PSI- Issue 9

Jesús Toribio / Procedia Structural Integrity 9 (2018) 323–328 Author name / Structural Integrity Procedia 00 (2018) 000–000

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In addition to these general trends of drawing-induced microstructural orientation, some exceptions can be found in the most heavily cold drawn steel. The most remarkable fact is the presence in some areas in the drawn steel of a special microstructural unit, cf. Fig. 3, the so-called pearlitic pseudocolony , a term coined by Toribio et al. (1997) to define a particular colony that is fully oriented and aligned (parallel to the main axis) in the wire direction but inside of which the ferrite/cementite lamellae are not oriented at all in such a drawing axis, so that they have an anomalous (extremely high) local interlamellar spacing , making them preferential fracture units with minimum local strength and producing anisotropy in fracture resistance and the subsequent effect on crack path deflection .

Fig. 3. Pearlitic pseudocolony in heavily cold drawn pearlitic steels after six drawing steps (longitudinal section).

The role of pseudocolonies promoting anisotropic fracture and crack path deflection has been studied thoroughly by Toribio et al. (1997), Toribio and Ayaso (2001, 2002a, 2002b, 2003, 2004) and Toribio (2004, 2008). A systematic compendium on microstructural evolution with cold drawing of pearlitic steel wires and on the relationship between microstructure and strength has been recently published by Toribio et al. (2016), including information about the microstructural evolution during strain hardening by cold drawing.

3. Anisotropic fracture behavior and crack path deflection

The special microstructural arrangement of cold drawn pearlitic steels (with orientation and alignment of colonies and lamellae in direction parallel to the wire axis or drawing direction) creates a hierarchicaly structured material with anisotropy regarding plastic behavior, fatigue and fracture and affecting the crack paths by preducing deflection and deviation in relation to the initial crack path in mode I. Research by Toribio and Toledano (2000) and Toribio (2002) analyzed the micro-mechanisms of fracture in pearlite that are also affected by the manufacturing process by cold drawing, changing from predominant cleavage in slightly drawn steels to an increasing presence of micro-void coalescence (MVC) in the most heavily drawn wires. With regard to this, it is well known that the fracture behavior of pearlite mainly depends on the size of the prior austenite grain. If the pearlite is drawn, then the colony, rather than the prior austenite grain, is the critical fracture unit determining the size of the cleavage facet. With regard to plasticity and yielding , the anisotropic plastic behavior of cold drawn pearlitic steels has been studied by Toribio et al. (2011). In the matter of anisotropy in fatigue and fracture resistance and its effect on crack paths , extensive research work has been developed by Toribio and co-workers during the last twenty years. With regard to fatigue performance of progressively cold drawn pearlitic steels, the main papers were published by Toribio et al. (2007b, 2007c, 2009, 2010, 2014, 2015a, 2016, 2017), showing that anisotropic fatigue behavior ( locally multiaxial fatigue crack growth ) takes place in cold drawn pearlitic steel, i.e., a microscopic ( real ) anisotropic effect arise in the matter of fatigue crack propagation linked with the lamellae alignment and orientation. In the matter of fracture, the most relevant papers were published by Toribio and Valiente (2004, 2006) and by Toribio et al. (2007a, 2010, 2011a, 2012, 2013, 2015b, 2016) showing the key impact of the colonies and lamellae alignment and orientation, producing anisotropic fracture behavior with its related crack path deflection .