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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Fig. 8. Microstructure-based model of fracture in the case of heavily cold drawn pearlitic steel (on the basis of the pearlitic pseudocolony as the weakest fracture unit promoting crack deflection). 5. Curling of cementite lamellae during cold drawing. Another effect appearing during cold-drawing of pearlitic microstructures is the curling of cementite lamellae (Fig. 9), 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. 9.

Fig. 9. Curling of cementite lamellae in heavily cold-drawn pearlitic steels after six drawing steps (transversal section).

If the ferrite matrix in pearlitic steel is assumed to behave as a softer element in comparison to the cementite plates, the model represented in Fig. 10 would be consistent with the mechanical behaviour of the steels undergoing a cold drawing process. In case (b), the compressive stresses act on the whole in combination with tensile stress, and this may result in a compression of the layers and a closer spacing due to the softer ferrite matrix which makes easier the radial straining (packing effect). In case (c), the cementite plates should act as the resistant component of the colony to bear the stress. Thus these colonies behave as stronger particles inside the material and exhibit a higher transverse resistance to becoming deformed by the rest of the material, producing the aforesaid curling effect. In case (a) a mixed situation is present, which produces firstly a turning effect and later a packing event, since when the arrangement is oriented close to the wire axis the situation approaches the case (b).