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Metallurgical Characterization of Dissimilar Metal Welds in Grade F65 Steel to Grade F22 Steel Overlaid with Low Alloy Steel Filler Metal Using Inconel Alloy 625

Dissimilar metal welds (DMW) are used regularly in the oil and gas industry as transition joints to join high strength steels in order to eliminate the need for post weld heat treatment (PWHT) following field welding. Over the last two decades, the oil and gas industry has reported brittle failures of Alloy 625 overlays on high strength steel forgings during subsea service while under cathodic protection. These failures have been attributed to hydrogen assisted cracking (HAC) that nucleates at brittle microstructural constituents that form at the DMW fusion boundary during welding and PWHT. A major problem with Alloy 625 overlays on high strength steel is the formation of brittle carbides in the dissimilar transition zone due to carbon diffusion during PWHT. Low alloy steel overlays have been used in place of Alloy 625 overlays in order to mitigate the effects of carbon diffusion during PWHT. This research investigated the HAC susceptibility of low alloy steel (LAS)–Alloy 625 and F65-Alloy 625 dissimilar metal combinations using three different welding procedures. All three procedures involve F22 forgings that have been overlaid with low alloy steel welding consumable using submerged arc welding along with a PWHT. The low alloy steel overlay is then welded to F65 pipes using Alloy 625 welding consumable using three different welding procedures. The welding procedures utilize: 1) narrow groove GTAW with weaving, 2) narrow groove GTAW without weaving using a layered stringer bead structure, and 3) SMAW using a standard V-groove preparation. A final PWHT is not used in any of the three welding procedures. The delayed hydrogen cracking test (DHCT) was used to rank the HAC susceptibility of the three welding procedures. The DHCT uses time to failure (TTF) under constant tensile load and simultaneous electrolytic charging with hydrogen to rank susceptibility to HAC. Initial DHCT results have found the HAC susceptibility to be strongly dependent on welding procedure. Metallurgical characterization of the dissimilar metal welds was performed using light optical microscopy, scanning electron microscopy (SEM), hardness testing, nano indentation, SEM fractography, energy dispersive spectroscopy (EDS) and electron backscatter diffraction (EBSD). Key microstructural features include large regions of planar growth, weld metal swirls, partially mixed zones, decarburization in the CGHAZ and a sharp compositional gradient at the dissimilar fusion boundary. Hardness values in excess of 400 HV have been observed in the planar growth region directly adjacent to the fusion boundary. 


Industry Sponsor: OneSubsea

Faculty: Boian Alexandrov (OSU)

Graduate Student: Ryan Buntain

Industry Contact: Dean Hannam