Quantitative Investigation Of Microstructural Evolution During Cold Wire Drawing Of A Pearlitic Steel Wire And Its Relationship With Mechanical Properties

This thesis reports a quantitative investigation of the evolution of microstructure and micro-texture during the cold-drawing of the pearlite steel wire. Microstructure observation and orientation measurements were performed in a scanning electron microscopy (SEM) with a electron backscatter diffracrion (EBSD) detector and data acquisition system and in a transmission electron microscopy (TEM) to investigate the ferrite micro-texture distribution, the deformation of ferrite, the deformation and fracture of cementite and the relationship between deformation and fracture of cementite and ferrite orientation. Nano-beam diffraction and relative orientation-indexing software have been used to quantify the change of ferrite orientation in pearlite colonies. X-ray diffraction was applied to quantify the change of lattice parameters of cemnetite during colding drawing for the analysis of cementite dissolution. The evolution of microstructure and micro-texture and its relationship with the mechanical properties were discussed.The investigation of microstructure and ferrite micro-texture shows:⑴the microstructure turns to the drawing axis during the cold drawing. The lamellar structure parallel to the drawing direction in the longitudinal section and curling structure in the cross section are observed.⑵the strong ferrite <110> fiber texture parallel to the drawing axis forms with increasing strain, but the intensity of ferrite <110> fiber texture is imhomogeneous from the center to surface in longitudinal section with the strongest in the center and the weakest near the surface.The quantitative and systimatic investigation of the deformation microstructure shows the following results:⑴the dislocation density in ferrite increases with drawing strain. In the initial state, dislocations of high density exist on the ferrite/cementite boundaries. The dislocation density in ferrite increases rapidly at the beginning of the wire-drawing, then rises slowly with increasing strain and keeps on the level of 1016 m-3 at high strains. The ratio of the misorientation between neighboring ferrite lamellae in the pearlite colony and the interlamellar spacing increases with increasing strain. The reason for the formation of high angle misorientation doundaries of ferrite in pearlite colonies differs with the strain. It is related to the local shear and S-bands at low strains. At high strains, it is not only related to the local shear and S-bands, but also to the increase of misorientation due to the extinction and re-arrangement of dislocations at the ferrite/cementite interfaces.⑵the deformation of cementite, closely related to the ferrite orientation, has its direction parallel to {110}α-Fe or {112}α-Fe plane traces determined by the largest Schmid factors. The cementite breaks gradually and its single-crystal structure changes into poly-crystal with increasing strain. The interlamellar spacing and thickness of the cementite lamellae decrease linearly, proportional to the wire diameter during the cold wire-drawing.⑶a new formula, including two major microstructral factors (the thickness of ferrite lamellae and the dislocation density in ferrite), is proposed to calculate the flow stresses of wires. This calculation leads to flow stress values in good agreement with those determined experimently. The shear stesses of cementite (calculated based on a crack nucleation model), with experimental flow stresses and tensile stresses, explain the observed microstructural evolution during the cold wire drawing well.