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 was due to hardening of the base material during welding, and recommendation was made to temper or anneal the welded regions to reduce the effects of hydrogen under pressure.