___________ cracking has been a major problem in coke drum shells.

Thermal fatigue is recognized as a major failure and cracking mechanism in coke drum shells operating in delayed coking units. Coke drums undergo severe cyclic heating and cooling during their regular operating cycle, which alternates between high-temperature hydrocarbon feed injection (up to 900°F) and high-pressure steam stripping combined with rapid cold-water quenching. These extreme thermal swings induce localized cyclic thermal stresses that exceed the yield strength of the material. Over time, these repeated strain cycles lead to low-cycle thermal fatigue cracking, predominantly at circumferential welds, skirt-to-shell attachment welds, and structural discontinuities. API RP 571 identifies thermal fatigue as the primary structural integrity concern for coke drum shell integrity.

A. Stress corrosion cracking requires a specific corrosive agent and tensile stress, whereas coke drum shell cracking is driven primarily by severe thermal cycling.
B. Carburization alters surface chemistry and promotes embrittlement at high temperatures, but is not the primary cracking mechanism affecting coke drum shells.
D. Sulfide stress cracking occurs in wet H2S services at low temperatures, whereas coke drum shell cracking results from cyclic thermal stresses during coking operations.

- American Petroleum Institute, API RP 571: Damage Mechanisms Affecting Fixed Equipment in the Refining Industry, 3rd Edition, 2020, Section 3.63 Thermal Fatigue, pp. 3-305–3-308.

- ASM International, Handbook Volume 11: Failure Analysis and Prevention, Cyclic Thermal Stresses in Pressure Vessels, DOI: https://doi.org/10.31399/asm.hb.v11.a0003541.