A Finite Element Analysis of the Cyclic Plasticity Behavior of Circumferential Weld of Coke Drum under Moving Axial Temperature Gradient

摘要

Coke drams undergo cyclic thermal and mechanical loading during operation, which tend to cause damage like bulging and cracking near circumferential welds in coke drums. In this paper, considering the existence of the overmatch welds and uneven temperature field during oil-feeding and water quenching as well as quenching conditions, the bulging behavior of the coke drum is investigated. Firstly, the uniaxial tension and ratchetting experiments of parent metal and weld of coke drum are carried out at 20 °C, 200 °C, 300 °C, and 400 °C. Secondly, the ratchetting strains are predicted by OW-II kinematic hardening model and the predicted ratchetting strain rates of steady stage are in qualitative agreement with the experimental results. By comparison, weld has significantly smaller ratchetting strain rate than parent metal under the same loading conditions, which shows that the anti-ratchetting ability of weld is better than that of parent metal. Thirdly, the transient temperature field during oil-feeding and water quenching are numerically simulated with the help of dynamic coordinate system method, which yields the axial temperature difference and its characteristic value for the simplified characteristic load to be used in the cyclic plastic analysis of coke drum cycled between oil-feeding and water quenching. Fourthly, the ratchet boundaries are determined under moving axial temperature gradient by cyclic plasticity analysis for the parent metal. It is found that, when temperature difference range of water quenching is smaller, the ratchet boundaries are heavily influenced by the cyclic internal pressure. Finally, considering the strength differences between parent metal and weld, cyclic plasticity analysis is done for the circumferential weld connection region of coke drum. The results indicate that the bulging is preferentially near and about 10 mm above the weld under given loading conditions. It is also observed that, for cylinder with or without the circumferential weld, the co-existence of cyclic internal pressure can increase the ratchetting strain of the structure and the ratchetting strain rate of structure also increases with the cyclic internal pressure range.