Issue 30

J. Toribio et alii, Frattura ed Integrità Strutturale, 30 (2014) 424-430; DOI: 10.3221/IGF-ESIS.30.51 427 The yield strength in the pearlite is consistent with the stress required to move the dislocations in ferrite between two impenetrable walls made of cementite, where ferrite-cementite interfaces act as barriers to dislocational movement [11]. The stress increases with the refinement of the interlamellar spacing leading to an increase in the resistance, so that the yield strength increases with decreasing interlamellar spacing following a Hall-Petch type relationship [1, 2], with the exponent -1/2. This expression sometimes results in an internal frictional stress in the ferrite with a negative value, which is why some authors prefer to use the exponent -1 in this equation [4, 5, 7]. Increasing the cooling rate, used in the heat treatment of the steel, reduces the maximum uniform elongation attained in standard tension tests, while increasing the total elongation in the necking. Deformation (dislocational structure) has a size effect on interlamellar spacing [15]. Fracture surface Macroscopically, the steels’ fracture surface shows a more brittle appearance with a decreasing cooling rate (Fig. 4). While in the air-cooled steel a necking and cup-cone fracture is observed, steel cooled in the furnace broke without even a necking, and with a more brittle fracture and with a more irregular topography. For steel cooled in the partially-open furnace, the aspect is in between the other two steels analyzed. (a) (b) (c) (d) (e) (f) Figure 4 : Fracture surfaces of the steels. Top view: (a) PA; (b) PS; (c) PF. Front view: (d) PA; (e) PS; (f) PF. The fracture begins in the process zone, continues with an unstable propagation zone with a radial form to the wire surface, and ends at the external ring. The fibrous process zone, situated in the central area of the wire (of a lighter color in the photographs), is deviated from the center of the wire; to a greater extent as the cooling rate used in the treatment decreases. The catastrophic propagation zone shows radial micro-cracking, which grows more winding with increasing cooling time, while the external ring thickness decreases (ductile shear lip inclined 45°). The fracture surface’s roughness increases with decreasing cooling rate (Fig. 4), and, in some tests, for the steel cooled inside the furnace, secondary cracking contained in the cross section of the wire was also observed (Fig. 4f). Fractographic analysis For the air-cooled steel, the process zone at high magnification shows the presence of microvoids (ductile fracture mechanism) of various sizes (round and elongated) along with small regions where pearlite lamellae are observed (Fig. 5a). In steel cooled inside the closed furnace, regions formed by pearlite lamellae are more extensive than in the air-cooled steel, thus presenting a more brittle fractography (Fig. 5b). In the unstable propagation zone, the fractography consists of cleavage facets, with microvoids regions among them (Fig. 6). For air-cooled steel the size of these cleavage facets (Fig. 6a) is smaller than for steel cooled in the closed furnace (Fig. 6b), a phenomenon related to the size of the prior austenite grain. The microvoid regions between the cleavage facets are most abundant in the air-cooled steel (with more ductile fractography) than in steel cooled inside the furnace.