Microstructural aging and property degradation of service-exposed 1Cr–1/2Mo Coke Drum steel

After long-term service, boat samples of 1Cr-1/2Mo steel seemed to have aged, evidenced by the spheroidisation of carbides in the pearlite region, and a dispersion of fine M₂₃C₆ precipitation in the ferrite matrix. The yield strength of the cracked boat samples was two to three times the reported values of new A387 Grade 12 Class 2 material. The ductility is less than 0.5 times the reported values of the new grade. The intergranular fracture mode was likely due to the fine carbide precipitation inside the ferrite grains that strengthened the metal matrix, and the coarse intergranular CrMo-rich carbides that weaken the precipitate/grain boundary interfaces. Cavities similar to creep were observed along the grain boundaries in some of the boat samples, suggesting a possible creep-oxidation damage mechanism for coke drum steels.