To mitigate the impact of severe thermal loads during operation, two different design methodologies are typically adopted when selecting base metal materials for coke drum fabrication. The first approach, which favors the use of high-chromium content chromoly steels, prioritizes strength characteristics to improve resistance to bulging. The second approach, which favors carbon steel and C-1/2Mo, tacitly acknowledges the inevitability of bulging, and targets materials with lower strength and finer grain size instead, to optimize fracture ductility and accommodate larger bulges prior to the onset of cracking. The inherent trade-off involved in optimizing bulge resistance versus resistance to cracking is a crucial one to be weighed, especially in light of additional considerations that may bias material selection one way or another, such as the greater reparability of C-1/2Mo. The current study investigates eight coke drums from the same upgrader unit, six of which were fabricated using a C1/2Mo alloy, with the remaining two fabricated using a 1Cr-1/2Mo alloy. The two sets of coke drums have many design/operating features in common, and therefore provide a reasonable methodological framework with which to compare the two base metal materials directly. To this end, an extensive experimental testing program on boat samples excavated from all eight coke drums is undertaken along with an assessment of historical data available for both sets of drums, to quantify the performance of each base metal material.
