Reheat cracking failure of a welded alloy 803 outlet pigtail tube used in a steam hydrocarbon reforming furnace

摘要

An intensive failure investigation of the alloy 803 outlet pigtail tube, used in a steam hydrocarbon reforming furnace, was conducted in the current study, in order to clarify the failure root causes. In a fabrication stage, the outlet pigtail tube was unintentionally overheated during post weld heat treatment for solution annealing. The overheating temperature of about 1250 degrees C, exceeding a designed temperature of 1150 degrees C, was detected by thermal measuring devices. As a result of overheating, the grains of the material were remarkably coarsened. An average grain size of 598 mu m was found for the overheated region, which was about 2.6 times larger than the grain size of material in the unaffected regions. Additionally, overheating resulted in the partial dissolution of Ti-rich stabilized carbides in the grain interiors. During service at a temperature between 800 and 850 degrees C, Cr-rich carbide networks were formed at the grain boundaries. A combination of the coarsened grains and the Cr-rich carbide networks at grain boundaries strongly resulted in a decrease of the materials cracking resistance. Therefore, the welded outlet pigtail tube failed by the occurrence of circumferential rupture after being in service for about 500 hours. Based on these evidences, the reheat cracking mechanism has to be anticipated as to be important for controlling the rupture of the welded outlet pigtail tube.