PSI - Issue 3

M.A. Beltrán et al. / Procedia Structural Integrity 3 (2017) 57–67 Author name / Structural Integrity Procedia 00 (2017) 000–000

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3.4 Mesotexture analysis. The grain boundaries having different orientations of 15° and higher are high-angle grain boundaries (HAGB), and this boundaries are generally considered to be effective ones, which can play a role in blocking the cleavage crack propagation as Sang et al. (2007) said. Although, it has been stablished that the HAGB are high energy and high mobility boundaries, they are considered weak boundaries, Venegas et al. (2007). The intergranular cracks would propagate most of the cases, across the HAGB, which are high susceptible to fracture. On the other hand, the special boundaries with low angle (LAGB) are considered like low energy boundaries, according to Mahn et al (2016). The Fig. 7 presents a histogram of the distribution of misorientation angle in the longitudinal and circumferential sections, where could be seen a larger presence of LAGB in the circumferential section than in the longitudinal section with a 20% difference between sections.

Fig. 7. Histogram of grain boundary type distribution. A) Longitudinal section B) Circumferential section.

The effective grain sizes of the longitudinal and circumferential sections were measured. The circumferential section has the larger effective grain size and might have better low-temperature fracture toughness and the largest fracture toughness in the ductile fracture regimen, as stablished Hai et al. (2014). Table 3 presents the effective grain sizes and the ASTM grain size of longitudinal and circumferential sections.

Table 3. Effective grain size of longitudinal and circumferential sections.

Direction

Grain size

Effective grain size

ASTM

μm

μm

Longitudinal

7.5

26.7 31.8

24.62 36.73

Circumferential

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3.5 Tensile properties Room-temperature tensile stress-strain average curves are presented in Fig. 8, from which tensile properties were obtained as listed in Table 4. The steel showed a yield strength (YS) over 340 MPa (49 ksi), satisfying the tensile strength requirements of API X46 grade pipeline steel. According to the tensile tests, the specimens oriented in the C direction have a higher yield and tensile strengths than the L direction. However, the C oriented specimens showed a low elongation percentage, comparing to the L oriented ones, which could be attributed to the orientation of the microstructural banding respect to the sample’s load axis. On the other hand, the C direction does not show the yield point phenomenon, whereas the L direction do. Kim et al (2005), explained the yield behavior in terms of presence and amount of secondary phases contained in the acicular ferrite structure. They reported that the quasi-continuous yield behavior occurred with increasing volume fraction of the polygonal ferrite.