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• The unexpected fracture of tube-to-tubesheet joints of a circulating oil steam generator is studied. • Caustic stress corrosion cracking is the root cause of the failure. • Proposals are given about material selection, manufacture and water quality assurance of the circulating oil steam generator. Tube-to-tubesheet joints are generally considered as one of the vulnerable part of heat exchangers, as they are sensitive to stress corrosion owing to residual stress generated during manufacture processes. As a result, it is crucial to carry out failure analysis on tube-to-tubesheet joints to ensure the safety of heat exchanger operations. The unexpected fracture of tube-to-tubesheet joints of a circulating oil steam generator in a petrochemical plant is studied. In order to find out the cause of the failure, the failed tube is investigated by visual inspection, base metal inspection and fracture morphologies observation. The results show that due to insufficient expansion of the tube, a gap is formed between the tube and tubesheet, which concentrates the alkali lye and chloride deposits, leading to the formation of caustic stress corrosion cracks in the outer wall of the failed tube before fracture, and the cracks rapidly propagate under tensile stress resulting in the failure. Finally, proposals are given about material selection, manufacture and water quality assurance of the circulating oil steam generator. [ABSTRACT FROM AUTHOR]