PSI - Issue 3

Jesús Toribio et al. / Procedia Structural Integrity 3 (2017) 3–10

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Author name / Structural Integrity Procedia 00 (2017) 000–000

4. Microstructural integrity of pearlitic steels Apart from the general trends of 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 appear during cold drawing, so that the new concept of microstructural integrity could be introduced. Between the most relevant phenomena taking place during the drawing process, cementite dissolution has been described by Languillaume et al. (1997) and Borchers and Kirchheim (2016). In addition, breaking of cementite lamellae or special interlamellar spacing appears in the research by Toribio et al. (1997) who in addition identified, denoted and described a new (non-conventional) microscrostructural unit: the pearlitic pseudocolony (Fig. 4), a special pearlitic colony in which the lamellae are not oriented along the wire axis or cold drawing direction, thereby producing an anomalous (extremely high) pearlitic interlamellar spacing. These characteristics make them weakest areas or potential fracture initiation units, i.e., places able to produce fracture path deflection.

Fig. 4. Pearlitic pseudocolonies in heavily cold-drawn pearlitic steels after six drawing steps (longitudinal section).

Another effect appearing during cold-drawing of pearlitic microstructures is the curling of cementite lamellae (Fig. 5), a seemingly paradoxical phenomenon since the cementite (Fe 3 C) phase is the hardest (and more brittle) phase, surrounded by the softer (and more ductile) ferrite (Fe) phase, but it seems that the cementite is ductile enough to allow a curved shape to appear, thus producing the curved features shown in Fig. 5, with many areas with very high curvature, so that the initially flat cementite plates become cylindrical surfaces or even more complicated shapes, with changes in the sign of the curvature.

Fig. 5. Curling of cementite lamellae in heavily cold-drawn pearlitic steels after six drawing steps (transversal section).