PSI - Issue 81

Jesús Toribio et al. / Procedia Structural Integrity 81 (2026) 140–142

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2. Materials and microstructural evolution with 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. In addition, there are some exceptional pearlitic pseudocolonies , cf. (Toribio et al., 1997), which are extremely slender, aligned quasi-parallel to the drawing direction (wire axis) and whose local interlamellar spacing is clearly anomalous — specially high — in comparison with the average (or global) spacing in the specific steel due to the fact that the cementite plates are not oriented along the wire axis direction and in some cases are pre-fractured by shear during the manufacturing process. These characteristics make them local precursors of micro-cracking, i.e., preferential fracture paths with minimum local resistance to damage, fracture or cracking. 3. Experimental programme Slow strain rate tests were performed on transversely precracked steel wires in a corrosion cell containing aqueous solution of 1g/l Ca(OH) 2 plus 0.1g/l NaCl (pH=12.5). The experimental device consisted of a potentiostat and a three-electrode assembly: metallic sample (working electrode), platinum counter-electrode and saturated calomel electrode (SCE: reference). Tests were performed at constant electrochemical potential with the values of – 600 mV vs. SCE, linked with the anodic regime of cracking for which the environmental mechanism is localized anodic dissolution (LAD) or pure stress corrosion cracking (SCC). 4. Anisotropy of SCC behaviour The experiments showed a progressive anisotropy of SCC behaviour (Fig. 1), so that the SCC resistance is a directional property depending on the angle in relation to the drawing direction ( strength anisotropy with regard to SCC behaviour). This anisotropic SCC behaviour of the drawn steels can be evaluated by means of the crack path or fracture profile after the tests. Fig. 1 shows the evolution of crack paths with cold drawing under SCC conditions. Fig. 1a offers a 3D-view of these fracture surfaces. For the slightly drawn steels (0, 1 and 2), the crack paths are macroscopically plane and oriented perpendicularly to the loading axis (mode I propagation). Steel 3 exhibits a certain deflection angle evolving to mixed mode cracking. In the most heavily drawn steels (4, 5 and 6) the deflection angle is even higher. Fig. 1b shows the geometric parameters describing the crack path. Fig. 1c offers the evolution of the fracture profile towards the weakest crack path.

II F

(a)

F

f

f I II

f I

F

II x x I

(b)

x I

F

h

f

θ

f

F

(c)

0

1

2

3

4

5

6

Fig. 1. Evolution of SCC behaviour with the cold drawing degree: (a) general appearance of the fracture surfaces; (b) geometric parameters describing the crack path; (c) evolution of fracture profiles; f: fatigue crack growth; I: mode I cracking; II: mixed mode cracking; F: final fracture. 5. Discussion The mechanism of SCC in cold drawn steel could be explained as follows: dissolution is produced in mode I along a distance x I (see Fig. 2). The crack continues in mode I along the initial plane and only deviates when it reaches a defect (pre-damage) in the material: a pearlitic pseudocolony (Toribio et al., 1997) that is a potential fracture site and represents the weakest crack path .