Chloride Stress Corrosion Cracking of an Austenitic Stainless Steel Pipe Fitting in a Hydroprocessing Unit

摘要

Cracking occurred in a stainless steel branch nozzle fitting at a low point in the reactor effluent piping in a Gasoil Hydrotreater unit following start up. Normal preventive measures were implemented to protect the reactor effluent piping system (made of HF modified material) against Polythionic acid stress corrosion cracking (PTASCC) during shut down (neutralization & nitrogen purging), however cracking occurred in the 321 (UNS S32100) type austenitic stainless steel (SS) weld-olet right after the start up. Materials selection of the low point drain, which used Alloy 825 (UNS N08825) piping and a 321 SS weld-olet fitting, is in accordance with good engineering practice. While there have been numerous cases of drain lines suffering cracking when made of 321 or 347 SS (UNS S34700), failures of fittings in similar applications are unusual and are not found in literature. The failed sample was removed for metallurgical analysis and determination of the damage mechanism(s). This paper provides the case history and the investigation into the root cause.