PSI - Issue 28

Jesús Toribio et al. / Procedia Structural Integrity 28 (2020) 2390–2395 Jesús Toribio et al. / Procedia Structural Integrity 00 (2020) 000–000

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2

The described phenomenon, also known as hydrogen embrittlement (HE) or hydrogen assisted cracking (HAC) modifies the mechanical performance of pearlitic steels in a harmful way, thereby affecting their structural integrity and leading to catastrophic failure in some cases, as described elsewhere (Vehovar et al., 1998; Woodtli and Kieselbach, 2000; Capelle et al., 2011). To study this phenomenon, constant extension rate tests (CERTs) are widely used and commonly accepted. Notched specimens, where different triaxial stress-strain states are generated, are frequently used in experimental studies of fracture mechanics, not only in inert but also in harsh environment (Toribio and Ayaso, 2004; Wang et al., 2007; Capelle et al., 2011). This paper analyzes micro-fracture maps (MFMs) in round-notched samples of cold drawn pearlitic steel subjected to CERTs under HAF conditions, to show the role of the microstructural anisotropy generated by cold drawing in the fracture performance of the steels immersed in a hydrogen environment and at the same time undergoing a triaxial stress state. Since the time of exposure to hydrogen (harsh environment) influences the HAF process of this type of specimens (Wang et al., 2007), several loading rates were considered in this work.

Nomenclature A

notch depth notch type A notch type B notch type C notch type D notch radius wire diameter cleavage

A B C C D R Ø

2. Experimental Procedure Pearlitic steels wires corresponding to each one of the dies of a cold drawing chain are used in the present study, from the initial hot rolled bar (not cold drawn at all) to the final commercial product (prestressing steel wire undergoing six steps of a commercial cold drawing process). The chemical composition of these pearlitic steels is the same (C 0.800%, Mn 0.690%, Si 0.230%, P 0.012%, S 0.009%, Al 0.004%, Cr 0.265 %, V 0.060%). Cold drawn prestressing steel wires are materials with a markedly oriented microstructure as a consequence of the manufacture process affecting material fracture behavior (Nam and Bae, 1995; Toribio and Ovejero, 1997; Zelin, 2002; Zhang et al., 2011; Guo et al., 2013; Zhang et al., 2013). To get general results, round notched (axisymmetric) specimens with very different geometries were used, as sketched in Fig. 1 in which dimensionless variables A /Ø y R /Ø are used, where A is the notch depth, R the notch radius and Ø the wire diameter. Therefore, different notch root radii and depths and, therefore, very distinct constraint levels in the vicinity of the notch tip were considered. The samples were subjected to CERTs in an aqueous solution of Ca(OH) 2 with 0.1 g/l of NaCl (pH = 12.5). The cell was connected to a potentiostat to maintain during the tests a constant cathodic potential of –1200 mV SCE (saturated calomel electrode). According to previous research works, these electrochemical conditions enhance HAF process in pearlitic steels (Parkins et al., 1982). Two different crosshead speeds were applied: (i) 0.001 mm/min and (ii) 0.01 mm/min. Specimens were identified with a number indicating the drawing steps undergone by each material followed by a letter representing the notch type (A, B, C o D), and finally, after a hyphen, a number (1, 2) indicating the applied displacement rate (0.001 mm/min and 0.01 mm/min, respectively).