Study On The Effect Of Annealing Treatment On Microstructure And Mechanical Properties Of Cold-drawn Pearlitic Steel Wire

In this paper,annealing treatments were used to change the microsturcture of pearlitic steel wires,and the microsturcture influence mechanism on mechanical properties was studied by using Scanning Electron Microscope(SEM),Transmission Electron Microscope(TEM),X-ray Diffraction analysis method(XRD)and related mechanical property testing methods.The evolution of microstructure for pearlitic steel wires was stuied,and the influence of the annealing treatment on the mechanical properties was studied.The evolution of torsion performances under different drawing strains was studied.Combined with the observation on cementite microstructures and dislocation patterns microstructures for tension fracture samples and torsion fracture samples,the influence mechanism of organization and microstructure of pearlitic on mechanical properties for steel wires was dicussed.The influence of the annealing treatment on the structure of pearlite was studied.After annealed 60 min at 210℃ or 350℃,the pearlite remained integral lamellar structures.The peralite kept lamellar structure while cementite came out fragmentation mildly after annealed2 min at 450℃.The annealing treatments make the dislocation recovery graduately which is tangled in cold drawn pearlitic steel wires,and the cementite turns from amorphous to nanocrystalline.With the increase of annealing temperature,the size of the cementite nanocrystals increased gradually.The influence of the annealing treatment on the mechanical properties was studied.The tensile strength of cold-drawn pearlitic steel wires was 2060 MPa and it increased apparently after annealing treatments.After annealing 60 min at 210℃ the tensile strength increased by 131 MPa.After annealing 60 min at 350℃,the strength and plasticity was found both increased obviously.After annealing 2min at 450℃,the strength decreased mildly while plasticity increased visibly.For the annealing steel wires which continued to cold draw,the advantage of tensile strength still maintained while the advantage of plasticity vanished compared with cold-drawn steel wires.The changes of strength and plasticity are the result of two factors which are the interaction of cementite bound dislocation and the dislocation recovery.The nanocrystal cementite which comes from amorphous cementite because ofannealing treatments impedes dislocation motion,as a consequence the strength for annealing steel wires increases.The dislocation recovery accompanied by annealing treatments makes the plasticity for steel wires better,and the plasticity is increasing with the rise of annealing temperature.The evolution of torsion performances during drawing process was studied.With the drawing strain increasing accordingly,the torsional strength gradually increased while the torsional plasticity increased first and then decreased.In this paper the torsional plasticity reached up to the maximum when the drawing strain is 0.68.The steel wires under annealing treatment started to present delamination fracture when the drawing strain reach a biggish value such as 1.80 in this paper.The torsion palsticity is related to the the initial orientation of the pearlite group.The higher of the overlap ratio between preferred orientation for torsion process with the initial orientation of the pearlite group is,the better of torsion palsticity is.The delamination is related to the states of cementite.After annealing treatments,the crystallization cementite block dislocation motion,which causing regional severe plastic deformation during torsion process.Under the circumstances,the torsion fracture present delamination fracture.