Failure investigation of AISI 321 austenitic stainless steel corrugated flexible hose in a reforming plant

A failure investigation was conducted on a 321 stainless steel corrugated flexible hose in a reforming plant. Several of these flexible hoses failed while in service after being subjected to elevated temperatures and unexpected shutdowns. Visual inspections showed various surface deposits, and energy‐dispersive spectroscopy elemental analysis detected the presence of chlorine, sodium, potassium, and sulfur, which are indicative of stress corrosion cracking and possible caustic stress corrosion cracking. Optical and scanning electron microscope imaging showed that the cracks were typically stress corrosion cracking‐type with a branched pattern, initiating from the inner surface and propagating intergranular to the outer surface. Coarse and elongated δ‐ferrite islands in the heat‐affected zone were identified as crack propagation pathways. The average hardness measurements were approximately 240 HV0.5 ± 3 HV0.5 for the base metal, 257 HV0.5 ± 3 HV0.5 for the heat‐affected zone, and 265 HV0.5 ± 3 HV0.5 for the weld metal. These variations, influenced by welding process parameters such as cooling rate, made the heat‐affected zone a potential failure region in the flexible hose. The root causes of the failure of the flexible hoses were determined to be improper service conditions in terms of temperature control, unexpected shutdowns, and insufficient control of welding process parameters.