High Temperature Pipeline Cracking Cause Analysis of a Synthetic Ammonia Reforming Furnace

Some cracks were detected in elbow welds of a high-temperature pipeline in a synthesis ammonia reforming furnace which has served for about two years at 650°C. The pipeline material is TP321H stainless steel. The maximum crack with a length larger than 130mm is located in the center of the circumferential weld. In this paper, FEM analysis of this pressure pipeline is carried out by the ABAQUS software, and the results indicate that the design parameters satisfy Chinese standard. By such measures as welding process analysis, chemical analysis, mechanical properties testing, optical microscopy, scanning electron microscopy, X-ray diffraction, the pipeline cracking cause is found to be the relatively poor welding quality, namely welding is fully deviated from the welding process, resulting in severe stress concentration and some welding reheat cracks. Additionally, during the installation and operation periods of the pipeline, abundant σ-phases were precipitated in grain boundaries, inducing a great plasticity and toughness losses of welds. Micro-cracks initiated from inside at some locations with relatively high stress, propagated intermittently towards outside along the grain boundary with rich σ-phases, and finally penetrated the whole pipeline wall. In the end, some suggestions are also presented to ensure the safe operations of this pressure pipeline.