PSI - Issue 39

Jesús Toribio et al. / Procedia Structural Integrity 39 (2022) 475–478 Author name / Procedia Structural Integrity 00 (2021) 000–000

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1. Introduction Commercial prestressing steel wires are manufactured by cold drawing previously hot rolled pearlitic steel rods to increase both the yield strength and the ultimate tensile strength (UTS), in order to be used as the main constituents of prestressed concrete structures (Borchers and Kirchheim, 2016). The manufacture technique by cumulative cold drawing of a pearlitic steel wire through a series of dies with diameters progressively thinner produces important microstructural changes in the drawn material that could influence its posterior performance (Embury and Fisher, 1966; Langford, 1977; Ridley, 1984; Toribio and Ovejero, 1997; 1998a, 1998b, 1998c). Evidence exists in the scientific literature showing the anisotropic fracture behaviour of prestressing steel in air (Toribio et al., 1997) as well as in aggressive environments (Cherry and Price, 1980; Sarafianos, 1989, Toribio and Lancha, 1998). This paper offers a materials science approach to the modelling of stress corrosion cracking (SCC) behaviour of cold drawn pearlitic steel wires. The approach is based on linking the microstructure of the steels (progressively oriented as a consequence of the manufacture process by cumulative cold drawing) with their macroscopic SCC behaviour (increasingly anisotropic as the degree of cold drawing increases). Special attention is paid to the evolution Materials were high-strength pearlitic steels taken from a real manufacturing process. Wires with different degrees of cold drawing were used. The different steels were named with digits 0 to 6 indicating the number of drawing steps undergone, so steel 0 is the hot rolled bar (base material) which is not cold drawn at all, and steel 6 represents the prestressing steel wire (final commercial product) which has suffered six cold drawing steps. The hierarchical microstructural evolution in pearlitic steels during cold drawing (at the two levels of pearlitic colonies and lamellae) was studied by Toribio and Ovejero (1997, 1998a, 1998b, 1998c), showing the following trends: slenderizing of the colonies, decrease of interlamellar spacing and orientation in the direction of cold drawing (wire axis) of both the colonies and the lamellae. 3. Experimental programme Slow strain rate tests were performed on transversely precracked steel wires immersed in aqueous environment under axial loading. After fatigue precracking, samples were placed 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. Consequence of cold drawing on crack paths The experimental results showed a fundamental fact: the SCC behaviour becomes more anisotropic as the degree of cold drawing increases, so a transverse crack tends to change its propagation direction to approach that of the wire axis, and thus a mode I growth evolves towards a mixed mode propagation. It may be assumed that the microstructural orientation in drawn steels influences the macroscopic behaviour, so that the SCC resistance is a directional property depending on the angle in relation to the cold 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. of the macroscopic crack path as the degree of cold drawing increases. 2. Materials and microstructural evolution with cold drawing