Duplex stainless steels (DSSs) are complex steels. These materials were originally manufactured to exploit the benefits of high strength and corrosion resistance arising from the duplex microstructure, i.e., ferrite and austenite. However, this very duplex nature can make these steels susceptible to failures commonly observed in both austenitic and ferritic stainless steels. In addition, improper fabrication and welding techniques can lead to the formation of deleterious sigma and chi phases, which can cause brittle failure.
In this paper, case studies related to DSS cracking are presented specific to the refining industry. Two cases of chloride stress corrosion cracking (Cl-SCC) of 2205 DSS tubing are presented. One of them occurred in an overhead exchanger tube of the crude tower, while the other failure occurred in a top pump-around exchanger tube. The mode of failure was intergranular and transgranular cracking respectively. A third case discusses sulfide stress cracking (SSC) on a 2205 DSS outlet piping of a high pressure separator in a hydrocracker unit. Improper fabrication procedures were found to be responsible for the failure of this material. Recommendations and mitigation measures to avoid such failures have been provided.
Keywords: Duplex stainless steel, chloride stress corrosion cracking, sulphide stress cracking, SCC, SSC, intergranular attack, transgranular cracking