The welded seams of three integral-clad catalytic-cracking unit reactors have been found to contain cracks after about 7 years of service. Examination has detected graphite in the carbon-steel base metal and heat-affected zones. This paper reports the findings of investigations made on these three and other reactors, both clad, lined and unlined, in similar service. The results of mechanical tests and microscopic examinations made on specimens removed from reactors and other high-temperature equipment in the same service are presented. Possible causes of the cracks are discussed with particular emphasis on the role of thermal stresses in bimetallic construction and the effect of graphite on mechanical properties. Recommendations are included relative to existing equipment and new construction for service at elevated temperatures.

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