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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2.4. XRD analysis

X-ray diffraction (XRD) analyses were used to determine different phases existed in the deposit components. To do this, the deposit sample picked from the internal surface of the failed tube was prepared on zero scattering foil and step-scanned in a STOE Stadi P diffractometer using unfiltered Cu Ka radiation. The scanning speed was 0.015 degree/second in the range of 5°- 120°. The machine was operated at 40 kV and 30 mA. According to Fig. 6 in which the diffractogram of the deposit has been shown, it can be concluded that the analysed deposit consists mainly of hematite (Fe 2 O 3 ) and magnetite (Fe 3 O 4 ).

Fig. 6. The X-ray diffractogram of the deposit on the internal surface of the failed tube indicated that mainly Fe 2 O 3 and Fe 3 O 4 exist.

3. Discussion

Visual examination of cross section of failed tube revealed that the opening area had tinned by 30% (Fig.1) in comparison to the opposite side of failure opening which typically considered thin-lipped related to short-term overheating. Also, there was 10% ovality which can be another feature of short-term overheating. Although the optical and SEM micrographs showed that the microstructures of samples adjacent to the opening consist of martensite blocks in ferrite matrix and few carbides, the microstructures of samples in the opposite side of the failure opening consist of normal ferrite and pearlite, corresponding to the chemical composition of the tube. Therefore, the microstructural changes and phase transformation are evident in the area of the failure. Since, the formation of martensite requires the temperature to rise into the intercritical (A1 to A3) range, the temperature of the phase change in the steels (above 700 °C), and then get cool rapidly (Erisir et al., 2013), the tube may experience temporarily increasing temperature. This is corresponded to the information reported by the plant inspection about the operation of tubes without water for a period of time with temperature higher than operating temperature by intercritical range, and may approve the hypothesis of short-term overheating. Additionally, hardness test results showed that the maximum hardness is seen in areas near to the failure opening which can be the consequence of the formation of harder phase like the martensite instead of the pearlite in the microstructure and confirmed short-term overheating as a failure mechanism, as well. Furthermore, XRD analysis indicated that iron oxides including hematite (Fe 2 O 3 ) and magnetite (Fe 3 O 4 ) were the main constituents of deposit on the internal surface of the tube and so there is no evidence that corrosion has been led to failure.

4. Conclusion

Research presented in this paper was concerned with root cause analysis of super-heated boiler tube failure. The fractured tube had suffered short-term overheating, as supported by partial phase transformation and the classic thin lipped rupture characteristics. In this study microscopic interpretation and visual observations were used to distinguish