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Using Duplex Stainless Steel to Join X65 Pipe Internally Clad with Alloy 625 for Subsea Applications

Suma, Emeric Emmanuel

Abstract Details

2017, Master of Science, Ohio State University, Welding Engineering.
Oil reserves off of the coast of Brazil have been discovered under a geological layer of salt. These pre-salt sea oil fields present high material requirements for extraction. The oil itself is high in H2S and other contaminants that make it extremely corrosive. The reserves are below 2 km water, 2km rock, and 2km requiring a stronger pipeline material. X65 pipe internally clad with Ni-based Alloy 625 was chosen for the risers and pipelines to meet these requirements. Joining of these pipelines will occur on-shore, after which the pipes will be loaded onto ships by being reeled onto spools with diameter of 20m. A high deposition rate process is required to make production efficient. The welds cannot be post-weld heat treated (PWHT) and their yield strength must over-match the base metal’s by 100 MPa (550 MPa), so that plastic strain occurs in the base metal, not the weld. The yield strength requirement is determined by DNV-OS-F101, a standard for offshore pipeline systems [1]. The primary issue is that weld consumables that meet this strength requirement have a higher melting point than Alloy 625. The increased energy required to melt the consumable results in greater melting of the substrate and increased dilution. If a low alloy steel were utilized, the increased dilution from Alloy 625 results in extensive solidification cracking in the weld metal. From a fundamental perspective, this project is about welding a higher melting point consumable over a lower melting point substrate. The objective of this project was to evaluate the applicability of Duplex and Super Duplex Stainless Steel (DSS, SDSS) filler metals for welding of X65 steel pipes internally clad with Alloy 625 utilizing low heat input Gas Metal Arc Welding (GMAW) process. The problem of solidification cracking in welding with higher melting point consumable over lower melting point substrate was addressed by developing a comprehensive consumable selection and evaluation procedure. The latter included: 1) material selection based on reported yield strength, 2) solidification simulations to determine solidification cracking risk, 3) bead on plate welding parameter variation trials to assess general weldability, 4) flat position narrow groove welding for parameter development, 5) pipe narrow groove welding, and 6) metallurgical characterization and mechanical testing. Alloy 686 was considered alongside Alloy 625 for the root pass. DSS 2209, SDSS 2594, & SDSS 2594-CuT filler metals were considered to fill the groove or act as a buffer layer between the Ni-based alloy root and a low alloy steel fill consumable. Because dilution from the Ni-based alloy root pass could increase solidification cracking risk, low heat input weld processes GMAW-Pulse, Cold Metal Transfer (CMT), and CMT+Pulse were used to minimize dilution. Solidification computational modelling was performed using ThermoCalcTM. Pseudo-binary solidification phase diagrams were generated using the Scheil Solidification Module by assuming an ideal mixing between a weld consumable and substrate in 10% dilution increments. Solidification cracking risk was evaluated in terms of solidification temperature range and dilution at which austenitic solidification cracking began. Alloy 686 was predicted to have a better crack resistance than Alloy 625. Solidification cracking risk was not affected by choice of duplex alloy. Modelling predicted a higher risk of intermetallic formation with increasing alloying content in the duplex material. SDSS 2594-CuT exhibited the highest risk, followed by SDSS 2594 and DSS 2209. Parameter variation trials were performed with various material and process combinations in the bead-on-plate configuration. Solidification cracking in every Ni-based & duplex combination was eliminated by maintaining dilution below 20%. At the interface between Alloy 625 and any given duplex alloy, inter-dendritic voids were encountered. These are hypothesized to be a form of shrinkage porosity that formed at the end of solidification. Pockets of liquid were isolated between multiple solidification fronts and the negative volume change of solidification resulted in voids. This hypothesis was validated by development of a model of the solidification process at the dissimilar fusion boundary that accounted for the local composition gradient and for the difference in solidification temperature ranges of the base metal and weld metal. The model predicted two opposite solidification fronts moving towards a dip in the solidus temperature in front of the dissimilar fusion boundary where shrinkage porosity typically forms. Alloy 686 did not experience a dip in the solidus temperature as a function of dilution and experienced no shrinkage porosity. Using CMT+Pulse to deposit SDSS 2594 over Alloy 686 resulted in no defects over a wide dilution range in bead on plate welding and was applied to the narrow groove geometry in the flat position. No shrinkage porosities or solidification cracks were encountered in narrow groove welds, but undercutting into the sidewall and lack of fusion defects at the sidewall between passes were common. These defects were eliminated by careful parameter optimization. By creating a shorter, narrower arc, wider weaves at low frequencies were able to result in no lack of fusion defects or undercutting. More severe lack of fusion defects due improper fixturing could not be corrected and development was continued on actual pipe. This same material and process combination was applied to X65 pipe and optimized. Full groove were performed using SDSS 2594 or DSS 2209 using CMT+Pulse over Alloy 686. A single buffer pass of SDSS 2594 using CMT+Pulse over Alloy 686 was utilized for fill passes of SDSS 2594 using GMAW-Pulse or ER100 using GMAW-Spray. Gas porosity was common in the both DSS 2209 and SDSS 2594. Shrinkage porosity was encountered between ER100 and the buffer pass of SDSS 2594. The SDSS 2594 weld with the least amount of gas porosity underwent mechanical testing and further metallurgical characterization. This weld passed the yield strength, hardness, and bending requirements outlined in DNV-OS-F101 . Of the three tensile samples, one did not meet the ductility requirement. The lower ductility sample had a gas pore approximately 15% of the fracture surface area. The weld exhibited low ferrite content, with 21% ferrite in the 6th pass and 33% ferrite in the final pass. Using CMT+Pulse to deposited SDSS 2594 over an Alloy 686 root pass to join X65 steel pipes met the
Boian Alexandrov, Ph.D. (Advisor)
Carolin Fink, Ph.D. (Committee Member)
235 p.

Recommended Citations

Citations

  • Suma, E. E. (2017). Using Duplex Stainless Steel to Join X65 Pipe Internally Clad with Alloy 625 for Subsea Applications [Master's thesis, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1492696298842902

    APA Style (7th edition)

  • Suma, Emeric. Using Duplex Stainless Steel to Join X65 Pipe Internally Clad with Alloy 625 for Subsea Applications. 2017. Ohio State University, Master's thesis. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1492696298842902.

    MLA Style (8th edition)

  • Suma, Emeric. "Using Duplex Stainless Steel to Join X65 Pipe Internally Clad with Alloy 625 for Subsea Applications." Master's thesis, Ohio State University, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=osu1492696298842902

    Chicago Manual of Style (17th edition)