Cracking of Stainless Steel Fitting in an Offshore Environment

摘要

This article investigates the failure of a Type 316L stainless steel (UNS S31603) fitting in an offshore environment. Visual examination of the cross section revealed branching cracks initiating from the outer surface of the fitting close to the area containing coating damage. The cracks occurred on the fitting base material away from the weld and heat-affected zone. Based on the laboratory tests conducted, it was found that the fitting failed by external stress corrosion cracking under the combined action of stress, sensitized microstructure, and exposure to a severely corrosive offshore atmosphere containing high levels of salinity. A 38.1-mm (1.5-in) outside diameter piping system failed after ～35 years of service. The failure was located on the fitting joined to a pipe using a fillet weld. The piping system is exposed internally to liquid butane. The design and operating temperatures were -45.6 °C (-50 ℉ ) and -42.8 °C (-45 ℉), while the design and operating pressures were 1.2 MPa (170 psig) and 0.02 MPa (0.3 psig), respectively. A leak was reported from the piping system, and it was found that the leak source was from the fitting. External coating damage was noticed on the fitting and liquid was leaking from the location with coating damage. No leakage was observed in areas where the coating was intact. This article details the metallurgical investigation conducted on the failed fitting welded to a pipe.