| Author(s) |
Shutong Zhang, Sebastian A. Romo , Kaleb U. Ponder, Rafael A. Giorjao, Jorge Penso, Haixia Guo, Simon Yuen, Lisa M. Ely, Antonio J. Ramirez |
| Abstract Scope |
Introduction
Welding repair technologies have been widely used in the petrochemical industry to restore the structural integrity of pipelines and pressure vessels, which are compromised by severe operation and environment conditions. This study focuses on investigating the welding repair technologies used for coke drums, which are large pressure vessels for the oil refining process. To maximize oil production, coke drum usually undergoes extreme thermal and mechanical loadings, which lead to low-cycle fatigue failures such as bulging and cracking at the vessel body and attachments. External welding repair is implemented to restore the integrity of the damaged area, but previous studies show the welded area is highly susceptible to subsequent failures caused by low-cycle fatigue. Hence, evaluating the fatigue behavior of weld joints becomes an important topic to improve welding repair technologies. In this study, two Ni-Steel dissimilar joints were investigated through isothermal low-cycle fatigue testing (ILCF), microstructural characterization and failure analysis. The nickel-base weld metals were alloy 182 and alloy 625 welded with 1.25Cr-0.5Mo low alloy steel. Fatigue tests were performed at weld metal (WM), base metal (WM) and weld transition (WT) at 1.0%, 1.5% and 2.0% strain amplitudes. Failure analysis was performed to investigate the relationship between strength mismatch and fatigue resistance.
Experimental Procedures
The two Ni-Steel dissimilar weld mockups were fabricated using temper-bead welding (TBW) technique. The WMs are alloy 82/182 and alloy 625 and the BM is 1.25Cr-0.5Mo LAS plate with 410s stainless steel clad layer. The welds are double V-groove shape with 110˚ angle. The root passes of the welds were made by Gas Tungsten Arc Welding (GTAW) process followed by Shield Metal Arc Welding (SMAW) process to weld filling and cap passes. The microstructure of weld cross-section was examined using optical microscopy and micro-hardness machine. Dogbone samples were extracted at the WM, BM and WT regions perpendicular to the welding direction. Fatigue tests were performed in the Gleeble 3800 thermo-mechanical simulator. The ILCF tests were strain-controlled fully-reversed tests (Strain Ratio R = -1) at 1.0%, 1.5% and 2.0% strain amplitudes. The sample gage section was heated isothermally at 250˚C during ILCF test. The fatigue results from alloy 182 and alloy 625 weld mockups were summarized to compare the cyclic behaviors of the two welds. Micro hardness measurements were performed at the WT regions of an as-received sample and samples tested at 1.0% and 1.5% strain amplitudes.
Fatigue cracks were characterized using optical microscopy (OM) and scanning electron microscopy (SEM) methods. Electron Backscattered Diffraction (EBSD) analysis was performed at the WT regions of the as-received sample and the WT sample tested at 1.0% strain. Features of the facture surfaces were characterized using SEM technique and the 3D topography information was acquired using a depth-of-field digital microscope.
In addition, finite element analysis (FEA) of WT samples were performed using the Abaqus software. A 2D finite element model was built upon the cyclic hardening data from ILCF test results. A simulation was performed at 1.5% strain amplitude to reveal the stress and strain distributions at the WT region.
Results and Discussions
Hardness mappings show that alloy 182 WM exhibits similar hardness to the 1.25Cr-0.5Mo BM while the hardness of alloy 625 WM overmatches the BM. High hardness regions were detected at the Coarse Grain Heat Affected Zone (CGHAZ), which consists of lath martensitic structure. In addition, the first layer of alloy 182 WM exhibits lower hardness than the subsequent passes and the HAZ region. Cyclic responses from ILCF tests show that both nickel-base WMs exhibit significant cyclic hardening in the first 10 ~ 50 cycles. As a result, the cyclic stress of alloy 182 WM matches that of the BM at stabilized cycles, while the cyclic stress of alloy 625 significantly overmatches that of the BM. The cyclic behavior of the WT sample is determined by weaker area of the joint, where most of the plastic deformation is accumulated. According to the strain-life curve, BM has longer fatigue life over the two WMs, while alloy 182 WM exhibit longer fatigue life over alloy 625 WM. In terms of the cyclic behaviors of WTs, alloy 625 shows better fatigue resistance over alloy 182 despite the overmatching strength.
Failure analysis shows that fatigue cracks of alloy 182 WT predominantly occur at the first layer of WM and propagate along the fusion boundary in comparison with BM cracks in alloy 625 WT. EBSD strain analysis of alloy 182 WT region reveals plastic strain in the WM region next to the fusion boundary due to the local strength mismatches. Fractography analysis of WM and WT samples show the solidification dendritic structures at crack initiation sites and crack paths of secondary cracks.
The FEA simulation of alloy 182 WT at 1.5% strain amplitude predicts the strains in WM and BM are 1.36% and 1.67% respectively, as opposed to 0.7% and 2.6% of alloy 625 WT.
Conclusions
Two Ni-Steel dissimilar joints were evaluated using low-cycle fatigue tests, failure analysis and finite element simulations. ILCF tests show that the cyclic strength of alloy 182 and alloy 625 against BM to be matching and overmatching. The BM has the longest fatigue life over alloy 182 WM and alloy 625. Despite the strength mismatch, alloy 625 WT exhibits better fatigue resistance over alloy 182 WT. Failure analysis shows that the first layer of alloy 182 WM is susceptible to fatigue cracking due to the local stress concentration, whereas the fatigue cracks of alloy 625 WT tend to occur at the BM region next to HAZ region. The solidification structures at the fracture surface demonstrate that the propagation of small cracks could be affected by solidification microstructures and inclusion particles. FEA analysis shows similar strain distribution at alloy 182 WT as opposed to the uneven strain distribution in alloy 625 WT. |