PSI - Issue 28

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Jesús Toribio et al. / Procedia Structural Integrity 28 (2020) 2424–2431 Jesús Toribio / Procedia Structural Integrity 00 (2020) 000–000

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Fig. 4. “Virtual” fractograph of the 90º-fracture propagation step in sample 5B (steel 5, geometry B), obtained by computer-assisted elongation (following the cold drawing direction, i.e., parallel to the wire axis) of the real fractograph of the step in sample 3B (steel 3, geometry B). In this “virtual” fractograph the wire axis or cold drawing direction (along which it was elongated) correspond to the vertical side of the picture. Furthermore, a quantitative analysis of the real (actual) and “virtual” (elongated) fractographs proves their similitude by considering the similar sizes of the cleavage facet in both cases (real CFS and “virtual” CFS*), as shown in Table 1 that includes also the pearlitic colony size PCS evaluated by Toribio and Ovejero (1997) by using quantitative metallography and measuring in the longitudinal metallographic sections of the different steels (progressively cold drawn), thereby demonstrating that the three sizes (PCS, CFS and CFS*) are approximately the same for a given steel with certain cold drawing degree.

Table 1. Sizes of the pearlitic colonies and the cleavage facets (real and “virtual”) in the different steels. ________________________________________________________________________ Steel 4 5 6 ________________________________________________________________________ PCS (  m) 28.9 36.5 42.2 CFS (  m) 30.5 34.3 41.4 CFS* (  m) 30.0 38.6 42.9 ________________________________________________________________________

PCS: pearlitic colony size (maximum dimension in the wire axis direction) CFS: real cleavage facet size (maximum dimension in the wire axis direction) CFS*: “virtual” cleavage facet size (maximum dimension in the wire axis direction)

In the case of conventional cleavage taking place in an initially pearlitic eutectoid steel, the cleavage facet size is a strong function of the prior austenite grain size, although it is always somewhat less (Park and Bernstein, 1979) This size is the zone in which the pearlite colonies inside the same grain share a common crystallographic orientation of ferrite, following the research by Park and Bernstein (1979). It represents the critical fracture unit in randomly oriented pearlitic microstructures (i.e., in an isotropic pearlitic steel) and determines the intrinsic material toughness linked to its microstructure.