PSI - Issue 79

Jorge Luis González-Velázquez et al. / Procedia Structural Integrity 79 (2026) 526–533

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metalloscope and a Hitachi Su6600 field-emission gun scanning electron microscope (FEG-SEM) equipped with a Nordlys nano electron backscatter diffraction (EBSD). The optical and SEM images were analyzed by an Image Pro Plus 6.2 image analyzer, and EBSD data weas done by TSL-OIM and ATEX software. For the hydrogen microprint technique (HMT), rectangular blocks of 20×20 ×2 mm cut from uncharged steel plates specimens were ground and polished on both sides. One side was etched with Nital and coated with an emulsion of 5 gr of silver bromide (AgBr) in 10 ml of 1.4 M sodium nitrite (NaNO2), and the uncoated face was exposed to a solution of 0.5 M sulfuric acid (H2SO4) and 3 g/l ammonium thiocyanate (NH4SCN). The procedure starts by pouring the electrolyte solution into the charging cell and applying a DC current density of 5 mA/cm2 for 1.0 h. The hydrogen atoms produced by the acid attack diffuse into the steel specimen, where silver is precipitated as droplets on the sample´s surface. After one hour, the specimen was washed with a solution of 0.6 mol/l sodium thiosulfate (Na2S2O3) and 1.4 mol/l sodium nitrite (NaNO2) for 1 min, to remove the AgBr crystals that were not reduced, and to protect the surface from further corrosion. Finally, the samples were observed in the SEM with EDS to observe the hydrogen release regions revealed by the precipitated silver particles. 3. Results and discussion Fig. 2 shows OM images of the microstructure of the NG and OG pipeline steels samples used in the experimentation. The microstructure of OG steels consisted of equiaxed ferrite grains and pearlite colonies that exibited different degrees of banding along the rolling direction. The microstructures of the NG pipeline steels consisted of a ferrite matrix, with blocky martensite/austenite (M/A) as second phase, whereas the X80 steel had ferrite matrix, containing lath bainite (LB) and scattered areas (less than 5%Vol) of blocky martensite/austenite islands (M/A). The average grain size and content of second phase are given in Table 3.

Table 3. Area fraction of non-metallic inclusions and grain size of the studied steels. Specimen Grain size (μm) Second phase (%Vol) Type X46 22.0 18.5 Pearlite X52 16.0 22.0 Pearlite X56 11.8 10.8 Pearlite X70-1 3.4 23.0 Martensite/Austenite X70-2 2.3 23.0 Martensite/Austenite X80 5.3 55.0 Lath Bainite

Fig. 3 shows the crack contours of two OG (X46 and X56) and two NG (X70-1 and X70-2) cathodically charged steel plates. These plates were chosen because they represent the typical behavior of HIC observed in the experimentation. At first, in both steel types, it is observed that HIC started as randomly scattered individual cracks, identified as Stage I, until almost no new individual cracks appear, and the growth is by interconnection, identified as Stage II, until the cracks eventually interconnected to cover almost the entire surface exposed to cathodic charging. In the X46 (OG) steel, HIC nucleation occurred after 96 hours, while in the X56 (OG) and X70-2 (NG) steel plates, the HIC was fully developed at 24 hours of CC. Furthermore, in the X-46 (OG)steel HIC started after 100 hours of CC. Regarding morphology, the HIC cracks in OG steels had elongated shapes, with the long side parallel to the rolling direction, while the NG steels displayed elliptical cracks. It was observed also that the crack nucleation slowed significantly after 168 hours and completely stopped after 384 hours for OG steels, but in NG steels (X70-1 and X70 2), the HIC nucleation stopped after 264 hours, and the HIC growth was only by interconnection of the existing cracks.