Reduced ductility of in-service 9Cr1Mo piping revealed during welding procedure qualification

摘要

Plant reliability and inspection systems identify critical process units and components for detailed inspections during shutdowns. Regular visual inspections at suitable periods remain the primary method for initial detection of failure modes. Once a mode of deterioration is identified or suspected, non-destructive examination such as surface methods, hardness testing, radiography and surface replication are normal means for further comparison and evaluation. Where welding to in-service material is required, one of the most common and practical methods is to apply existing qualified welding procedures. Process related visual and metallurgical changes in materials are two main factors to be considered when existing welding procedures, qualified on new material, are proposed. Such procedures may not adequately assess weldability and/or the properties of the in-service welded joint. Systematic visual inspection and the advantage of welding procedure qualification testing using ex-service materials, particularly in low alloy materials where risk is a factor are reinforced. A case study is presented which highlights that process conditions can significantly affect material properties, particularly air-hardenable ferritic alloy piping often used in high temperature utility and process applications.