Failure analysis on 304 stainless steel tube of semi water gas preheater in coal chemical plant

A semi water gas preheater of coal chemical plant suffered a serious leakage failure, and a large numbers of fracture cracks were found on the tube bundles. A series of failure investigation including chemical composition, tensile strength, metallographic inspection and corrosion deposits have been conducted to identify the root cause of failure. It can be found that the cracks were initiated from the outer wall of the tube whose morphology was branched pattern. Macro observation showed that the ends were even and no obvious plastic deformation existed. With the scanning electron microscopy (SEM) attached energy dispersive spectroscopy (EDS) analysis, it can be concluded that the surface of the crack area is covered by a thick layer of corrosion products with high sulfur and oxygen content. Therefore, the reason of the fracture may be that the ambient temperature at the lower tubesheet is rapidly quenched to below the dew point temperature when the two gas mediums are exchanging heat energy. The dew point condensation of the shift gas on the outer wall of the tube forms a wet hydrogen sulfide environment, which leads to the typical sulfide stress corrosion cracking (SSCC). By the way, some recommendations have been proposed to avoid the occurrence of the similar failure again. Considering that this kind of cracks failure in heat exchanger tubes caused by single sulfide medium is seldom reported in previous research, this paper provides essential reference for improving the safety maintenance management of similar equipment.