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Cracked Hastelloy U Bend

Background: Cracking of C 276 U bend samples exposed to Sodium Methlaminobutyrate, 60% Sodium Hydrosulfide, and 40% Water with operating temperature between 210-435 °F.

Figure 1: Visual Inspection of As-Received U Bend Sample

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Figure 2: Optical and Scanning Electron Microscopic Examination of U Bend Cross-sections

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Summary: This U bend tube damage was associated with environmentally assisted cracking likely due to hydrogen sulfide. High levels of S were detected on the crack fracture surface. Ni-Cr-Mo alloys have been found to be susceptible to stress corrosion cracking in aggressive conditions viz. T>200 °C, acidic pH, and presence of hydrogen sulfide. Take Away: Even in alkaline pH the stress corrosion attack can possibly occur on Ni-Cr-Mo alloys in presence of hydrogen sulfide and operating temperatures higher than 200 °C.
Take Away: Temperature excursions coupled with improper cooling can lead to premature failures of TLEs. The microstructural changes in the material due to thermal exposure can further facilitate the loss of material under synergistic mechanisms of metal dusting and erosion.

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