PSI - Issue 81

Jesús Toribio et al. / Procedia Structural Integrity 81 (2026) 18–22

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Previous research by Toribio and Ovejero (1997, 1998a, 1998b, 1998c) showed that progressive cold drawing of pearlitic steel affects the microstructural arrangement in the form of slenderizing of the colonies, decrease of interlamellar spacing and orientation in the direction of cold drawing (wire axis) of both colonies and lamellae. This paper offers a philosophical approach to the multiscale microstructural evolution in progressively cold drawn pearlitic steel based on the idea of palimpsestus , analyzing in particular: (i) the prior austenitic grain ( zero, or “virtual”, or “palimpsestus” microstructural level ); (ii) the pearlitic colony ( first microstructural level ); (iii) the pearlite lamellae ( second microstructural level ). The final aim of the article is the clarification of the critical fracture unit governing cleavage fracture in cold drawn pearlitic steels supplied in the form of prestressing steel wires for prestressed concrete. 2. Critical fracture unit in cold drawn pearlitic steels: a “palimpsestus” approach The prior austenitic grain of the steel ( zero, or “virtual”, or “palimpsestus” microstructural level ) disappears during the eutectoid transformation. Nevertheless, its boundary as a set of material points represents a geometrical domain that can be analyzed (the “ previous writing ” in the material in this kind of palimpsestus approach ) and such a set of points (domain) evolves during cold drawing and becomes more slender and elongated in the cold drawing direction (wire axis), in a sort of “ rewriting ” in the material over the previous text, although the latter still remains in the steel as a heritage. This is a palimpsestus approach, from the conceptual point of view, or an updated lagrangian formulation , from the continuum mechanics viewpoint. Fig. 1 sketches the palimpsestus approach by showing the evolution of the boundary of the prior austenitic grain during cold drawing from the hot rolled pearlitic steel (Fig. 1a) to the heavily cold drawn pearlitic steel (Fig. 1b). The pearlitic colonies also evolve with cold drawing from regular shape in the hot rolled material (Fig. 1a) to elongated and slenderised in the heavily cold drawn steel (Fig. 1b), as described by Toribio and Ovejero (1997, 1998a).

(a)

(b)

Fig. 1. 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.

In the matter of the pearlite (Fe/Fe 3 C) lamellae ( second microstructural level ), they are randomly oriented in any direction of alignment in the hot rolled material (Fig. 1a), becoming markedly oriented along the cold drawing direction (wire axis) in the heavily cold drawn steel (Fig. 1b). In addition, there is an increase of packing closeness associated with a decrease of pearlite interlamellar spacing (compare Figs. 1a and 1b), as analyzed and reported by Toribio and Ovejero (1998b, 1998c). In order to evaluate the critical fracture unit , it is interesting to analyze the crystallographic orientation of ferrite. In the hot rolled material (Fig. 1a) that has a randomly oriented pearlitic microstructure in the matter of colonies and lamellae (first and second microstructural levels), all pearlite colonies belonging to the same prior austenite grain share a common crystallographic orientation of ferrite. This is the reason why, in the case of conventional cleavage taking place in an isotropic pearlitic steel the conventional cleavage facet size is a function of the prior austenite grain size (Park and Bernstein, 1979). On the other hand, in the case of the heavily cold drawn material (Fig. 1b) that has a markedly oriented pearlitic microstructure in the matter of colonies and since both levels have evolved (in particular, rotated ) during cold drawing, that rotation being the cause of the microstructural orientation, in the same manner as the prior austenite grain is also, in certain sense, cold drawn (even though it does not exist any more after the eutectoid transformation), but one can imagine a “virtual” cold drawing of it, during which the evolution of its boundary (material points defining it) can be analyzed in an updated lagrangian formulation or a “palimpsestus” approach (metaphorically as rewriting over a previous text in an old table). 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, elongated and oriented shape, as suggested in Fig. 1b, 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