| Abstract Scope |
This study investigates the failure of a branch connection in an oxygen service line, specifically focusing on the dissimilar metal weld joint connecting a Monel 400 weldolet to an SS316L run pipe. The joint, fabricated using Gas Tungsten Arc Welding (GTAW) with Alloy 625 filler wire, was initially selected to ensure safety in oxygen environments by utilizing materials with high ignition temperatures and low reactivity. Although initial Dye Penetrant Testing (DPT) indicated acceptance with no visible defects, leakage was subsequently detected at the weld toe during pneumatic testing. Subsequent re-verification of the weld joints via DPT revealed multiple linear indications, necessitating a detailed failure analysis.
To determine the root cause, a comprehensive laboratory examination was conducted, encompassing macroscopic inspection, microstructural analysis, chemical analysis, hardness testing, Scanning Electron Microscopy (SEM), and Energy Dispersive X-ray Spectroscopy (EDS). The analytical results identified the primary damage mechanism as liquation cracking. This cracking was localized within the Partially Melted Zone (PMZ) on the Monel 400 weldolet side of the dissimilar joint.
This investigation further elaborates on the metallurgical conditions that contributed to this failure mode. It discusses potential causal factors, including the complex interaction between the base metals and the filler material during the welding process, which may have led to the formation of low-melting-point phases susceptible to liquation. Finally, the study recommends specific remedies to prevent recurrence, emphasizing strict control over welding parameters and heat input to mitigate the risk of PMZ liquation. These recommendations aim to enhance the integrity of dissimilar metal weld joints in critical oxygen service applications, ensuring long-term reliability and safety. |