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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5. Critical fracture unit in cold drawn pearlitic steels: a “palimpsestus” approach. Fig. 5 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. 5a) to the heavily cold drawn pearlitic steel (Fig. 5b). Although the prior austenitic grain ( zero, or “virtual”, or “palimpsestus” microstructural level ) disappears during the eutectoid transformation (producing ferrite and cementite plates, i.e., a pearlitic microstructure), 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 enlarged 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. Inside the described prior austenitic grain, the pearlitic colony ( first microstructural level ) is defined as a set of ferrite (Fe) & cementite (Fe 3 C) lamellae that share a common orientation inside the colony, and different from the lamellar orientation in the neighboring colonies. The pearlitic colonies also evolve with cold drawing from regular shape in the hot rolled material (Fig. 5a) to enlarged and slenderised in the heavily cold drawn steel (Fig. 5b), as described by Toribio and Ovejero (1997, 1998a). 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. 5a), becoming markedly oriented along the cold drawing direction (wire axis) in the heavily cold drawn steel (Fig. 5b). In addition, there is an increase of packing closeness associated with a decrease of pearlite interlamellar spacing (compare Figs. 5a and 5b), as analyzed and reported by Toribio and Ovejero (1998b, 1998c). It is worth to mention that, as sketched in the enlarged “virtual” grain (left-hand part of Fig. 5b), there are some special colonies that, in spite of the fact that the colony itself is properly oriented and enlarged (slenderised) with the drawing process, the interior lamellae do not follow the same trend, i.e., they do not behave properly and become oriented in the cold drawing direction but, contrarily, they remain transverse with the undesired consequence of an anomalous (very high) local interlamellar spacing , making them weakest domains in the material with minimum local resistance to fracture. These special microscopical units have been firstly mentioned by Toribio et al. (1997) as perlitic pseudocolonies . Recent papers go further in the analysis of the aforesaid special units, by providing rigorous identification and definition (Toribio, 2020a), as well as studying the consequences of their presence in the material regarding anisotropic fracture behaviour of cold drawn pearlitic steel (Toribio, 2020b). In order to evaluate the critical fracture unit , it is interesting to analyze the crystallographic orientation of ferrite. In the hot rolled material (Fig. 5a) 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 ( zero or “virtual” or “palimpsestus” microstructural level ) 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 (hot rolled that is not drawn at all or slightly cold drawn material), the conventional cleavage facet size is a strong function of the prior austenite grain size , although it is always somewhat less (Park and Bernstein, 1979). On the other hand, in the case of the heavily cold drawn material (Fig. 5b) that has a markedly oriented pearlitic microstructure in the matter of colonies and lamellae (first and second microstructural levels) 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 ( zero or “virtual” or “palimpsestus” microstructural level ) 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). Following this approach, such a prior austenitic grain (or “virtual” grain) becomes progressively enlarged (slenderized) and oriented along the wire axis as the drawing degree increases (see Fig. 5b), but both the pearlitic colonies and the pearlite lamellae inside the boundaries of the “virtual” grain also rotate, thereby creating a new crystal distribution of ferrite in which there is no common crystallographic orientation of ferrite in all colonies of the same prior austenite grain.