PSI - Issue 13

Reza Khadem Hosseini / Procedia Structural Integrity 13 (2018) 232–237 Reza Khadem Hosseini / Structural Integrity Procedia 00 (2018) 000 – 000

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2.2. Tube material

Tube sample was chemically analysed using spark emission spectrophotometry with worldwide Analytical systems AG (model PMI – Master). The chemical composition of the tube material is presented in Table 2 that is consistent with the standard (DIN 17175-79). The chemical composition of tube material conforms to the specifications for the steel grade ST.45.8. Additionally, hardness test was performed in 13 points indicated in Fig. 1, using Karl Frank 38106 hardness tester and the Rockwell Type B (HRB) method (EN ISO 6508:2005 ) . According to Table 2, the nearest areas to the opening have the maximum hardness.

Table 2. Chemical composition of the steam tube material compared to the standard (wt. %). Elements C Mn Si P S Cr Fe Tube material 0.13 0.72 0.24 0.022 0.031 0.25 Balance DIN 17175-79 0.21 0.4 - 1.2 0.1 - 0.35 0.040 Gr. ST. 45.8 0.040 - Balance Table 3. Rockwell B hardness values of the cross section of tube according to Fig. 1 Point Thickness ( mm ) Point Thickness ( mm ) A 94.5 H 76.5 B 93.0 I 79.5 C 88.5 J 83.0 D 87.0 K 87.0 E 80.5 L 90.5 F 77.5 G 73.5 M 93.0

2.3. Microscope analysis In order to perform microscope analysis optical microscope (OM) and scanning electron microscope (SEM) were used. Optical microscopy and scanning electron microscope were done using Olympus BH2-MJL microscope and TESCAN MIRA3 field emission gun scanning electron microscope (FEG – SEM), respectively . Specimens were cut from the failed tube along the crack and away from the failure opening, at 180° apart along the circumference, to examine variations if any in microstructure along the tube circumference. Both transverse and longitudinal sections were examined namely, 1 to 4 as shown in Fig. 2. The samples were polished using a series of emery papers and finished with diamond polishing and then etched with Nital etchant (2 ml HNO3 in 100 ml of ethanol).

Fig. 2. Samples prepared from the adjacent and the opposite side of opening for microstructural analysis.

The optical micrographs of the specimens in the adjacent areas to the failure opening, namely 1 and 2 were shown in Fig. 3a and b. The microstructures represent martensite blocks (M) in the ferrite matrix (F) and few carbides (C) near the grain boundaries.