Coke drums are vertical pressure vessels used in the delayed coking process in petroleum refineries. Significant temperature variation during the delayed coking process causes damage in coke drums in the form of bulging and cracking. It was also observed that most cracks were near or within the weld seams. In order to better understand fatigue damage mechanisms of the coke drum materials with their weld sections, an experimental investigation of fatigue lives of these materials are carried out. It is a common consideration that the yield strength of the weld should be within a close percentage to that of base material in order to keep strength uniformity throughout the drum shell sections. However, this effect of the yield strength matching on the shell durability has not yet clarified quantitatively. From this point of view, two different types of specimens have been designed and manufactured: base material only, base with base weld. In addition, three groups of base with weld materials with different ratios of weld to base yield strengths are prepared. Low cycle fatigue tests at elevated temperature of 250°C are carried out on the above specimens. Through study on the recorded stress-strain hysteresis loops and observation on the fracture surfaces, damage mechanisms of different types of specimens are analyzed. Their strain range - fatigue life curves are also compared. Furthermore, finite element analyses based on the actual specimens' geometries and properties are conducted to help in understanding the experimental observations. This experimental evaluation of fatigue life of coke drum materials with welds may provide a good reference for better design of coke drums in the future.
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