PSI - Issue 37

Jesús Toribio et al. / Procedia Structural Integrity 37 (2022) 1007–1012 Jesús Toribio / Procedia Structural Integrity 00 (2021) 000 – 000

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The appearance of leopard skin micro-damage in the form of dark specks is achieved due to the chemical attack produced by (nital) acid, which enters the micro-voids generated by the breaks in the cementite lamellae, thereby producing an over-attack. The micro-damage, called leopard skin , can evolve throughout the different stages (passing through the dies) of the drawing process towards: – Leopard claw marks : formed by the union between the micro-damage of leopard skin forming micro breaks with directions close to 45º in relation to the direction of the drawing process, as shown in Fig. 3 (b). – Tearing : occurring either between colonies or between the lamellae of the same colony, sometimes generating the fracture of the perlite colonies in question. The process consists of the union of micro leopard skin defects, generating a tear in the material composed of a hollow without noticeable cementite lamellae inside. In the stage that precedes the formation of the tear, from this micro-defect, it can be seen how the leopard skin deforms in the direction of the drawing, Fig. 3 (c). – Constraint : due to the great constraint between colonies, the cementite lamellae that have been fragmented can come together, when they move, generating morphologies similar to labyrinths, Fig. 3 (d). 3.2. Leopard claw marks It is a micro-defect produced by the union of the microcavities generated by the breakage of the cementite lamellae during wire drawing. It can be seen in colonies whose lamellae are oriented close to the main direction of the wire, Fig. 4 (a). The aforesaid micro-defect has an approximate orientation of 45  , due to a Miller & Smith type formation mechanism, and it can be observed even in micrographs belonging to the first drawing step, with which the drawing of a single row would be enough to generate micro-damage in colonies whose lamellae are oriented in the direction of the longitudinal axis of the wire. Leopard claw marks appear in all wires belonging to all drawing steps. As has been observed in the analyzed micrographs, these claws as well as its precursor micro-damage, leopard skin, are shown in colonies whose lamellae already are oriented in the direction of the longitudinal axis of the wire. As the drawing process progresses, the micro defect called leopard claw, generated in the early stages, can evolve towards: – The formation of an intercolonial tear: especially in those colonies with lamellae that barely undergo rotation during the process of conformation by being oriented in the longitudinal direction of the wire, Fig. 4 (b). – As microcracks quasi-parallel to the longitudinal direction of the wire: in those colonies whose lamellae have rotated notably towards the direction of the longitudinal axis of the wire during the drawing process, Fig. 4 (c).

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Fig. 4. Micrographs pertaining to the evolution of the micro-damage called leopard claw marks .