A heat exchanger failed five years after going into service in an ammonia synthesis plant. Its container, made of Cr-Mo alloy steel (Material No. 1.7362), operated in an environment that did not exceed 400 deg C or 600 atm of hydrogen partial pressure. X-ray examination revealed a fissure in one of the welded seams, which according to microscopic examination, originated in the base material of the container. Higher magnification revealed a narrow zone adjacent to the weld seam permeated with intergranular cracks, the result of hydrogen attack. It also showed the structure to be completely martensitic. Thus, the failure...