Characteristic of Near-surface Microstructure and Its Effects on the Torsion Performance of Cold Drawn Pearlitic Steel Wires for Bridge Cables

The characteristic of near-surface microstructure and its effects on the torsion performance of cold-drawn pearlitic steel wires for bridge cables were investigated by focused ion beam-scanning electron microscope, transmission electron microscopy and differential scanning calorimetry. The samples with similar tensile strength before and after hot-dip galvanizing process are, respectively, characterized as delaminated and non-delaminated in torsion test which indicates that the tensile strength is independent of the toughness value ( i e , reduction area and torsion ability). It is interesting to find that there exists submicron granular ferrite on near-surface of the wires, which can be attributed to dislocation rearrangement and sub-grains rotation during cold drawing and hot-dip galvanizing process. And their distribution can suggest homogeneousness of deformation degree to a certain extent: the closer to the surface of their distribution, the more homogeneous deformation of the wires. There is a close relationship between the thermal stability of the cementite layer and distribution of granular ferrite: differential scanning calorimetry (DSC) analysis shows that the sample is accompanied by submicron granular ferrite which is located closer to the surface has higher thermal stability under galvanizing temperature (450 °C). A new mechanism of the torsion delamination of pearlitic steel wires is discussed in terms of the thermal stability of the cementite layer and distribution of granular ferrite.