micrographs. Figures 3a and 3b plot the quantification of this orientation effect at the levels of colonies and lamellae respectively. Figure 1: Microstructure of steel 0 (L-section) Figure 2: Microstructure of steel 6 (L-section) 0 10 20 30 40 50 60 Angle(º) 1.1 1 0.9 0.8 0.7 0.6 0.5 Di / Do T L 0 10 20 30 40 50 60 Angle (º) 1.1 1 0.9 0.8 0.7 0.6 0.5 Di / Do L T (a) (b) Figure 3: Evolution with cold drawing of the orientation angles of pearlite colonies (a) and lamellae (b), in the longitudinal (L) and transverse (T) metallographic sections. In (a) the angle was measured between the major axis of the colony (modeled as an ellipsoid) and the axial direction (L section) or the radial one (T section). In (b) the angle was measured between the direction of the lamellae in the metallographic cut and the axial direction (L section) or the radial one (T section). EXPERIMENTAL PROGRAM To analyze the hydrogen assisted cracking (HAC) behavior of the different steels, slow strain rate tests were performed on precracked steel wires. Samples were precracked by axial fatigue in the normal laboratory air environment to produce a transverse precrack, so that the maximum stress intensity factor during the last stage of fatigue precracking was Kmax = 0.30KIC, and the crack depth was a = 0.30D in all cases, with D as the wire diameter. After precracking, samples were placed in a corrosion cell containing aqueous solution of 1g/l Ca(OH)2 plus 0.1g/l NaCl (pH=12.5). All tests were conducted under potentiostatic control at –1200 mV vs. SCE at which the environmental mechanism is HAC [1]. The applied displacement rate in axial direction was constant during each test and proportional to each wire diameter: 1.7 x10–3 mm/min for steel 6 and 3.0 x10–3 mm/min for steel 0. The experimental results showed an important fact: the HAC behaviour becomes more anisotropic as the cold drawing degree increases, as depicted in Figure 4 by means of the fracture profile (macroscopic topography of the fracture surface). Thus a transverse crack tends to change its propagation direction to approach the wire axis or cold
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