Hydrogen-Assisted Cracking of Austenitic Stainless Steel 321 Pipes Used in Petrochemical Plants

During startup operations, the oxygen bypass line made of austenitic stainless steel AISI 321 in a secondary reformer unit developed a crack. A clamp box repair was implemented to maintain operations, but the pipe completely fractured near the clamp box after 4 days. Macroscopic observations indicated a brittle fracture with minimal thinning. Further examination revealed transgranular crack growth, confirming a brittle fracture mechanism. The chemical composition of SS 321 and its bulk mechanical properties, including strength, ductility, and hardness, met the required standards. However, microstructural analysis showed dispersed TiN particles within the austenitic matrices. SEM examination of the fracture surface identified TiN as the weakest point in the microstructure, suggesting a hydrogen-assisted cracking mechanism. Hydrogen from steam reformate gas lines, catalyzed by deposits containing Mg and Ca on the internal tube surface, diffused and accumulated at the interface between TiN and the austenitic matrices, leading to hydrogen-assisted cracking. Therefore, it is recommended that the oxygen line pipe be replaced with austenitic stainless steel grades that do not contain titanium, such as AISI 316L.