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Gearbox Shaft Failure

Background:

Snap failure of a new gearbox shaft was experienced after a very short time in service. A failure analysis was conducted to identify the failure mechanism(s) and assess possible causes.

Figure 1: Visual Inspection of As-Received Gearbox Shaft

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Figure 2: Optical and SEM Micrographs of the Shaft Material and Fracture Surface

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Summary: TLE tube-tubesheet failure was associated with excessive localized metal loss due to high temperature erosion and metal dusting. Carbide phases at the tubesheet ID known to facilitate metal dusting were observed. Inlet gas flow accelerated metal loss. High levels of carbon were detected via EDS analysis. A flaky oxide scale on the shell side suggested film boiling due to improper cooling. Scale forming elements like Ca, Mg, Al were also detected at the maximum thickness loss regions.

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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