Failure analysis of cracks in the sealing slot of the inlet flange of a stainless steel residual oil hydrogenation reactor
Abstract
This paper addresses the issue of circumferential cracks appearing at the bottom of the trapezoidal sealing groove of the 321H stainless steel elbow flange at the inlet of reactor R103I in a residual oil hydrogenation unit at a certain refining and petrochemical company. Using methods such as macroscopic observation, chemical composition analysis, room-temperature mechanical property testing, metallographic analysis, and scanning electron microscopy (SEM) combined with energy dispersive spectroscopy (EDS), and ANSYS finite element simulation, the study systematically investigated the morphological characteristics, causes of formation, and propagation mechanisms of the cracks. The results indicate that while the chemical composition of the flange base metal complies with the requirements of the NB/T 47010-2010 standard, the tensile strength (approximately 450 MPa) and elongation after fracture (approximately 24%) are below the lower limit of the standard, and the hardness of the overlay weld (approximately 210 HBW) significantly exceeds the standard limit. The δ-ferrite content in the overlay layer (5.2%–7.8%) is much higher than that in the base metal (<1%), which is the main reason for the increased hardness and SCC sensitivity of the overlay layer. The cracks originated in the 5–6 mm-thick overlay layer on the surface of the sealing groove. Cracking within the overlay layer was primarily intergranular and occurred along quasi-cleavage planes, indicating that these were heat-affected zone cracks or reheat cracks formed during the manufacturing stage. During service, chlorides (0.49%–0.61% wt) and sulfides in the medium induced stress corrosion. Cracks propagated dendritically through the grain from the overlay weld layer across the fusion line into the base metal, ultimately leading to the failure of the sealing groove. The research findings provide important evidence for the manufacturing quality control and in-service safety monitoring of flanges in similar pressure-bearing equipment.