Therefore the main aim of the present paper is to determine in a quantitative manner the effect of the external uniaxial stress in the range from zero to s/so = 0.5 on the kinetics of hydrogen induced cracking. 2. EXPERIMENTAL PROCEDURES The material used in the experimentation was a carbon steel, type API-5L-X52, from a pipe of 91.4 cm diameter and 2.0 cm thickness. The chemical composition was: 0.14 C, 0.25 Si, 0.99 Mn, 0.01 P and 0.03 S (wt.%). At first, the pipe was inspected with an ultrasonic flaw detector to verify the absence of pre existing cracks and internal defects. Plates of 7.6 cm wide and 17.8 cm long, were cut out from the pipe and ground to get parallel faces and a thickness of 0.9 cm. The greater length of the test plates was parallel to the rolling direction of the pipe. The plate faces were finished with grade 400 SiC grit-paper and cleaned by immersion in an ultrasonic bath for 10 minutes. using a commercial cleaning solution (Ultramet). The plates were stressed using a loading frame as shown in Figure 1. The stress was measured with a strain gauge placed on the face of the plate. Finite element stress field calculation showed that plate exposed test area was under a uniform stress. The applied stress were 15.3, 68.3 and 165.9 MPa, which correspond to a s/so ratio of 0.05, 0.2 and 0.5 respectively (s is the applied stress and so is the yield stress of the material). Power supply C.D. CD Load apparatus Electrolyte Specimen Surface to inspection Acrylic cell Electrode of platinum Load adjustment nut Figure 1. Loading frame and cathodic charging set up. The cathodic charging set up is shown schematically in Figure 1. This set up, promotes hydrogen absorption through only one face of the plate, just as happens in a pipeline in service. The test solution was 0.4 wt % sulfuric acid in bidistilled water plus five drops of a “poison solution", that consists of 2g of phosphorus dissolved in 40 ml of carbon disulfide. The average pH of the test solution was 2.9. The applied current density was 2.5 mA/cm2 and the test were ran at ambient temperature (average 22 °C). The solution was changed every six days, adding five drops of the “poison solution” daily. The growth of cracks in the plate was monitored from the free side by ultrasonic inspection. The inspection technique assured a resolution for crack length increments of at least 1 mm. The test were ran until most of the exposed area was cracked. 3. RESULTS AND DISCUSSION Ultrasonic system used in these studies is a sensitive technique for detecting cracks in samples exposed to hydrogen cathodic charging. Since ultrasonic inspections were conducted before an after to cathodic charging, and those obtained before charging had no defect indications, all defects detected after cathodic charging were due to HIC. Ultrasonic indications were used to identify locations of cracking and quantitative data to characterize the extend of cracking, such as crack length ratios, were obtained. To verify the presence of the cracks, the samples were sectioned for metallographic examination. The cracks observed metallographically were found at the locations indicated from the ultrasonic flaw detector. Figure 2 shows the cracks contours (dashed areas) after the HIC tests at
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