Failure analysis of austenitic stainless steel tubes in a gas fired steam heater

Highlights: ► 304H stainless steel is more susceptible to caustic SCC compared to SA335 alloy steel. ► Caustic attacks the protective layer of stainless steel superheater tubes. ► Sigma phase formation at the weld zone causes crack initiation in fired heater tubes. -- Abstract: Carryover of caustic soda (NaOH) in the steam path caused catastrophic failure of superheater 304H stainless steel tubes in a gas fired heater and led to an unexpected shutdown after just 5 months of continuous service following the start of production. The cause of the failure was studied, with a focus on the effect of caustic embrittlement on stress corrosion cracking (SCC). The cracks were examined at the seam weld, heat affected zone (HAZ), and U-bend areas. Hardness was measured for the base metal, HAZ, and weld metal, and microstructures were examined using optical microscopy and scanning electron microscopy (SEM). Crack initiation is attributed to gouging on the precipitated carbide at the HAZ and also the formation of sigma phase in the weld metal, as shown by energy dispersive X-ray (EDX) analysis. In addition, cracking was propagated by caustic embrittlement because of residual stresses and hammering. Finally, the characteristic feature of fracture was illustrated by SEM fractography, and consists mostly of intergranular SCC and some quasi-cleavage transgranular.