Failure Analysis of a Stainless Steel Pipe Elbow in a Purge Gas Line

摘要

A failure analysis was performed on a SA-312 TP304 stainless steel pipe elbow in a purge gas line at a petrochemical plant. Fatigue was confirmed to be the mechanism of the failure. Visual, dimensional, fractography, macrostructural, and vibrational evaluations; as well as microindentation hardness measurements, material characterization, and morphology analysis, were performed. The contributing factors for the fatigue were extreme steady-state piping vibration in the line of the elbow, a non-conformance weld joint, and improper pipe installation which caused external stress on the elbow. The fatigue most likely was originated by a crack at an inclusion in the grain boundary of the austenitic microstructure. The results emphasize that by not complying with adequate piping installation, with welding practices and without reducing or attenuating vibrations in equipment or at its proximity; failures associated with fatigue in this material will tend to be present, increasing the need of costly plant shutdowns.