The methanator vessel of a MW Kellogg designed ammonia plant developed severe material imperfections by hydrogen-induced damage. This was most likely due to the doubtful acceptability of C0.5Mo for hydrogen service, it is more than 30 years service life and some above-design temperature excursions. Ultrasonic inspection during the 2001 turnaround revealed several discontinuities within 1 ft (30 cm) above the bottom circumferential weld seam. In following years these imperfections further deteriorated to form large sized voids within the shell material. Repair of the affected area was evaluated to be not feasible. Consequently, the vessel was replaced in the 2004 turnaround with a 1.25Cr0.5Mo vessel. After replacement, the metallographic examination of the samples from affected portions of the old vessel confirmed the findings of the ultrasonic flaw detection and disclosed the presence of mainly transgranular and a few intergranular cracks and signs of overheating at some locations. The details of the ultrasonic inspection and micrographs have also been presented. Although the ultrasonic inspection was quite delayed, it prevented a catastrophic incident. (C) 2011 American Institute of Chemical Engineers Process Saf Prog, 2012