LAUSR

Abstract In this paper, a damaged fire tube of H-500 vessel in Naftshahr operational area was investigated. The damages were reported as circumferential cracks at the joint of the fire tube to oval flange, which was led to leakage at U-turn section, local corrosion at the welding lines and observing scales at the outer surfaces of the fire tube. Microstructure studies were conducted at different zones (base metal, HAZ, welding line and cracked areas) for a cracked U-turn of the fire tube using optical microscopy, scanning electron microscopy (SEM) and SEM/EDS analysis. The results showed that the initial microstructure of the fire tube was laminar ferritic-pearlitic, whereas, the heat affected zone of the welding line contained grain boundary and Widmanstatten ferrite. MnS particles were also seen in the microstructure with elongated morphology and circumferential cracks. At the HAZ of welding lines, step-wise cracking was observed as an accumulation of the voids around MnS particles. This could be an indicative of hydrogen induced cracking (HIC). Mechanical properties (hardness, impact energy and tensile strength) of the four studied zones were investigated and showed that all the properties were in agreement with the standard values for ASTM A516 Gr 70 N, the steel of the fire tube, and their variation was based on the microstructural evolution. Therefore, these results showed no probability of hydrogen embrittlement.