PSI - Issue 68

Reza Khadem Hosseini et al. / Procedia Structural Integrity 68 (2025) 409–414 R. K. Hosseini / Structural Integrity Procedia 00 (2025) 000–000

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Table 2. The EDAX results of elemental analysis of regions "A", "E", and "F", as shown in Sig. 5a

Regions

C

O

Cr

Mo 2.4 1.0 1.0

Fe

A E

8.0

2.5 3.9 1.3

12.0 11.4

75.0 54.5 74.1

29.3 15.3

F

7.9

2.5. Micro-hardness Testing The Vickers micro-hardness test was performed using a Leitz micro-hardness tester with a 200 g load at various points on the failed sample subjected to SEM analysis, as illustrated in Fig. 6. The lowest hardness recorded was 116 HV near zone "E," while the highest hardness reached 325 HV near zone "A."

Fig. 6. The micro-hardness results of different parts of a section of the failed tube sample B, as shown in Fig. 5a.

3. Discussion Stereographic microscope images of cavities on the inner surface of the tube sample A reveal fully rounded and hemispherical morphologies, closely resembling those reported for metal dusting mechanisms (Orlikowski, 2018; Hucińska and Gajowiec, 2010). The corrosion reaction initiates with the saturation of the metal matrix by carbon, which reacts with iron to form iron carbide in the form of cementite precipitating at the metal surface and grain boundaries (Orlikowski, 2018; Grabke, H.J., 1995). Macro and micro-structural evaluations of tube samples indicate a high level of carburization near both the inner and outer surfaces of the tubes. It appears that the deep hole on the inner surface of sample B resulted from the coalescence of smaller cavities. The highest intensity of metal dusting is reported to occur at temperatures around 700 °C (Orlikowski, 2018), suggesting that localized temperature increases may have occurred in certain areas of the tubes. This hypothesis is supported by observed changes in SEM micrographs transitioning from ferrite-pearlite to bainitic microstructures. Furthermore, micro-hardness results demonstrated significant variations in hardness that corresponded with microstructural observations. The area exhibiting a bainitic microstructure (zone “A”) showed the highest hardness, nearly two and a half times greater than that of the area with a ferritic-pearlitic microstructure (zone “F”). In addition to these observations, changes in alloying element concentrations across different areas of the tube wall profile—based on EDAX analysis results—have been previously documented in instances of metal dusting damage. Specifically, in 9Cr-1Mo steel, carbon transport is accompanied by outward diffusion of chromium and molybdenum, which are stronger carbide formers than iron. This leads to the formation of high-alloy carbides beneath the steel surface (Hucińska and Gajowiec, 2010), as evidenced by the higher values of chromium in zones "A" and, "B" (12 wt.% and 11.4 wt.%, respectively) than the original chemical composition of the tube (9.1 wt.%) as shown in Table 2. Regarding the root cause of the hole located just below the Thermo Skin Shield welding joint, it is likely that the welded sheets acted as fins, thereby accelerating heat transfer from that point to other areas of the tube. Consequently, it is plausible that heat absorption in that region exceeded that in surrounding areas.