PSI - Issue 26

Jesús Toribio et al. / Procedia Structural Integrity 26 (2020) 360–367 Toribio / Structural Integrity Procedia 00 (2019) 000 – 000

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Cold drawn eutectoid pearlitic steels can be considered as high-performance structural materials (Toribio, 2006) because: (i) they have an extremely high tensile strength (Gil-Sevillano, 1986) only limited by cleavage fracture ( cleavage limited strength ), (ii) they behave as micro-composites (Toribio, 2006) from the mechanical point of view, so that their oriented lamellar microstructure influences their fracture behaviour and a materials science link can be established between the micro- and the macro-levels, (iii) they react in a non-conventional manner due to their inherent anisotropy induced by the manufacturing process in the form of multi-step (progressive) cold-drawing. This anisotropy is related to yielding and plastic behaviour ( anisotropic plastic behaviour ), as reported by Toribio et al. (2011) and also linked with fatigue and fracture performance ( anisotropic fracture behaviour ), as described elsewhere (Toribio et al. (1997), Toribio and Toledano (2000), Toribio and Ayaso (2001), Toribio and Ayaso (2003), Toribio et al. (2013) with the result of mixed-mode fracture and strength anisotropy . In the matter of fractography associated with the afore-said anisotropic behaviour, a special type of cleavage has been described by Toribio and Ayaso (2002a). The concept of exfoliation fracture has been used by Sarafianos (1989) and by Toribio and Ayaso (2004), and the idea of delamination has been proposed by Toribio (2008) and by Tanaka et al. (2016). This paper provides microstructural bases of the afore-mentioned anisotropic fracture behaviour of heavily cold drawn pearlitic steels (i.e., the steels of the final steps of the manufacturing chain) by analysing the relationship between microstructural peculiarities induced by heavy cold drawing and the sudden change in crack propagation direction which appears in this steels when a crack becomes critical as a consequence of a special micromechanism of fracture consisting of shear cracking of pearlitic plates. In particular, a new (non-conventional) microstructural unit in heavily cold drawn pearlitic steels is identified and defined: the pearlitic pseudocolony . 2. Cold drawing of pearlitic steels Manufacture of prestressing wires is made by progressive (multi-stage) cold drawing of pearlitic steels to increase the strength by a strain hardening mechanism. Fig. 1 shows two views of a real cold drawing chain, whereas Fig. 2 shows a scheme of a multi-pass cold drawing process.

Fig. 1. Manufacture of prestressing wires by progressive cold drawing: two views of a real cold drawing chain.

Fig. 2. Scheme of a cold drawing process with multiple passes (six in this case) through the dies.