Study On Anisotropic Mechanical Properties And Microstructure Of Cold Drawn Pearlitic Steel Wire

The anisotropic mechanical properties of cold drawn pearlitic steel wires were investigated by tensile test,scanning electron microscopy(SEM),transmission electron microscopy(TEM).The effect of low temperature annealing on the mechanical properties of steel wire in different directions was studied and the influence of ferrite / cementite interface on the mechanical properties of steel wire was investigated.The compressive properties of different directions of steel wire were studied and the microstructure morphology was observed after compression test.The deformation characteristics of different orientation pearlite lamellae under compressive stress were discussed.Results indicated that the distinctions of tensile strength between three directions(parallel to the tensile axis,inclined to the tensile axis(45°),vertical to the tensile axis)were amplified with increasing strain.When ε=0,the tensile strength was found isotropic;As strain increasing,the tensile strength reached peaks in parallel direction and valleys in vertical direction.The obvious effect of strain hardening was observed in parallel and inclined directions while the vertical direction remained strength stability.In TEM,the phenomenon was discovered that due to non-homogeneous distribution in vertical direction,dislocations piled up at the boundaries resulting in stress concentration.On the contrary,the dislocations were uniformly distributed which led to homogeneous transformation in parallel direction.As annealing temperature increasing,the tensile strength in parallel and vertical directions raised in the first stage.When t = 250 ℃,tensile strength reached peaks with 1715 MPa and raised about 59%.Before annealing,the lattice of the cementite was disordered and the interface was highly disordered.After annealing,the cementite turned into nanocrystals with different orientation and a clear interface was formed between cementite and ferrite.Recrystallization of cementite during annealing may be the key factor to enhance the tensile strength in vertical direction.The compressive properties of different directions of steel wire were studied and the microstructure morphology was observed after compression test.The results showed that in the compression test,an X shaped shear band was produced in the parallel samples(pressure stress parallel to cementite)and the shear band caused uneven deformation which resulted in the nonhomogeneous hardness.The hardness value of the center area was higher than that of the edges at both ends of the sample.In vertical samples(pressure stress vertical to cementite),the hardness value increased with the increase of the relative reduction and the pearlite vertical to the compressive stress still remained lamellar.