PSI - Issue 81

Jesús Toribio et al. / Procedia Structural Integrity 81 (2026) 547–551

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2. Effect of cold drawing of the pearlitic steel wires Materials were high-strength pearlitic steels with different degree of plastic strain ( different cold drawing level ), named with digits 0 to 6 indicating the number of drawing steps undergone, from the initial hot rolled bar (not cold drawn at all) to the final commercial product (commercial prestressing steel wire). 2.1. Strength evolution during progressive (multi-step) cold drawing One important issue in cold-drawn pearlitic steel wires is the relationship between microstructure and strength, i.e., the effect of microstructural evolution on the (macroscopic) mechanical properties, since the final aim of the manufacture process by cold drawing is the improvement of mechanical properties (increase of strength) by a strain hardening mechanism. Fig. 1 (left) plots the stress-strain curves for the progressively cold-drawn pearlitic steels from A0 (hot rolled steel, not cold drawn at all, 0 drawing steps) to the commercial prestressing steel wire A6 (heavily cold drawn pearlitic steel that has undergone 6 drawing steps). A strain-hardening behaviour is observed, i.e., both the yield strength  Y and the UTS  max increase with the drawing degree.

2.2. Hierarchical microstructural evolution during progressive (multi-step) cold drawing

Progressive cold drawing of pearlitic steel affects the microstructural arrangement in the form of slenderizing of the colonies, decrease of interlamellar spacing and orientation in the direction of cold drawing (wire axis) of both colonies and lamellae (Toribio and Ovejero, 1997, 1998a, 1998b, 1998c), i.e., inducing microstructural anisotropy, as shown in Fig. 1 (right) in the most heavily cold drawn pearlitic steel wire.

2.0

1.5

1.0

0.0 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 A6 A5 A4 A3 A2 A1 A0  (GPa)  0.5

Fig. 1. Stress-strain curves of the progressively drawn pearlitic steels A0 to A6 (from 0 to 6 steps of cold drawing; left) and microstructure of the cold drawn wire A6 (right). Vertical side of the micrograph is parallel to the wire axis or drawing direction and horizontal side is associated with the radial direction. 3. Fractographic analysis After standard tension tests up to fracture performed in wires associated with all steps of cold drawing, fracture surfaces were observed by means of scanning electron microscopy (SEM). The micro-fracture maps (MFMs) containing the fractographic appearance of the whole cross section of the wires after tensile testing of two steel families (B and E) are shown in Figs. 2 and 3 respectively. The MFMs show an increasing trend towards a more anisotropic fracture as the cold drawing degree increases. However, it is only a locally anisotropic fracture behaviour in the form of axial (or longitudinal) micro-cracking, micro-splitting, micro delamination, micro-deflection or micro-kinking in the fracture surface. This local anisotropy of fracture behaviour (increasing with the cold drawing degree) in the progressively cold drawn pearlitic steel wires does not become a global anisotropy , even in the most heavily drawn steels.