Issue 30

J. Toribio et alii, Frattura ed Integrità Strutturale, 30 (2014) 424-430; DOI: 10.3221/IGF-ESIS.30.51 428 (a) (b) Figure 5 : Process zone: (a) air-cooled steel, (b) steel cooled inside the closed furnace. (a) (b) Figure 6 : Unstable propagation zone: (a) air-cooled steel, (b) steel cooled inside the closed furnace. The critical fracture unit has demonstrated to be a region where the ferrite (and cementite) of neighboring colonies share a common orientation {100}. The size of this orientation unit is controlled by the prior austenite grain size and can be calculated by measuring the extension of cleavage facets appearing on the fracture surface [5, 19]. Fracto-metalographic analysis The specimens’ cuts along longitudinal sections allow the observation, in areas close to the fracture surface, of the micro- structural damage that the material has undergone because of the plastic strain occurring during the standard tension test. In many of the colonies whose lamellae have an orientation close to the axial one of the wire (direction in which the load is applied in the standard tension test) the existence of abundant microcracking is observed (Fig. 7). Therefore, the fracture is determined by processes in the pearlite colony with the lamellae being parallel to the tensile axis, where the deformation occurs in narrow bands of locally intense shear stress [11] according to the Miller-Smith mechanism [20]. In this phenomenon the slip bands in the ferrite produce microcracking in the cementite plates, followed by tearing in the ferrite lamellae. In the colonies with lamellae of thin thickness (air-cooled steel) microcracks with uneven appearance (with regards to the crack opening) and shorter in length than the colony’s size are observed (Fig. 7a). On the other hand, the colonies with greater lamellae thickness (steel cooled inside the closed furnace), the inclined cracking is generally of greater length (even across the complete colony) and looks more uniform (Fig. 7b).