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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This paper offers a palimpsestus approach to the multiscale microstructural evolution in progressively cold drawn pearlitic steel, analyzing in particular: (i) the prior austenitic grain ( zero, or “virtual”, microstructural level ); (ii) the pearlitic colony ( first microstructural level ); (iii) the pearlite lamellae (second microstructural level). 2. Multiscale microstructural evolution with cold drawing: a palimpsestus approach The hierarchical microstructural evolution in pearlitic steels during cold drawing (at the two levels of pearlitic colonies and lamellae) was studied by Toribio and Ovejero (1997, 1998a, 1998b, 1998c). 2.1. Evolution of the pearlitic colony (first microstructural level) Specific analyses by Toribio and Ovejero (1997, 1998a) demonstrated the progressive slenderizing and orientation (in the wire axis or cold drawing direction) of the pearlitic colonies ( first microstructural level ). Thus the pearlitic colony, initially assumed to have a quasi-spherical shape in the three-dimensional (3D) analysis or quasi-circular in the two-dimensional (2D) analysis, evolves towards an ellipsoidal shape in the 3D-approach or an elliptic shape in the 2D-approach, with enlargement and slenderizing of the colony, i.e., increase of the colony aspect ratio (Toribio and Ovejero, 1997) and orientation towards an alignment quasi-parallel to the wire axis or cold drawing direction (Toribio and Ovejero, 1998a). 2.2. Evolution of the pearlite lamellae (second microstructural level) Previous research by Toribio and Ovejero (1998b, 1998c) studied the evolution with the manufacturing process by cold drawing of the pearlitic lamellae ( second microstructural level ), showing a progressive densification of the ferrite/cementite (Fe/Fe 3 C) lamellae associated with decrease of interlamellar spacing (Toribio and Ovejero, 1998b), together with an orientation of the plates in a direction parallel or quasi-parallel to the wire axis or cold drawing direction (Toribio and Ovejero, 1998c). 2.3. Evolution of the prior austenitic grain (zero, or “virtual”, microstructural level) The austenitic grain (zero or “ virtual” microstructural level) is associated with the microstructure that was present in the material before the eutectoid transformation, so that it does not exist any more. However, the material points defining the boundary of the austenitic grain before the eutectoid transformation define a sort of virtual grain that also evolve with cold drawing, thereby defining a new boundary of the cold drawn virtual grain, that becomes thus an enlarged and oriented virtual grain (after virtual cold drawing). 2.4. A palimpsestus approach The aforesaid evolution of the material points defining the prior austenitic grain represents, from the continuum mechanics viewpoint, an updated lagrangian formulation in which material points evolve with cold drawing, in the same manner as material properties also evolve towards higher resistance with cold drawing, so that a new (improved) material –the cold drawn pearlitic steel wire– appears, but it is not really a new material, but the old one (hot rolled pearlitic steel bar as the base material before drawing) with improved properties. It is a sort of palimpsestus approach , similar to rewriting over an old, previously written, table or re-building over and old city in ruins (or destroyed), i.e., both the prior (“ virtual” ) austenitic grain (i.e., the material points defining its boundary) and the material itself are metaphorically re-written, i.e., evolve with cold drawing and create the new material over the old ( it being paradoxically old and new ) in the same manner as the Catholic Rome appears (and develops in a complementary manner) over the ancient (purely roman) Rome , or the Baroque Rome does the same over the Rome of the Caesars .