Investigating Weld Interface Cracking of 1.25Cr-0.5Mo Steel External Weld Repair with Alloy 625 Filler Metal due to Low-cycle Fatigue Failure

Abstract

Welding repair has been widely used to restore the structural integrity of coke drums compromised by cracks due to low-cycle fatigue failure among other damaging mechanisms. Nevertheless, due to the heterogeneous mechanical and metallurgical properties, repair welds are susceptible to re-cracking during subsequent operations. The API 1996 report has shown that 88% of outer-diameter (OD) repair welds have experienced re-cracking in coke drums. The re-cracking susceptibility downgrades the efficiency of welding repair to extend coke drum operation life and may increase the complexity of coke drum maintenance. Hence, addressing the low-cycle fatigue failure of weld becomes a critical issue to optimize welding repair for coke drum maintenance. Alloy 625 is a commonly used Ni-based filler metal to perform coke drum repairs due to its good thermal and metallurgical compatibility with the base metal. However, weld metal dilution close to the weld interface changes local microstructure and mechanical properties from the rest part of the weld. In this study, an external repair welds of 1.25Cr-0.5Mo steel under as-received (Normalized and Tempered) and service-aged conditions with Alloy 625 using HP-GTAW process are evaluated based on low-cycle fatigue tests and failure analysis. Weld transition samples are extracted to encompass the transition from weld metal to base metal at the gage section. Low-cycle fatigue tests are performed at 0.7%, 1.0%, 1.5% and 2.0% strain amplitudes. Interfacial cracks are observed at lower strain amplitudes (0.7% and 1.0%) and failures at higher strain amplitudes tend to occur at the base metal region. Metallurgical characterizations are performed to characterize the microstructure at weld interface and measure the mechanical properties through micro hardness. Interfacial cracks are examined using Scanning Electron Microscopy. This study helps clarify how microstructure and mechanical properties contribute to the interfacial cracking of the Ni-base alloy and steel dissimilar joint.