PSI - Issue 17

Flavio Pereira de Moraes et al. / Procedia Structural Integrity 17 (2019) 131–137 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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the ASPM A 262 standard, has been used. Electrolytic etching has been performed using oxalic acid (10%) with a current density of 1 A/cm 2 for 90 seconds. All executed mechanical tests followed standard procedures and were performed at room temperature.

3. Results and discussion

3.1 Microstructural characterization

Fig. 1 and 2 give a general view of the microstructure of the pipe using optical microscopy (Fig. 1) and scanning electron microscopy with backscattered electrons (Fig. 2). It may be observed that an extensive precipitation occurred both at grain boundaries as in the grain interior, as well as on the twin boundaries. Furthermore, the delta ferrite (elongated phase) present before exposure to high temperature transformed itself into a whitish phase. This elongated and white phase and the larger precipitates placed at the grain boundaries allowed chemical microanalysis using energy dispersion with small matrix interference. Table 2 presents the EDS results for the white phase that substituted the delta ferrite (DF) and for a white particle at the grain boundary (GB), both identified as sigma phase, in comparison with the results obtained by Weiss et al. (1971).

Fig. 1. Optical micrography of the pipe (etched with Glyceregia) after exposure for 100,700 hours at 640ºC.

Fig. 2. SEM using backscattered electrons of the pipe (polished and unetched sample) after exposure for 100,700 hours at 640ºC.