Development of non-destructive tools for in-situ condition assessment of austenitic stainless steel boiler tubes

The steel alloy UNS S30432, commonly referred to as Super 304H, is increasingly being utilized in the high temperature superheater and reheater sections of the boiler of coal-based power plants, where the metal wall temperature does not exceed 700 °C. In case of accelerated overheating caused by flow restrictions within the boiler furnace or regular temperature fluctuations in the region where these tubes are located, it becomes essential to identify the affected tube sections for planning their timely replacement to avoid any forced outage and consequential economic loss to the Power Utility. This paper elaborates on electromagnetic-based methods that hold potential for monitoring creep degradation in austenitic stainless-steel materials. Further, case studies of five sample tubes of Super 304H are also discussed, wherein a significant correlation between microstructural changes, such as the proliferation and growth of sigma (σ) phases within the Super 304H tubes, and alterations in electromagnetic responses in the eddy current and magnetic coercive force (MCF) based testing has been observed. Due to the features of rapid scanning, portability and easy field deploy-ability offered by these techniques, along with their reliability, they are useful and effective in identifying selected portions of such degraded tubes for their timely replacement.