Non-destructive techniques for the detection of sigma phase in duplex stainless steel: A comprehensive review

Sigma phase is a deleterious phase which can be formed in duplex stainless steels (DSSs) during heat treatment, manufacturing, or welding processes, leading to a decrease of mechanical properties and corrosion resistance. This issue is highly concerning for numerous industries because the presence of σ–phase in the austenite/ferrite microstructure of DSSs reduces their lifetime and promotes a brittle fracture in service. This review focuses on the non-destructive techniques (NDT) for the detection of σ–phase and their application in engineering failure analysis and field study cases in recent years. The most used NDT as inspection tools are ultrasound (US) and magnetic techniques; there are further alternative methods such as electrochemical potentiokinetic reactivation (EPR) technique and replication metallography (RM) analysis. Their advantages and drawbacks are discussed and exemplified with case studies. The current trend for NDT applied to the σ–phase is thoroughly studied and represented, showing the key mechanisms and factors influencing σ–phase formation and its consequences on DSSs. In addition, a detailed description of important failure cases wherein σ–phase has been responsible is presented.