The source of cracking in the circumferential weld seam in a JIS-SM50B carbon-manganese steel pipe used in a CO2 absorber was investigated, the absorber had been in service for 18 years. The seam had been weld-repaired twice, and the repair welds had been locally stress relieved. Longitudinal seams in the same vessel, which had been stress relieved in a furnace, showed no tendency toward cracking. The solution passing through the vessel contained CO2-CO-H20, KHCO, and Cl− ions. Nondestructive testing revealed that the cracks originated in the heat-affected zone and propagated into the base metal and weld. Severe branching of the cracks characteristic of stress-corrosion cracking was observed. Microexamination revealed that crack propagation was transgranular further supporting the possibility of stress-corrosion cracking. Simulation tests carried out in the vessel confirmed this mode of cracking. It was recommended that weld seams be furnace heat treated at a temperature of 600 to 640 deg C (1110 to 1180 deg F) for a minimum of 1 h per inch of section thickness.