Evaluating Low-cycle Fatigue Behavior of Low-alloy Steels and Welded Joints for Coke Drum Fabrication and Repair

Coke drums are large pressure vessels used in the oil refining process, which generate lightweight oil products and solid cokes as a byproduct from the heavy residual oil. Coke drums are critical units in the delayed coking process and often undergo severe thermo-mechanical loadings that cause plastic deformation in the vessels. The accumulative damage in coke drums over operation cycles leads to low-cycle fatigue failure that significantly reduces the lifetime of coke drums. Many factors can contribute to the low-cycle fatigue damage in coke drums, including design, fabrication, operation, and repair. One of the inherent reasons is the thermal and mechanical incompatibility between the base metal and weld metal, as low-cycle fatigue damages are frequently observed adjacent to the weld seams. The structural integrity is significantly compromised at a bulging or cracking region, so coke drum repair is required before continuing the operation. Welding repair has been widely adopted in industry to restore the structural integrity of damaged regions. However, the repaired regions are also susceptible to subsequent failures due to welding defects and mechanical incompatibility.A careful selection of filler metal and the welding process is critical to improve the efficacy of welding repair. To address the issue of selecting the optimal filler metal and welding process for coke drum repair, a study regarding coke drum welding repair was initiated in the Manufacturing & Materials Joining Innovation Center (MA2JIC) at OSU in the year of 2016. In Phase-I, a novel isothermal low-cycle fatigue (ILCF) testing approach was developed using the Gleeble® thermo-mechanical simulator, and a wide range of filler metals and welding processes were evaluated using this technique. The project entered Phase II in the year 2019. The Phase-II study continues Phase-I efforts on filler metal selection based on low-cycle fatigue evaluation and expands the materials selection scope to coke drum fabrication. In addition, fundamental analysis of the low-cycle fatigue performance of coke drum base materials and welded joints was implemented through mechanical testing, materials characterization, in situ testing, and finite element analysis. This document covers the achievements made in Phase II and includes a summary of key findings in both Phase I and Phase II.The document consists of 11 chapters. Chapter 1 discusses the motivation and objectives of Phase II study, and Chapter 2 presents the background of this work. The industrial background of cokedrum is first discussed in Section 2.2 and followed by the discussions of a literature review in low-cycle fatigue testing approaches, materials performance, and weld simulation. Chapter 3 presents the low-cycle fatigue testing results of C-0.5Mo steel and a comparison with the 1.25Cr-0.5Mo steel results (Phase I) in terms of low-cycle fatigue performance. Chapter 4 shows the low-cycle fatigue evaluation of Ni-steel dissimilar joints and failure analysis. The chapter is constructed by three conference proceeding papers published regarding the topic. Chapter 5 evaluates the cyclic deformation of a dissimilar welded joint, in which the weld failure predominantly occurred in the weld metal region adjacent to the fusion boundary. Part of the contents in Chapter 5 overlap those in Section 4.1 and Section 4.2, but Chapter 5 presents a more in-depth analysis of the failure mechanism using the advanced characterization and the in-situ testing approach. Chapter 6 presents a published paper regarding cyclic deformation and strain localization in ferritic-pearlitic C-0.5Mo steel. This chapter aims at fundamentally understanding the impact of the ferritic-pearlitic microstructure on the cyclic behavior observed in Chapter 3. Chapter 7 introduces the measurement of local mechanical performance in similar and dissimilar welded joints using the digital image correlation technique. Chapter 8 introduces a novel optimization algorithm for cyclic hardening parameters, which are obtained from the isothermal low-cycle fatigue testing results. Chapter 9 presents a summary of the achievements in Phase I and Phase II regarding the selection of the material for coke drum fabrication and welding repair. Chapter 10 and 11 presents the major conclusions and future work for the study.