Hydrogen induced stress cracking in UNS S32750 super duplex stainless steel tube weld joint

• Hydrostactic stress fields enhances the hydrogen solubility and permeability.
• The welded microstructure accelerates the hydrogen diffusion.
• Super duplex stainless steel structures subjected to cathodic protection are prone to hydrogen embrittlement.

In this work, the hydrogen embrittlement of super duplex stainless steel tubes UNS S32750 under different hydrogen charging conditions was investigated. Hydrogen charging was performed both in unloaded and in elastically stressed specimens. The tubes were charged and tested under two conditions: as-received and welded. The samples were submitted to tensile tests in order to evaluate how the mechanical properties are affected by the hydrogen inserted into the microstructure. The fracture surfaces were analyzed in order to identify the failure active micro mechanisms. Combined tensile test results and microstructural investigations indicated that elastic stress applied to samples during hydrogen charging enhanced hydrogen solubility and permeability, resulting in greater loss of ductility. As pointed by fractographies of welded H-charged tubes, the microstructural changes induced by welding increased hydrogen diffusivity. The fracture surface of the hydrogen charged as-received tubes exhibited a quasi-cleavage aspect, indicating a mixed failure mechanism composed by plastic strain in austenite and cleavage on ferrite. Otherwise, the main mechanism of failure on welded tubes was cleavage.