strains above 1.5.4)High-strength multiphase steels consisting of pearlite surrounded by tempered martensite were prepared by pre-quenching and ultrafast tempering heat treatment of high-carbon pearlitic steels?0.81%C?.With an increasing quenching temperature from 120°C to 190°C,the quenched martensite variants nucleated via autocatalytic nucleation along the interface.Furthermore,the tempered nodules exhibited a distinct symmetrical structure,and the tempered martensite and pearlitic colonies in the group also showed a symmetrical morphology.In addition,a reasonable model was formulated to explain the transformation process from quenching martensite to the multiphase microstructure.When the quenching temperature was set to 120°C,followed by ultrafast heating at 200°C/s to 600°C and subsequent isothermal treatment for 60 s,the multiphase structure showed highest strength,and the pearlite volume fraction after tempering was the lowest.The microhardness softening mechanism for the tempered structures consisted of two stages.The first stage is related to martensitic sheets undergoing reverse transformation and the nucleation of cementite on dislocations.The second stage involves the transformation of austenite into pearlite and continued carbide coarsening in the martensitic matrix.