13th International Conference on Fracture June 16–21, 2013, Beijing, China 2. Experimental details A section of a new 610-mm (24-inch) API 5L X52 pipe was used to obtain RD-oriented strips in order to have identical starting samples of this steel (Table I). These samples were subjected to different thermomechanical treatments and a total of 5 usable samples, that is, with similar microstructures but different crystallographic textures, were selected for further testing. HIC tests were conducted by cathodic hydrogen charging for 48 hours in a hydrogen sulfide (H2S) saturated solution under a current density of 50 mA/cm2. The solution was selected very close to the TM 0177-B solution recommended in the NACE TM0284-96 standard [8]. Table 1. Chemical composition, in wt%, of the starting API 5L X52 steel C Mn S P Si Cu Cr 0.212 1.334 0.032 0.028 0.037 0.021 0.009 Table 2 shows the thermomechanical paths followed by the investigated samples, which can be grouped into two categories: warm- rolled samples (WRA, WB), and cold- and hot-rolled samples (HRA, HRB, CRA). All samples were first austenitized at 1040°C for 1 hour, them rolled as indicated in Table 2, and finally treated at 850°C for about 100 seconds to ensure complete recrystallization with no substantial grain growth. Table 2. Thermomechanical path followed by the investigated samples Sample Thickness reduction (%) Path Final rolling temperature (°C) WRA 75 2 steps, 50% each 800 WRB 75 2 steps, 50% each 600 HRA 75 2 steps, 50% each 1040 HRB 50 15 steps, 5% each 1000 CRA 50 15steps, 5% each 27 The global or macrotexture X-ray texture of the samples was determined by measuring three incomplete pole figures: {110}, {200}, and {111} using a Mo texture goniometer. The orientation distribution function (ODF) of each sample was determined from the measured pole figures assuming a cubic-triclinic symmetry. The ADC method [8] was used to estimated the ODFs, which are represented here in the ϕ2 = 45 deg. section of Euler space [9]. The metallographic inspection and microtexture2 study of the samples were performed using scanning electron microscopy and automated FEG/EBSD, respectively. Orientation Imaging Microscopy (OIM) was used to analyze the EBSD measurements. Grain boundaries (GBs) were defined in the EBSD-derived orientations maps by the presence of a point-to-point misorientation greater than 3 degrees. To study the mesotexture2 of the samples, GBs were categorized into: (i) low-angle (LABs, with misorientation angles less than 15 deg.), (ii) high-angle (HABs), and (iii) coincidence site lattice (CSL, Σn [9]). 3. Results Figure 1 shows examples of typical optical micrographs taken from the studied samples. It is observed the characteristic banded pearlite/ferrite microstructure of low-strength carbon steels after 2 The term ‘microtexture’ refers to individual orientation measurements that can be related to their location in the sample. ‘Mesotexture’ refers to the distribution, or texture, of GBs [9]. -2-
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