Failure Analysis of Boiler Feedwater Heat Exchanger Tubes

Premature leakage occurred in a bundle of stainless steel tubes within a boiler feedwater reheater at a thermal power plant after approximately ten years of service. Roughly 4% of the tubes failed, primarily within the first unsupported span of the exchanger. A detailed metallurgical failure analysis was performed to determine the root cause of the failures and identify contributing factors.

The investigation included visual examination, metallography, scanning electron microscopy (SEM), X-ray diffraction (XRD), hardness testing, and chemical composition analysis. The tubes were confirmed to be TP304LN austenitic stainless steel with chemical composition and microstructure consistent with specifications. Fractographic examination revealed extensive fatigue striations across the fracture surface, indicating progressive crack growth under cyclic loading. Cracks initiated from the inner diameter surface of the tubes at shallow longitudinal surface indications that acted as stress concentrators.

Metallographic examination showed that these surface indications extended a short distance into the material and likely originated during tube manufacturing or forming. Once initiated, fatigue cracks propagated circumferentially until final overload fracture occurred. Deposits containing iron oxides and chlorides were identified on the fracture surface and inner diameter; however, these were interpreted as secondary environmental products rather than the primary failure mechanism.

The fatigue loading responsible for crack propagation was most likely associated with flow-induced vibration within the heat exchanger. Thermal stresses resulting from temperature gradients and operational fluctuations may have further contributed to crack initiation and growth. The concentration of failures within the first span of the tubes supports the role of localized vibration-induced stresses.

This case study demonstrates how relatively small manufacturing surface imperfections can significantly reduce fatigue resistance when combined with operational vibration. The findings highlight the importance of controlling surface quality during tube production, monitoring vibration in heat exchanger systems, and implementing periodic inspection programs to detect early fatigue damage.