PSI - Issue 81

Jesús Toribio et al. / Procedia Structural Integrity 81 (2026) 95–97

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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. 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 – 1200 mV vs. SCE, linked with the cathodic regime of cracking for which the environmental mechanism is hydrogen assisted cracking (HAC) or hydrogen embrittlement (HE). 4. Anisotropy of HAC/HE behaviour The experiments showed a progressive anisotropy of HAC/HE behaviour (Fig. 1), so that the HAC/HE resistance is a directional property depending on the angle in relation to the drawing direction ( strength anisotropy with regard to HAC/HE behaviour). This anisotropic HAC/HE 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 HAC/HE 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.

F

II F

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II x

x I

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Fig. 1. Evolution of HAC/HE 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 In the matter of HAC/HE, the proposed mechanism is hydrogen-enhanced delamination (HEDE), cf. Fig. 2. The lamellar structure of the steel (markedly oriented) which produces anisotropy regarding fracture and hydrogen diffusion, so that hydrogen diffuses mainly in the direction of the plates and can weaken the bonds or interfaces between the ferrite and the cementite lamellae (which are the weakest links even before the hydrogen presence) thus contributing to the hydrogen-induced fracture by delamination or debonding between two similar microstructural units, i.e, at the ferrite/cementite interface or at the pearlitic colony boundaries.