Coke drums are pressure vessels used in the delayed coking process at oil refineries, which transform heavy residual oil into light-weight hydrocarbon molecules and solid coke through thermal cracking. Due to the severe thermal and mechanical loadings during operation, these vessels experienced low-cycle fatigue failure, which led to shell and skirt damage such as bulging and cracking. External weld repairs using the temper bead technique have been widely applied to repair damaged regions caused by bulging and cracking for preventing the leaks of the residual oil contents. However, a substantial proportion of the external repairs have been reported to experience post-repair cracking issues. In this study, an external weld repair mockup with a backing plate was evaluated through metallurgical characterizations and isothermal low-cycle fatigue tests. The micro-hardness mapping identified the mismatching interfaces from base metal to weld metal (WM) and from root passes to internal clad. Four types of dog-bone samples were extracted from the weld: weld metal, heat affected zone, internal clad and backing plate. These samples were used to evaluate the fatigue resistance of weld metal and transition zone under low-cycle fatigue tests. Failure analysis showed that weld metal samples were susceptible to multiple-crack initiations, while other transition samples failed at mismatching interfaces or stress concentration points at weld toes.

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