Factors affecting stress assisted corrosion cracking of carbon steel under industrial boiler conditions
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Failure of carbon steel boiler tubes from waterside has been reported in the utility boilers and industrial boilers for a long time. In industrial boilers, most waterside tube cracks are found near heavy attachment welds on the outer surface and are typically blunt, with multiple bulbous features indicating a discontinuous growth. These types of tube failures are typically referred to as stress assisted corrosion (SAC). For recovery boilers in the pulp and paper industry, these failures are particularly important as any water leak inside the furnace can potentially lead to smelt-water explosion. Previous research shows that metal properties, environmental variables, and stress conditions are the major factors influencing SAC crack initiation and propagation in carbon steel boiler tubes. A significant volume of work has also been published to show that the use of carbon steel in high temperature water applications strongly depends upon the formation and stability of a protective magnetite oxide film on the waterside of boiler tubes. This present study is aimed at evaluating above stated variables individually and interactively to identify SAC crack initiation and crack propagation behavior in carbon steel boiler tubes. Other goal of this research is to understand the mechanism of bulbous SAC crack formation under industrial boiler operating conditions, thus to figure out a practical way to predict and prevent SAC type failures in the industrial boilers.