PSI - Issue 26

Jesús Toribio et al. / Procedia Structural Integrity 26 (2020) 360–367 Toribio / Structural Integrity Procedia 00 (2019) 000 – 000

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4. Micro- and nanostructural integrity of pearlitic steels during cold drawing: the pearlitic pseudocolony . 4.1. Factors affecting micro- and nanostructural integrity of cold drawn pearlitic steels Apart from the general microstructural evolution in drawn pearlite (progressive orientation of pearlitic colonies and lamellae in the wire axis or cold drawing direction and decrease of interlamellar spacing), special features also appear, so that the innovative concepts of micro- and nanostructural integrity could be introduced. Between the most relevant phenomena taking place during the drawing process, cementite dissolution has been described by Borchers and Kirchheim (2016). In addition to these general trends of drawing-induced microstructural orientation and increase of packing closeness, some exceptions can be found in the case of the most heavily cold drawn steels. The most remarkable fact is the presence in some areas in the drawn steel of a special ( non-conventional ) microstructural unit (see appearance in Fig. 5), the so-called pearlitic pseudocolony , a term coined (more than twenty years ago) by Toribio et al. (1997) in a pioneering paper on microstructural bases of anisotropic fracture behavior of heavily cold drawn pearlitic steels.

Fig. 5. Pearlitic pseudocolony in a heavily cold drawn pearlitic steel after six drawing steps (longitudinal section of the wire). The boundaries of the pseudocolony are marked in the micrograph through a white dashed line. The vertical side of the micrograph is parallel to the wire axis or cold drawing direction, while the horizontal side is associated with the radial direction of the wire. Fig. 6 offers a enlarged view at high magnification of a pearlitic pseudocolony like that described in the previous paragraph, showing evidence of cementite lamellae that are not properly oriented following a direction parallel to the wire (drawing) axis (they remain transverse to the wire), curling of cementite and anomalous ( extremely high when compared to the average) local interlamellar spacing, and even some evidence of local fracture of specific lamellae.

Fig. 6. Enlarged view (x8000 magnification) of a pearlitic pseudocolony in a heavily cold drawn pearlitic steel after six drawing steps (longitudinal section of the wire). The vertical side of the micrograph is parallel to the wire axis or cold drawing direction, while the horizontal side is associated with the radial direction of the wire.