PSI - Issue 28

Jesús Toribio et al. / Procedia Structural Integrity 28 (2020) 2396–2403 Jesús Toribio et al. / Procedia Structural Integrity 00 (2020) 000–000

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Fig. 1 shows a scheme of the cuts associated with the metallographic analysis. The horizontal axis of the micrograph corresponds to the radial direction in the wire, while the vertical axis is linked with the hoop cylindrical coordinate in the transverse section and associated with the axial cylindrical coordinate in the longitudinal section of the wire. After cutting, the samples were polished up to mirror quality to reveal material microstructure.

(a) (b) Fig. 1. Materialographic analysis in the: (a) transverse section; (b) longitudinal section.

The microstructure of both materials (hot rolled bar and cold drawn wire) appears in Figs. 2 and 3 in their respective transverse and longitudinal sections. Cold drawing induces important microstructural changes in the steel at the two basic microstructural levels. The colonies become progressively enlarged and oriented in axial direction with cold drawing (Toribio and Ovejero, 1997, 1998a). With regard to the lamellae, the interlamellar spacing decreases with cold drawing (thereby increasing the microstructural packing closeness) and at the same time the axial orientation also increases (Toribio and Ovejero, 1998b, 1998c).

(a) (b) Fig. 2. Microstructure of steel in transverse section: (a) hot rolled bar; (b) cold drawn wire.

(a) (b) Fig. 3. Microstructure of steel in longitudinal section: (a) hot rolled bar; (b) cold drawn wire.

Apart from these general trends, some special pearlitic pseudocolonies have been found and described by Toribio et al. (1997), they behaving as weakest units of minimum material resistance (local toughness ) for fracture initiation.