SUMMARY

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, environment factors, and stress conditions are the major factors in SAC crack initiation and propagation in carbon steel boiler tubes. A significant volume or 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. Research was conducted to evaluate above stated variables individually/interactively, and to identify SAC crack initiation and crack growth behaviors in carbon steel tubes. An autoclave with recirculation-loop was set up to simulate boiler-waterside environment in laboratory to systematically evaluate the role of key industrial boiler design and environmental parameters in initiation and propagation of SAC in the laboratory. Slow strain rate tests (SSRT) were conducted under boiler water conditions to study the effect of temperature and oxygen level on crack initiation and propagation on SA-210 carbon steel samples made out of boiler tubes. Magnetite film formed on the surface has a significant role in the SAC mechanism, so coupon exposure tests were conducted to study the characteristics of film formed; Heat treatments were also performed to examine the effect of material microstructure. Interrupted slow strain rate tests (ISSRT) were designed and conducted in lab, based on which, mechanism of SAC cracks initiation and propagation were proposed and validated. Through these activities, significant environmental, operational, and material characteristics were identified to reduce the frequency and severity of SAC. Finally, efforts was made on providing water chemistry control guidelines to reduce SAC susceptibility in recovery boilers and developing a fracture mechanical model to predict SAC failure on industrial boiler tubes.