PSI - Issue 28

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

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Cold drawing produced rotation of both the pearlitic colonies and the pearlite lamellae (Fig. 5b), thereby creating different crystallographic orientations of ferrite in the colonies of the same prior austenite grain (the “palimpsestus” or “virtual” grain). Therefore, a new critical fracture unit arises governing brittle fracture by special cleavage: the slender pearlite colony inside which all ferrite/cementite lamellae share a common orientation and, as a consequence of it and after rotation of colonies and lamellae during drawing, the common crystallographic orientation of ferrite is maintained only inside that colony (enlarged and slenderized in the drawing direction), cf. Fig. 5b.

(a) (b) Fig. 5. Scheme showing the crystallographic orientation of ferrite, the pearlite (ferrite/cementite) lamellae, the pearlitic colonies and the “virtual” boundary of the prior austenitic grain in a: (a) a hot rolled pearlitic steel; (b) heavily cold drawn pearlitic steel. Thus the prior austenitic grain (“virtual” or “palimpsestus” grain), in spite of the fact that it does not exist , is virtually cold drawn and takes a new, enlarged and oriented shape, as suggested in Fig. 5b, thereby modifying the previous crystal orientation of ferrite inside it, that becomes now governed by the parallel lamellae inside a slender pearlitic colony. Then the pearlite colony more than the prior austenite grain could be taken as critical fracture unit in the drawn material, because different pearlite colonies in the same grain follow distinct orientations paths along the manufacturing route. Therefore the slender pearlitic colony becomes the critical fracture unit and determines the size of the enlarged and oriented cleavage facet characteristic of the anisotropic fracture behaviour ( exfoliation ) in heavily cold drawn steels. In the cold drawing process described in previous paragraphs the boundary of the non-existing austenitic grain plays the role of the firstly-written old text whose heritage remains in the material (more or less present) during drawing, but the rewriting almost (if not fully) deletes such a heritage or presence, creating a new text (which is really the old corrected, improved or modified), in the same manner as the cold drawn pearlitic steel wire is an improvement of the previously hot rolled bar but, in certain sense, the cold drawn wire is not a new material, but an improved version of the old one , conditioned by the circumstance of the manufacturing process by cold drawing. From the philosophical viewpoint, this is an orteguian approach in the sense proposed by the Spanish philosopher Ortega y Gasset, as explained by Toribio (2020c). 6. Conclusions. For anisotropic materials such as the cold drawn steels analyzed in this paper, there is an orientation of all microstructural units: (i) the prior austenitic grain (zero, “virtual” or “palimpsestus” microstructural level); (ii) the pearlitic colony (first microstructural level); (iii) the pearlite lamellae (second microstructural level). The slender pearlite colony more than the prior austenite grain (that is also virtually cold drawn in the palimpsestus approach) could be taken as critical fracture unit in the drawn material, because different pearlite colonies in the same grain follow distinct orientations paths along the manufacturing route. Thus the slender pearlitic colony becomes the critical fracture unit and determines the size of the enlarged and oriented cleavage facet characteristic of the anisotropic fracture behaviour (exfoliation) in heavily cold drawn steels.