PSI - Issue 17

C. Medrea et al. / Procedia Structural Integrity 17 (2019) 526–531 C. Medrea, E. Pappa, D.G. Papageorgiou, C. Stergiou / Structural Integrity Procedia 00 (2019) 000 – 000

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Fig.7 . (a) General aspect of internal area of the third fragment including defect details.

Inspection with penetrant liquids held on both sides of all fragments revealed many defects, some of them already detected with naked eye; toe cap crack on elliptical welding (Fig. 8a). A network of secondary cracks initiated on the welding root and developed on the flow direction of the high temperature steam (Fig. 8b). The microstructural changes of the material during the welding process, rends it prone to cracks. Vertical cracks between the welding, the reinforcing metal pad and the pipe lining of the entrance of high temperature steam were observed (Fig. 8c).

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Fig. 8. Defects detected with liquid penetrant testing; (a) Toe cap crack on elliptical welding, (b) Network of cracks developed on gases flow direction, (c) Vertical cracks on the interface between reinforcing pad and pipe.

The fragments were further sectioned and samples were selected for further examination. Figure 10a shows a representative specimen selected for inspection after macro etching. As can be seen on the figure, they are concomitant visible sections of all welded zones. The welding between high temperature inlet pipe and the reinforcing pad was named “Weld 1”, the welding between high temperature inlet pipe and outlet process pipe was named “Weld 2” and the welding between outlet process pipe and the reinforcing pad, “Weld 3” respectively. A closer view on welding 1 reveals a discontinuity of the welding area and the basic metal (Fig. 9b). The separation of the two regions is related to the chemical affinity of the jointed materials, as well as to the deposition conditions of the electrode. Small cracks can be observed on the bottom (the last welding layers) areas of heat affected zone suggesting lower background temperature than the minimum required for the next run (details pointed with red arrows). The crack on the last run of the welding could be a sign of cold weld cracking (black arrow on Fig. 9b). Metallographic examination is suggested in order to confirm the founding. Moreover, the heat affected zone is distinguishable only on the horizontal area below the welding. (Fig. 9b,d). Spattered and large pores are observed in different spots of the weld (Fig. 9c). Furthermore, porosity related to poor cleaning between passes was detected on the opposite side of welding 1 (Fig. 9d). This defect is correlated with the welding conditions such as current strength and welding deposition rate.