Failure analysis of leakage at tube-to-tubesheet joints of a waste heat boiler

Abstract

Failure analysis of a waste heat boiler with leakage occurring at the tube-to-tubesheet joint is presented in this study. In order to identify the causes of leakage, a series of investigations were conducted including visual inspection, chemical composition analysis, metallographic examination, hardness evaluation and crack fracture profile analysis. The results showed that the leakage was caused by the caustic stress corrosion cracking (caustic SCC), which was mainly resulted from the welding residual stress and caustic concentration between the tube and tubesheet. The welding flaw promoted the cracks initiation and growth, and widmanstatten structure in the heat-affected zone deteriorated the corrosion resistance. Furthermore, the mechanism of the caustic SCC was discussed. Finally, practical countermeasures were put forward to prevent the similar failure from happening again.