Similar and Dissimilar Metal Weld Failures in Hydrocracking Service at a Refinery

Two welds on a primary process piping in a hydrocracker unit leaked after 2 years in service. Dye penetrant and magnetic particle inspection of the two tube-flange joints indicated circumferential cracking. The first connection was a similar metal weld (SMW) with ferritic steel materials. The second leak was at a dissimilar metal weld (DMW), with a ferritic steel tube welded to a 321 stainless steel (SS) flange. These lines were exposed to a stream consisting of 84 mol% H-2, 0.85 mol% H2S, 0.15 mol% H2O (balance hydrocarbon) operating at 123 bar and 248 degrees C. Following metallography and SEM analysis, it was determined that the SMW failed due to inadequate welding procedures that promoted high hardness and rendered the material highly susceptible to cracking. The DMW failed due to inadequate fit up and wrong filler metal selection, which led to development of hard zones in the weld deposit. Adequate pre- and post-weld heat treatment for SMWjoints, and weld design modifications and positive material identification (PMI) inspection for DMW joints, were recommended.