Intergranular Corrosion in Retaining Rings Made of X39CrMo17-1 and X39Cr13

Abstract

Abstract A number of retaining rings used in fuel gas piping assemblies of large stationary heavy-duty industrial gas turbines were found broken upon reception at the gas turbine manufacturer's plant. While the pipes were made of AISI 321 austenitic stainless steel, 1.4541, X6CrNiTi18-10, the specified material of the subject locking rings is the martensitic stainless chromium steel X39CrMo17-1, 1.4122. In fact, however, the failed snap rings consisted of the lower-chromium X39Cr13, 1.4031. The fractured retaining rings all originated from a particular North American supply chain. In contrast, the same pipe assemblies delivered by an alterna­tive second-source European supplier did not show these remarkable fractures in their retain­ing rings. Also, they were indeed made of the specified higher-chromium material. It was later discovered that the supplier that shipped the faulty product employed a peculiar wet cleaning process without proper post-cleaning drying. The conclusion of the metallurgical failure analysis was that the subject retaining rings failed by intergranular corrosion due to sensitisation from heat treatment and wet cleaning residues that remained on the product and caused corrosion during shipment. The erroneous material selection, violating the specifica­tion, contributed to the failure.